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| author | Guido van Rossum <guido@python.org> |
|---|---|
| date | Tue, 02 Dec 1997 20:38:11 +0000 |
| parents | 0f2498b847bf |
| children | 90edcb38fb00 |
| rev | line source |
|---|---|
| 3198 | 1 % libparser.tex |
| 2 % | |
| 3 % Copyright 1995 Virginia Polytechnic Institute and State University | |
| 4 % and Fred L. Drake, Jr. This copyright notice must be distributed on | |
| 5 % all copies, but this document otherwise may be distributed as part | |
| 6 % of the Python distribution. No fee may be charged for this document | |
| 7 % in any representation, either on paper or electronically. This | |
| 8 % restriction does not affect other elements in a distributed package | |
| 9 % in any way. | |
| 10 % | |
| 11 | |
| 12 \section{Built-in Module \sectcode{parser}} | |
| 13 \bimodindex{parser} | |
| 14 | |
| 15 The \code{parser} module provides an interface to Python's internal | |
| 16 parser and byte-code compiler. The primary purpose for this interface | |
| 17 is to allow Python code to edit the parse tree of a Python expression | |
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18 and create executable code from this. This is better than trying |
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19 to parse and modify an arbitrary Python code fragment as a string |
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20 because parsing is performed in a manner identical to the code |
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21 forming the application. It is also faster. |
| 3198 | 22 |
| 23 There are a few things to note about this module which are important | |
| 24 to making use of the data structures created. This is not a tutorial | |
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25 on editing the parse trees for Python code, but some examples of using |
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26 the \code{parser} module are presented. |
| 3198 | 27 |
| 28 Most importantly, a good understanding of the Python grammar processed | |
| 29 by the internal parser is required. For full information on the | |
| 30 language syntax, refer to the Language Reference. The parser itself | |
| 31 is created from a grammar specification defined in the file | |
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32 \file{Grammar/Grammar} in the standard Python distribution. The parse |
| 3198 | 33 trees stored in the ``AST objects'' created by this module are the |
| 34 actual output from the internal parser when created by the | |
| 35 \code{expr()} or \code{suite()} functions, described below. The AST | |
| 4084 | 36 objects created by \code{sequence2ast()} faithfully simulate those |
| 37 structures. Be aware that the values of the sequences which are | |
| 38 considered ``correct'' will vary from one version of Python to another | |
| 39 as the formal grammar for the language is revised. However, | |
| 40 transporting code from one Python version to another as source text | |
| 41 will always allow correct parse trees to be created in the target | |
| 42 version, with the only restriction being that migrating to an older | |
| 43 version of the interpreter will not support more recent language | |
| 44 constructs. The parse trees are not typically compatible from one | |
| 45 version to another, whereas source code has always been | |
| 46 forward-compatible. | |
| 3198 | 47 |
| 4084 | 48 Each element of the sequences returned by \code{ast2list} or |
| 49 \code{ast2tuple()} has a simple form. Sequences representing | |
| 50 non-terminal elements in the grammar always have a length greater than | |
| 51 one. The first element is an integer which identifies a production in | |
| 52 the grammar. These integers are given symbolic names in the C header | |
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53 file \file{Include/graminit.h} and the Python module |
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54 \code{symbol}. Each additional element of the sequence represents |
| 4084 | 55 a component of the production as recognized in the input string: these |
| 56 are always sequences which have the same form as the parent. An | |
| 57 important aspect of this structure which should be noted is that | |
| 58 keywords used to identify the parent node type, such as the keyword | |
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59 \code{if} in an \code{if_stmt}, are included in the node tree without |
| 4084 | 60 any special treatment. For example, the \code{if} keyword is |
| 3198 | 61 represented by the tuple \code{(1, 'if')}, where \code{1} is the |
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62 numeric value associated with all \code{NAME} tokens, including |
| 4084 | 63 variable and function names defined by the user. In an alternate form |
| 64 returned when line number information is requested, the same token | |
| 65 might be represented as \code{(1, 'if', 12)}, where the \code{12} | |
| 66 represents the line number at which the terminal symbol was found. | |
| 3198 | 67 |
| 68 Terminal elements are represented in much the same way, but without | |
| 69 any child elements and the addition of the source text which was | |
| 70 identified. The example of the \code{if} keyword above is | |
| 71 representative. The various types of terminal symbols are defined in | |
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72 the C header file \file{Include/token.h} and the Python module |
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73 \code{token}. |
| 3198 | 74 |
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75 The AST objects are not required to support the functionality of this |
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76 module, but are provided for three purposes: to allow an application |
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77 to amortize the cost of processing complex parse trees, to provide a |
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78 parse tree representation which conserves memory space when compared |
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79 to the Python list or tuple representation, and to ease the creation |
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80 of additional modules in C which manipulate parse trees. A simple |
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81 ``wrapper'' class may be created in Python to hide the use of AST |
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82 objects; the \code{AST} library module provides a variety of such |
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83 classes. |
| 3198 | 84 |
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85 The \code{parser} module defines functions for a few distinct |
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86 purposes. The most important purposes are to create AST objects and |
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87 to convert AST objects to other representations such as parse trees |
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88 and compiled code objects, but there are also functions which serve to |
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89 query the type of parse tree represented by an AST object. |
| 3198 | 90 |
| 91 \renewcommand{\indexsubitem}{(in module parser)} | |
| 92 | |
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93 |
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94 \subsection{Creating AST Objects} |
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95 |
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96 AST objects may be created from source code or from a parse tree. |
