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| author | Guido van Rossum <guido@python.org> |
|---|---|
| date | Thu, 17 Jul 1997 16:29:42 +0000 |
| parents | be31e7e19241 |
| children | 3468ea8645ca |
| rev | line source |
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| 3198 | 1 % libparser.tex |
| 2 % | |
| 3 % Introductory documentation for the new parser built-in module. | |
| 4 % | |
| 5 % Copyright 1995 Virginia Polytechnic Institute and State University | |
| 6 % and Fred L. Drake, Jr. This copyright notice must be distributed on | |
| 7 % all copies, but this document otherwise may be distributed as part | |
| 8 % of the Python distribution. No fee may be charged for this document | |
| 9 % in any representation, either on paper or electronically. This | |
| 10 % restriction does not affect other elements in a distributed package | |
| 11 % in any way. | |
| 12 % | |
| 13 | |
| 14 \section{Built-in Module \sectcode{parser}} | |
| 15 \bimodindex{parser} | |
| 16 | |
| 17 The \code{parser} module provides an interface to Python's internal | |
| 18 parser and byte-code compiler. The primary purpose for this interface | |
| 19 is to allow Python code to edit the parse tree of a Python expression | |
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20 and create executable code from this. This is better than trying |
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21 to parse and modify an arbitrary Python code fragment as a string |
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22 because parsing is performed in a manner identical to the code |
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23 forming the application. It is also faster. |
| 3198 | 24 |
| 25 There are a few things to note about this module which are important | |
| 26 to making use of the data structures created. This is not a tutorial | |
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27 on editing the parse trees for Python code, but some examples of using |
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28 the \code{parser} module are presented. |
| 3198 | 29 |
| 30 Most importantly, a good understanding of the Python grammar processed | |
| 31 by the internal parser is required. For full information on the | |
| 32 language syntax, refer to the Language Reference. The parser itself | |
| 33 is created from a grammar specification defined in the file | |
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34 \file{Grammar/Grammar} in the standard Python distribution. The parse |
| 3198 | 35 trees stored in the ``AST objects'' created by this module are the |
| 36 actual output from the internal parser when created by the | |
| 37 \code{expr()} or \code{suite()} functions, described below. The AST | |
| 4084 | 38 objects created by \code{sequence2ast()} faithfully simulate those |
| 39 structures. Be aware that the values of the sequences which are | |
| 40 considered ``correct'' will vary from one version of Python to another | |
| 41 as the formal grammar for the language is revised. However, | |
| 42 transporting code from one Python version to another as source text | |
| 43 will always allow correct parse trees to be created in the target | |
| 44 version, with the only restriction being that migrating to an older | |
| 45 version of the interpreter will not support more recent language | |
| 46 constructs. The parse trees are not typically compatible from one | |
| 47 version to another, whereas source code has always been | |
| 48 forward-compatible. | |
| 3198 | 49 |
| 4084 | 50 Each element of the sequences returned by \code{ast2list} or |
| 51 \code{ast2tuple()} has a simple form. Sequences representing | |
| 52 non-terminal elements in the grammar always have a length greater than | |
| 53 one. The first element is an integer which identifies a production in | |
| 54 the grammar. These integers are given symbolic names in the C header | |
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55 file \file{Include/graminit.h} and the Python module |
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56 \file{Lib/symbol.py}. Each additional element of the sequence represents |
| 4084 | 57 a component of the production as recognized in the input string: these |
| 58 are always sequences which have the same form as the parent. An | |
| 59 important aspect of this structure which should be noted is that | |
| 60 keywords used to identify the parent node type, such as the keyword | |
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61 \code{if} in an \code{if_stmt}, are included in the node tree without |
| 4084 | 62 any special treatment. For example, the \code{if} keyword is |
| 3198 | 63 represented by the tuple \code{(1, 'if')}, where \code{1} is the |
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64 numeric value associated with all \code{NAME} tokens, including |
| 4084 | 65 variable and function names defined by the user. In an alternate form |
| 66 returned when line number information is requested, the same token | |
| 67 might be represented as \code{(1, 'if', 12)}, where the \code{12} | |
| 68 represents the line number at which the terminal symbol was found. | |
| 3198 | 69 |
| 70 Terminal elements are represented in much the same way, but without | |
| 71 any child elements and the addition of the source text which was | |
| 72 identified. The example of the \code{if} keyword above is | |
| 73 representative. The various types of terminal symbols are defined in | |
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74 the C header file \file{Include/token.h} and the Python module |
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75 \file{Lib/token.py}. |
| 3198 | 76 |
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77 The AST objects are not required to support the functionality of this |
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78 module, but are provided for three purposes: to allow an application |
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79 to amortize the cost of processing complex parse trees, to provide a |
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80 parse tree representation which conserves memory space when compared |
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81 to the Python list or tuple representation, and to ease the creation |
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82 of additional modules in C which manipulate parse trees. A simple |
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83 ``wrapper'' class may be created in Python to hide the use of AST |