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97 When creating an AST object from source, different functions are used |
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98 to create the \code{'eval'} and \code{'exec'} forms. |
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99 |
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100 \begin{funcdesc}{expr}{string} |
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101 The \code{expr()} function parses the parameter \code{\var{string}} |
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102 as if it were an input to \code{compile(\var{string}, 'eval')}. If |
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103 the parse succeeds, an AST object is created to hold the internal |
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104 parse tree representation, otherwise an appropriate exception is |
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105 thrown. |
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106 \end{funcdesc} |
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107 |
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108 \begin{funcdesc}{suite}{string} |
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109 The \code{suite()} function parses the parameter \code{\var{string}} |
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110 as if it were an input to \code{compile(\var{string}, 'exec')}. If |
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111 the parse succeeds, an AST object is created to hold the internal |
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112 parse tree representation, otherwise an appropriate exception is |
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113 thrown. |
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114 \end{funcdesc} |
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115 |
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116 \begin{funcdesc}{sequence2ast}{sequence} |
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117 This function accepts a parse tree represented as a sequence and |
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118 builds an internal representation if possible. If it can validate |
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119 that the tree conforms to the Python grammar and all nodes are valid |
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120 node types in the host version of Python, an AST object is created |
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121 from the internal representation and returned to the called. If there |
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122 is a problem creating the internal representation, or if the tree |
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123 cannot be validated, a \code{ParserError} exception is thrown. An AST |
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124 object created this way should not be assumed to compile correctly; |
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125 normal exceptions thrown by compilation may still be initiated when |
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126 the AST object is passed to \code{compileast()}. This may indicate |
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127 problems not related to syntax (such as a \code{MemoryError} |
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128 exception), but may also be due to constructs such as the result of |
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129 parsing \code{del f(0)}, which escapes the Python parser but is |
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130 checked by the bytecode compiler. |
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131 |
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132 Sequences representing terminal tokens may be represented as either |
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133 two-element lists of the form \code{(1, 'name')} or as three-element |
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134 lists of the form \code{(1, 'name', 56)}. If the third element is |
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135 present, it is assumed to be a valid line number. The line number |
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136 may be specified for any subset of the terminal symbols in the input |
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137 tree. |
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138 \end{funcdesc} |
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139 |
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140 \begin{funcdesc}{tuple2ast}{sequence} |
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141 This is the same function as \code{sequence2ast()}. This entry point |
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142 is maintained for backward compatibility. |
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143 \end{funcdesc} |
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144 |
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145 |
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146 \subsection{Converting AST Objects} |
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147 |
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148 AST objects, regardless of the input used to create them, may be |
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149 converted to parse trees represented as list- or tuple- trees, or may |
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150 be compiled into executable code objects. Parse trees may be |
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151 extracted with or without line numbering information. |
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152 |
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153 \begin{funcdesc}{ast2list}{ast\optional{\, line_info\code{ = 0}}} |
| 4084 | 154 This function accepts an AST object from the caller in |
| 155 \code{\var{ast}} and returns a Python list representing the | |
| 156 equivelent parse tree. The resulting list representation can be used | |
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157 for inspection or the creation of a new parse tree in list form. This |
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158 function does not fail so long as memory is available to build the |
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159 list representation. If the parse tree will only be used for |
| 4084 | 160 inspection, \code{ast2tuple()} should be used instead to reduce memory |
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161 consumption and fragmentation. When the list representation is |
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162 required, this function is significantly faster than retrieving a |
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163 tuple representation and converting that to nested lists. |
| 4084 | 164 |
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165 If \code{\var{line_info}} is true, line number information will be |
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166 included for all terminal tokens as a third element of the list |
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167 representing the token. Note that the line number provided specifies |
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168 the line on which the token \emph{ends\/}. This information is |
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169 omitted if the flag is false or omitted. |
| 4084 | 170 \end{funcdesc} |
| 171 | |
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172 \begin{funcdesc}{ast2tuple}{ast\optional{\, line_info\code{ = 0}}} |
| 3198 | 173 This function accepts an AST object from the caller in |
| 174 \code{\var{ast}} and returns a Python tuple representing the | |
| 4084 | 175 equivelent parse tree. Other than returning a tuple instead of a |
| 176 list, this function is identical to \code{ast2list()}. | |
| 177 | |
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178 If \code{\var{line_info}} is true, line number information will be |
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179 included for all terminal tokens as a third element of the list |
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180 representing the token. This information is omitted if the flag is |
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181 false or omitted. |
| 3198 | 182 \end{funcdesc} |
| 183 | |
| 4084 | 184 \begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}} |
| 3198 | 185 The Python byte compiler can be invoked on an AST object to produce |
| 186 code objects which can be used as part of an \code{exec} statement or | |
| 187 a call to the built-in \code{eval()} function. This function provides | |
| 188 the interface to the compiler, passing the internal parse tree from | |
| 189 \code{\var{ast}} to the parser, using the source file name specified | |
| 190 by the \code{\var{filename}} parameter. The default value supplied | |
| 191 for \code{\var{filename}} indicates that the source was an AST object. | |
| 4084 | 192 |
| 193 Compiling an AST object may result in exceptions related to | |
| 194 compilation; an example would be a \code{SyntaxError} caused by the | |
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195 parse tree for \code{del f(0)}: this statement is considered legal |
| 4084 | 196 within the formal grammar for Python but is not a legal language |
| 197 construct. The \code{SyntaxError} raised for this condition is | |
| 198 actually generated by the Python byte-compiler normally, which is why | |
| 199 it can be raised at this point by the \code{parser} module. Most | |
| 200 causes of compilation failure can be diagnosed programmatically by | |
| 201 inspection of the parse tree. | |
| 3198 | 202 \end{funcdesc} |
| 203 | |
| 204 | |
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205 \subsection{Queries on AST Objects} |
| 3198 | 206 |
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207 Two functions are provided which allow an application to determine if |
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208 an AST was create as an expression or a suite. Neither of these |
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209 functions can be used to determine if an AST was created from source |
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210 code via \code{expr()} or \code{suite()} or from a parse tree via |
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211 \code{sequence2ast()}. |
| 3198 | 212 |
| 213 \begin{funcdesc}{isexpr}{ast} | |
| 214 When \code{\var{ast}} represents an \code{'eval'} form, this function | |
| 215 returns a true value (\code{1}), otherwise it returns false | |
| 216 (\code{0}). This is useful, since code objects normally cannot be | |
| 217 queried for this information using existing built-in functions. Note | |
| 218 that the code objects created by \code{compileast()} cannot be queried | |
| 219 like this either, and are identical to those created by the built-in | |
| 220 \code{compile()} function. | |
| 221 \end{funcdesc} | |
| 222 | |
| 223 | |
| 224 \begin{funcdesc}{issuite}{ast} | |
| 225 This function mirrors \code{isexpr()} in that it reports whether an | |
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226 AST object represents an \code{'exec'} form, commonly known as a |
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227 ``suite.'' It is not safe to assume that this function is equivelent |
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228 to \code{not isexpr(\var{ast})}, as additional syntactic fragments may |
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229 be supported in the future. |
| 3198 | 230 \end{funcdesc} |
| 231 | |
| 232 | |
| 233 \subsection{Exceptions and Error Handling} | |
| 234 | |
| 235 The parser module defines a single exception, but may also pass other | |
| 236 built-in exceptions from other portions of the Python runtime | |
| 237 environment. See each function for information about the exceptions | |
| 238 it can raise. | |
| 239 | |
| 240 \begin{excdesc}{ParserError} | |
| 241 Exception raised when a failure occurs within the parser module. This | |
| 242 is generally produced for validation failures rather than the built in | |
| 243 \code{SyntaxError} thrown during normal parsing. | |
| 244 The exception argument is either a string describing the reason of the | |
| 4084 | 245 failure or a tuple containing a sequence causing the failure from a parse |
| 246 tree passed to \code{sequence2ast()} and an explanatory string. Calls to | |
| 247 \code{sequence2ast()} need to be able to handle either type of exception, | |
| 3198 | 248 while calls to other functions in the module will only need to be |
| 249 aware of the simple string values. | |
| 250 \end{excdesc} | |
| 251 | |
| 252 Note that the functions \code{compileast()}, \code{expr()}, and | |
| 253 \code{suite()} may throw exceptions which are normally thrown by the | |
| 254 parsing and compilation process. These include the built in | |
| 255 exceptions \code{MemoryError}, \code{OverflowError}, | |
| 256 \code{SyntaxError}, and \code{SystemError}. In these cases, these | |
| 257 exceptions carry all the meaning normally associated with them. Refer | |
| 258 to the descriptions of each function for detailed information. | |
| 259 | |
| 260 | |
| 4084 | 261 \subsection{AST Objects} |
| 262 | |
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263 AST objects returned by \code{expr()}, \code{suite()}, and |
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264 \code{sequence2ast()} have no methods of their own. |
| 4084 | 265 Some of the functions defined which accept an AST object as their |
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266 first argument may change to object methods in the future. The type |
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267 of these objects is available as \code{ASTType} in the module. |
| 4084 | 268 |
| 269 Ordered and equality comparisons are supported between AST objects. | |
| 270 | |
| 271 | |
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272 \subsection{Examples} |
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273 \nodename{AST Examples} |
| 3198 | 274 |
| 4084 | 275 The parser modules allows operations to be performed on the parse tree |
| 276 of Python source code before the bytecode is generated, and provides | |
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277 for inspection of the parse tree for information gathering purposes. |
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278 Two examples are presented. The simple example demonstrates emulation |
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279 of the \code{compile()} built-in function and the complex example |
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280 shows the use of a parse tree for information discovery. |
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281 |
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282 \subsubsection{Emulation of \sectcode{compile()}} |
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283 |
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284 While many useful operations may take place between parsing and |
| 4084 | 285 bytecode generation, the simplest operation is to do nothing. For |
| 286 this purpose, using the \code{parser} module to produce an | |
| 287 intermediate data structure is equivelent to the code | |
| 288 | |
| 5818 | 289 \bcode\begin{verbatim} |
| 4084 | 290 >>> code = compile('a + 5', 'eval') |
| 291 >>> a = 5 | |
| 292 >>> eval(code) | |
| 293 10 | |
| 5818 | 294 \end{verbatim}\ecode |
| 295 % | |
| 4084 | 296 The equivelent operation using the \code{parser} module is somewhat |
| 297 longer, and allows the intermediate internal parse tree to be retained | |
| 298 as an AST object: | |
| 3198 | 299 |
| 5818 | 300 \bcode\begin{verbatim} |
| 3198 | 301 >>> import parser |
| 302 >>> ast = parser.expr('a + 5') | |
| 303 >>> code = parser.compileast(ast) | |
| 304 >>> a = 5 | |
| 305 >>> eval(code) | |
| 306 10 | |
| 5818 | 307 \end{verbatim}\ecode |
| 308 % | |
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309 An application which needs both AST and code objects can package this |
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310 code into readily available functions: |
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311 |
| 5818 | 312 \bcode\begin{verbatim} |
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313 import parser |
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314 |
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315 def load_suite(source_string): |
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316 ast = parser.suite(source_string) |
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317 code = parser.compileast(ast) |
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318 return ast, code |
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319 |
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320 def load_expression(source_string): |
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321 ast = parser.expr(source_string) |
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322 code = parser.compileast(ast) |
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323 return ast, code |
| 5818 | 324 \end{verbatim}\ecode |
| 325 % | |
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326 \subsubsection{Information Discovery} |
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327 |
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328 Some applications benefit from direct access to the parse tree. The |
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329 remainder of this section demonstrates how the parse tree provides |
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330 access to module documentation defined in docstrings without requiring |
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331 that the code being examined be loaded into a running interpreter via |
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332 \code{import}. This can be very useful for performing analyses of |
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333 untrusted code. |
| 3198 | 334 |
| 4084 | 335 Generally, the example will demonstrate how the parse tree may be |
| 336 traversed to distill interesting information. Two functions and a set | |
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337 of classes are developed which provide programmatic access to high |
| 4084 | 338 level function and class definitions provided by a module. The |
| 339 classes extract information from the parse tree and provide access to | |
| 340 the information at a useful semantic level, one function provides a | |
| 341 simple low-level pattern matching capability, and the other function | |
| 342 defines a high-level interface to the classes by handling file | |
| 343 operations on behalf of the caller. All source files mentioned here | |
| 344 which are not part of the Python installation are located in the | |
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345 \file{Demo/parser/} directory of the distribution. |
| 4084 | 346 |
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347 The dynamic nature of Python allows the programmer a great deal of |
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348 flexibility, but most modules need only a limited measure of this when |
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349 defining classes, functions, and methods. In this example, the only |
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350 definitions that will be considered are those which are defined in the |
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351 top level of their context, e.g., a function defined by a \code{def} |
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352 statement at column zero of a module, but not a function defined |
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353 within a branch of an \code{if} ... \code{else} construct, though |
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354 there are some good reasons for doing so in some situations. Nesting |
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355 of definitions will be handled by the code developed in the example. |
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356 |
| 4084 | 357 To construct the upper-level extraction methods, we need to know what |
| 358 the parse tree structure looks like and how much of it we actually | |
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359 need to be concerned about. Python uses a moderately deep parse tree |
| 4084 | 360 so there are a large number of intermediate nodes. It is important to |
| 361 read and understand the formal grammar used by Python. This is | |
| 362 specified in the file \file{Grammar/Grammar} in the distribution. | |
| 363 Consider the simplest case of interest when searching for docstrings: | |
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364 a module consisting of a docstring and nothing else. (See file |
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365 \file{docstring.py}.) |
| 4084 | 366 |
| 5818 | 367 \bcode\begin{verbatim} |
| 4084 | 368 """Some documentation. |
| 369 """ | |
| 5818 | 370 \end{verbatim}\ecode |
| 371 % | |
| 4084 | 372 Using the interpreter to take a look at the parse tree, we find a |
| 373 bewildering mass of numbers and parentheses, with the documentation | |
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374 buried deep in nested tuples. |
| 3198 | 375 |
| 5818 | 376 \bcode\begin{verbatim} |
| 4084 | 377 >>> import parser |
| 378 >>> import pprint | |
| 379 >>> ast = parser.suite(open('docstring.py').read()) | |
| 380 >>> tup = parser.ast2tuple(ast) | |
| 381 >>> pprint.pprint(tup) | |
| 382 (257, | |
| 383 (264, | |
| 384 (265, | |
| 385 (266, | |
| 386 (267, | |
| 387 (307, | |
| 388 (287, | |
| 389 (288, | |
| 390 (289, | |
| 391 (290, | |
| 392 (292, | |
| 393 (293, | |
| 394 (294, | |
| 395 (295, | |
| 396 (296, | |
| 397 (297, | |
| 398 (298, | |
| 399 (299, | |
| 400 (300, (3, '"""Some documentation.\012"""'))))))))))))))))), | |
| 401 (4, ''))), | |
| 402 (4, ''), | |
| 403 (0, '')) | |
| 5818 | 404 \end{verbatim}\ecode |
| 405 % | |
| 4084 | 406 The numbers at the first element of each node in the tree are the node |
| 407 types; they map directly to terminal and non-terminal symbols in the | |
| 408 grammar. Unfortunately, they are represented as integers in the | |
| 409 internal representation, and the Python structures generated do not | |
| 410 change that. However, the \code{symbol} and \code{token} modules | |
| 411 provide symbolic names for the node types and dictionaries which map | |
| 412 from the integers to the symbolic names for the node types. | |
| 3198 | 413 |
| 4084 | 414 In the output presented above, the outermost tuple contains four |
| 415 elements: the integer \code{257} and three additional tuples. Node | |
| 416 type \code{257} has the symbolic name \code{file_input}. Each of | |
| 417 these inner tuples contains an integer as the first element; these | |
| 418 integers, \code{264}, \code{4}, and \code{0}, represent the node types | |
| 419 \code{stmt}, \code{NEWLINE}, and \code{ENDMARKER}, respectively. | |
| 420 Note that these values may change depending on the version of Python | |
| 421 you are using; consult \file{symbol.py} and \file{token.py} for | |
| 422 details of the mapping. It should be fairly clear that the outermost | |
| 423 node is related primarily to the input source rather than the contents | |
| 424 of the file, and may be disregarded for the moment. The \code{stmt} | |
| 425 node is much more interesting. In particular, all docstrings are | |
| 426 found in subtrees which are formed exactly as this node is formed, | |
| 427 with the only difference being the string itself. The association | |
| 428 between the docstring in a similar tree and the defined entity (class, | |
| 429 function, or module) which it describes is given by the position of | |
| 430 the docstring subtree within the tree defining the described | |
| 431 structure. | |
| 3198 | 432 |
| 4084 | 433 By replacing the actual docstring with something to signify a variable |
|
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434 component of the tree, we allow a simple pattern matching approach to |
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435 check any given subtree for equivelence to the general pattern for |
|
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436 docstrings. Since the example demonstrates information extraction, we |
|
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437 can safely require that the tree be in tuple form rather than list |
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438 form, allowing a simple variable representation to be |
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439 \code{['variable_name']}. A simple recursive function can implement |
| 4084 | 440 the pattern matching, returning a boolean and a dictionary of variable |
|
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441 name to value mappings. (See file \file{example.py}.) |
| 4084 | 442 |
| 5818 | 443 \bcode\begin{verbatim} |
| 4084 | 444 from types import ListType, TupleType |
| 445 | |
| 446 def match(pattern, data, vars=None): | |
| 447 if vars is None: | |
| 448 vars = {} | |
| 449 if type(pattern) is ListType: | |
| 450 vars[pattern[0]] = data | |
| 451 return 1, vars | |
| 452 if type(pattern) is not TupleType: | |
| 453 return (pattern == data), vars | |
| 454 if len(data) != len(pattern): | |
| 455 return 0, vars | |
| 456 for pattern, data in map(None, pattern, data): | |
| 457 same, vars = match(pattern, data, vars) | |
| 458 if not same: | |
| 459 break | |
| 460 return same, vars | |
| 5818 | 461 \end{verbatim}\ecode |
| 462 % | |
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463 Using this simple representation for syntactic variables and the symbolic |
|
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464 node types, the pattern for the candidate docstring subtrees becomes |
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465 fairly readable. (See file \file{example.py}.) |
| 3198 | 466 |
| 5818 | 467 \bcode\begin{verbatim} |
|
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468 import symbol |
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469 import token |
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470 |
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471 DOCSTRING_STMT_PATTERN = ( |
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472 symbol.stmt, |
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473 (symbol.simple_stmt, |
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474 (symbol.small_stmt, |
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475 (symbol.expr_stmt, |
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476 (symbol.testlist, |
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477 (symbol.test, |
|
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478 (symbol.and_test, |
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479 (symbol.not_test, |
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480 (symbol.comparison, |
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481 (symbol.expr, |
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482 (symbol.xor_expr, |
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|
483 (symbol.and_expr, |
|
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484 (symbol.shift_expr, |
|
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|
485 (symbol.arith_expr, |
|
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|
486 (symbol.term, |
|
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|
487 (symbol.factor, |
|
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|
488 (symbol.power, |
|
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489 (symbol.atom, |
|
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490 (token.STRING, ['docstring']) |
|
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491 )))))))))))))))), |
|
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492 (token.NEWLINE, '') |
|
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|
493 )) |
| 5818 | 494 \end{verbatim}\ecode |
| 495 % | |
| 4084 | 496 Using the \code{match()} function with this pattern, extracting the |
| 497 module docstring from the parse tree created previously is easy: | |
| 498 | |
| 5818 | 499 \bcode\begin{verbatim} |
| 4084 | 500 >>> found, vars = match(DOCSTRING_STMT_PATTERN, tup[1]) |
| 501 >>> found | |
| 502 1 | |
| 503 >>> vars | |
| 504 {'docstring': '"""Some documentation.\012"""'} | |
| 5818 | 505 \end{verbatim}\ecode |
| 506 % | |
| 4084 | 507 Once specific data can be extracted from a location where it is |
| 508 expected, the question of where information can be expected | |
| 509 needs to be answered. When dealing with docstrings, the answer is | |
| 510 fairly simple: the docstring is the first \code{stmt} node in a code | |
| 511 block (\code{file_input} or \code{suite} node types). A module | |
| 512 consists of a single \code{file_input} node, and class and function | |
| 513 definitions each contain exactly one \code{suite} node. Classes and | |
| 514 functions are readily identified as subtrees of code block nodes which | |
| 515 start with \code{(stmt, (compound_stmt, (classdef, ...} or | |
| 516 \code{(stmt, (compound_stmt, (funcdef, ...}. Note that these subtrees | |
| 517 cannot be matched by \code{match()} since it does not support multiple | |
| 518 sibling nodes to match without regard to number. A more elaborate | |
| 519 matching function could be used to overcome this limitation, but this | |
| 520 is sufficient for the example. | |
| 521 | |
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522 Given the ability to determine whether a statement might be a |
|
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|
523 docstring and extract the actual string from the statement, some work |
|
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|
524 needs to be performed to walk the parse tree for an entire module and |
|
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|
525 extract information about the names defined in each context of the |
|
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|
526 module and associate any docstrings with the names. The code to |
|
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|
527 perform this work is not complicated, but bears some explanation. |
|
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|
528 |
|
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|
529 The public interface to the classes is straightforward and should |
|
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Final set of changes by Fred before 1.4beta3
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|
530 probably be somewhat more flexible. Each ``major'' block of the |
|
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|
531 module is described by an object providing several methods for inquiry |
|
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Final set of changes by Fred before 1.4beta3
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|
532 and a constructor which accepts at least the subtree of the complete |
|
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Final set of changes by Fred before 1.4beta3
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|
533 parse tree which it represents. The \code{ModuleInfo} constructor |
|
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|
534 accepts an optional \code{\var{name}} parameter since it cannot |
|
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|
535 otherwise determine the name of the module. |
|
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|
536 |
|
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|
537 The public classes include \code{ClassInfo}, \code{FunctionInfo}, |
|
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|
538 and \code{ModuleInfo}. All objects provide the |
|
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|
539 methods \code{get_name()}, \code{get_docstring()}, |
|
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|
540 \code{get_class_names()}, and \code{get_class_info()}. The |
|
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|
541 \code{ClassInfo} objects support \code{get_method_names()} and |
|
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Final set of changes by Fred before 1.4beta3
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|
542 \code{get_method_info()} while the other classes provide |
|
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Final set of changes by Fred before 1.4beta3
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|
543 \code{get_function_names()} and \code{get_function_info()}. |
|
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Final set of changes by Fred before 1.4beta3
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|
544 |
|
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|
545 Within each of the forms of code block that the public classes |
|
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Final set of changes by Fred before 1.4beta3
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|
546 represent, most of the required information is in the same form and is |
|
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547 accessed in the same way, with classes having the distinction that |
|
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|
548 functions defined at the top level are referred to as ``methods.'' |
|
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|
549 Since the difference in nomenclature reflects a real semantic |
|
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550 distinction from functions defined outside of a class, the |
|
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|
551 implementation needs to maintain the distinction. |
|
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|
552 Hence, most of the functionality of the public classes can be |
|
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|
553 implemented in a common base class, \code{SuiteInfoBase}, with the |
|
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Final set of changes by Fred before 1.4beta3
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|
554 accessors for function and method information provided elsewhere. |
|
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changeset
|
555 Note that there is only one class which represents function and method |
|
5032
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|
556 information; this parallels the use of the \code{def} statement to |
|
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557 define both types of elements. |
|
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|
558 |
|
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|
559 Most of the accessor functions are declared in \code{SuiteInfoBase} |
|
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Final set of changes by Fred before 1.4beta3
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|
560 and do not need to be overriden by subclasses. More importantly, the |
|
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changeset
|
561 extraction of most information from a parse tree is handled through a |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
562 method called by the \code{SuiteInfoBase} constructor. The example |
|
c18d52e06238
Final set of changes by Fred before 1.4beta3
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|
563 code for most of the classes is clear when read alongside the formal |
|
c18d52e06238
Final set of changes by Fred before 1.4beta3
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|
564 grammar, but the method which recursively creates new information |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
565 objects requires further examination. Here is the relevant part of |
|
c18d52e06238
Final set of changes by Fred before 1.4beta3
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changeset
|
566 the \code{SuiteInfoBase} definition from \file{example.py}: |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
567 |
| 5818 | 568 \bcode\begin{verbatim} |
|
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Final set of changes by Fred before 1.4beta3
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|
569 class SuiteInfoBase: |
|
c18d52e06238
Final set of changes by Fred before 1.4beta3
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changeset
|
570 _docstring = '' |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
571 _name = '' |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
572 |
|
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Final set of changes by Fred before 1.4beta3
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diff
changeset
|
573 def __init__(self, tree = None): |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
574 self._class_info = {} |
|
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Final set of changes by Fred before 1.4beta3
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|
575 self._function_info = {} |
|
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|
576 if tree: |
|
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changeset
|
577 self._extract_info(tree) |
|
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diff
changeset
|
578 |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
579 def _extract_info(self, tree): |
|
c18d52e06238
Final set of changes by Fred before 1.4beta3
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changeset
|
580 # extract docstring |
|
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Final set of changes by Fred before 1.4beta3
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diff
changeset
|
581 if len(tree) == 2: |
|
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Final set of changes by Fred before 1.4beta3
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parents:
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diff
changeset
|
582 found, vars = match(DOCSTRING_STMT_PATTERN[1], tree[1]) |
|
c18d52e06238
Final set of changes by Fred before 1.4beta3
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parents:
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diff
changeset
|
583 else: |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
584 found, vars = match(DOCSTRING_STMT_PATTERN, tree[3]) |
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585 if found: |
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586 self._docstring = eval(vars['docstring']) |
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587 # discover inner definitions |
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|
588 for node in tree[1:]: |
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589 found, vars = match(COMPOUND_STMT_PATTERN, node) |
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590 if found: |
|
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591 cstmt = vars['compound'] |
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592 if cstmt[0] == symbol.funcdef: |
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593 name = cstmt[2][1] |
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594 self._function_info[name] = FunctionInfo(cstmt) |
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595 elif cstmt[0] == symbol.classdef: |
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596 name = cstmt[2][1] |
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597 self._class_info[name] = ClassInfo(cstmt) |
| 5818 | 598 \end{verbatim}\ecode |
| 599 % | |
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600 After initializing some internal state, the constructor calls the |
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601 \code{_extract_info()} method. This method performs the bulk of the |
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602 information extraction which takes place in the entire example. The |
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603 extraction has two distinct phases: the location of the docstring for |
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604 the parse tree passed in, and the discovery of additional definitions |
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605 within the code block represented by the parse tree. |
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|
606 |
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607 The initial \code{if} test determines whether the nested suite is of |
|
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608 the ``short form'' or the ``long form.'' The short form is used when |
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609 the code block is on the same line as the definition of the code |
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610 block, as in |
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|
611 |
| 5818 | 612 \bcode\begin{verbatim} |
|
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|
613 def square(x): "Square an argument."; return x ** 2 |
| 5818 | 614 \end{verbatim}\ecode |
| 615 % | |
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616 while the long form uses an indented block and allows nested |
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617 definitions: |
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|
618 |
| 5818 | 619 \bcode\begin{verbatim} |
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|
620 def make_power(exp): |
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|
621 "Make a function that raises an argument to the exponent `exp'." |
|
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|
622 def raiser(x, y=exp): |
|
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|
623 return x ** y |
|
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|
624 return raiser |
| 5818 | 625 \end{verbatim}\ecode |
| 626 % | |
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627 When the short form is used, the code block may contain a docstring as |
|
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|
628 the first, and possibly only, \code{small_stmt} element. The |
|
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|
629 extraction of such a docstring is slightly different and requires only |
|
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630 a portion of the complete pattern used in the more common case. As |
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631 implemented, the docstring will only be found if there is only |
|
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|
632 one \code{small_stmt} node in the \code{simple_stmt} node. Since most |
|
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633 functions and methods which use the short form do not provide a |
|
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|
634 docstring, this may be considered sufficient. The extraction of the |
|
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|
635 docstring proceeds using the \code{match()} function as described |
|
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Final set of changes by Fred before 1.4beta3
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|
636 above, and the value of the docstring is stored as an attribute of the |
|
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|
637 \code{SuiteInfoBase} object. |
|
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|
638 |
|
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639 After docstring extraction, a simple definition discovery |
|
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640 algorithm operates on the \code{stmt} nodes of the \code{suite} node. The |
|
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|
641 special case of the short form is not tested; since there are no |
|
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Final set of changes by Fred before 1.4beta3
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|
642 \code{stmt} nodes in the short form, the algorithm will silently skip |
|
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changeset
|
643 the single \code{simple_stmt} node and correctly not discover any |
|
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Final set of changes by Fred before 1.4beta3
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|
644 nested definitions. |
|
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|
645 |
|
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|
646 Each statement in the code block is categorized as |
|
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|
647 a class definition, function or method definition, or |
|
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Final set of changes by Fred before 1.4beta3
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|
648 something else. For the definition statements, the name of the |
|
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|
649 element defined is extracted and a representation object |
|
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changeset
|
650 appropriate to the definition is created with the defining subtree |
|
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Final set of changes by Fred before 1.4beta3
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|
651 passed as an argument to the constructor. The repesentation objects |
|
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|
652 are stored in instance variables and may be retrieved by name using |
|
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changeset
|
653 the appropriate accessor methods. |
|
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Final set of changes by Fred before 1.4beta3
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|
654 |
|
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changeset
|
655 The public classes provide any accessors required which are more |
|
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Final set of changes by Fred before 1.4beta3
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|
656 specific than those provided by the \code{SuiteInfoBase} class, but |
|
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changeset
|
657 the real extraction algorithm remains common to all forms of code |
|
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|
658 blocks. A high-level function can be used to extract the complete set |
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659 of information from a source file. (See file \file{example.py}.) |
|
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changeset
|
660 |
| 5818 | 661 \bcode\begin{verbatim} |
|
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|
662 def get_docs(fileName): |
|
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changeset
|
663 source = open(fileName).read() |
|
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|
664 import os |
|
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changeset
|
665 basename = os.path.basename(os.path.splitext(fileName)[0]) |
|
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|
666 import parser |
|
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|
667 ast = parser.suite(source) |
|
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|
668 tup = parser.ast2tuple(ast) |
|
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|
669 return ModuleInfo(tup, basename) |
| 5818 | 670 \end{verbatim}\ecode |
| 671 % | |
|
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|
672 This provides an easy-to-use interface to the documentation of a |
|
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673 module. If information is required which is not extracted by the code |
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674 of this example, the code may be extended at clearly defined points to |
|
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|
675 provide additional capabilities. |
| 4084 | 676 |
| 677 | |
|
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678 \section{Standard Module \sectcode{symbol}} |
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|
679 \stmodindex{symbol} |
|
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|
680 |
|
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|
681 This module provides constants which represent the numeric values of |
|
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|
682 internal nodes of the parse tree. Unlike most Python constants, these |
|
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|
683 use lower-case names. Refer to the file \file{Grammar/Grammar} in the |
|
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|
684 Python distribution for the defintions of the names in the context of |
|
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|
685 the language grammar. The specific numeric values which the names map |
|
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|
686 to may change between Python versions. |
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|
687 |
|
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|
688 This module also provides one additional data object: |
|
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|
689 |
|
6648
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|
690 \renewcommand{\indexsubitem}{(in module symbol)} |
|
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diff
changeset
|
691 |
|
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changeset
|
692 |
|
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693 \begin{datadesc}{sym_name} |
|
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|
694 Dictionary mapping the numeric values of the constants defined in this |
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695 module back to name strings, allowing more human-readable |
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696 representation of parse trees to be generated. |
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697 \end{datadesc} |
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|
698 |
|
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|
699 |
|
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700 \section{Standard Module \sectcode{token}} |
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701 \stmodindex{token} |
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|
702 |
|
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|
703 This module provides constants which represent the numeric values of |
|
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704 leaf nodes of the parse tree (terminal tokens). Refer to the file |
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705 \file{Grammar/Grammar} in the Python distribution for the defintions |
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706 of the names in the context of the language grammar. The specific |
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707 numeric values which the names map to may change between Python |
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708 versions. |
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709 |
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710 This module also provides one data object and some functions. The |
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711 functions mirror definitions in the Python C header files. |
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712 |
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6648
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713 \renewcommand{\indexsubitem}{(in module token)} |
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714 |
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715 |
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716 \begin{datadesc}{tok_name} |
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717 Dictionary mapping the numeric values of the constants defined in this |
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718 module back to name strings, allowing more human-readable |
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719 representation of parse trees to be generated. |
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720 \end{datadesc} |
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721 |
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722 \begin{funcdesc}{ISTERMINAL}{x} |
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723 Return true for terminal token values. |
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724 \end{funcdesc} |
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725 |
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726 \begin{funcdesc}{ISNONTERMINAL}{x} |
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727 Return true for non-terminal token values. |
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728 \end{funcdesc} |
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729 |
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730 \begin{funcdesc}{ISEOF}{x} |
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731 Return true if \var{x} is the marker indicating the end of input. |
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732 \end{funcdesc} |
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733 |
| 4084 | 734 %% |
| 735 %% end of file |