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84 objects; the \code{AST} library module provides a variety of such |
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85 classes. |
| 3198 | 86 |
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87 The \code{parser} module defines functions for a few distinct |
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88 purposes. The most important purposes are to create AST objects and |
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89 to convert AST objects to other representations such as parse trees |
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90 and compiled code objects, but there are also functions which serve to |
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91 query the type of parse tree represented by an AST object. |
| 3198 | 92 |
| 93 \renewcommand{\indexsubitem}{(in module parser)} | |
| 94 | |
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95 |
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96 \subsection{Creating AST Objects} |
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97 |
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98 AST objects may be created from source code or from a parse tree. |
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99 When creating an AST object from source, different functions are used |
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100 to create the \code{'eval'} and \code{'exec'} forms. |
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101 |
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102 \begin{funcdesc}{expr}{string} |
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103 The \code{expr()} function parses the parameter \code{\var{string}} |
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104 as if it were an input to \code{compile(\var{string}, 'eval')}. If |
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105 the parse succeeds, an AST object is created to hold the internal |
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106 parse tree representation, otherwise an appropriate exception is |
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107 thrown. |
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108 \end{funcdesc} |
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109 |
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110 \begin{funcdesc}{suite}{string} |
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111 The \code{suite()} function parses the parameter \code{\var{string}} |
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112 as if it were an input to \code{compile(\var{string}, 'exec')}. If |
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113 the parse succeeds, an AST object is created to hold the internal |
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114 parse tree representation, otherwise an appropriate exception is |
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115 thrown. |
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116 \end{funcdesc} |
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117 |
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118 \begin{funcdesc}{sequence2ast}{sequence} |
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119 This function accepts a parse tree represented as a sequence and |
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120 builds an internal representation if possible. If it can validate |
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121 that the tree conforms to the Python grammar and all nodes are valid |
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122 node types in the host version of Python, an AST object is created |
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123 from the internal representation and returned to the called. If there |
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124 is a problem creating the internal representation, or if the tree |
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125 cannot be validated, a \code{ParserError} exception is thrown. An AST |
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126 object created this way should not be assumed to compile correctly; |
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127 normal exceptions thrown by compilation may still be initiated when |
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128 the AST object is passed to \code{compileast()}. This may indicate |
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129 problems not related to syntax (such as a \code{MemoryError} |
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130 exception), but may also be due to constructs such as the result of |
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131 parsing \code{del f(0)}, which escapes the Python parser but is |
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132 checked by the bytecode compiler. |
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133 |
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134 Sequences representing terminal tokens may be represented as either |
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135 two-element lists of the form \code{(1, 'name')} or as three-element |
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136 lists of the form \code{(1, 'name', 56)}. If the third element is |
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137 present, it is assumed to be a valid line number. The line number |
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138 may be specified for any subset of the terminal symbols in the input |
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139 tree. |
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140 \end{funcdesc} |
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141 |
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142 \begin{funcdesc}{tuple2ast}{sequence} |
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143 This is the same function as \code{sequence2ast()}. This entry point |
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144 is maintained for backward compatibility. |
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145 \end{funcdesc} |
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146 |
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147 |
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148 \subsection{Converting AST Objects} |
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149 |
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150 AST objects, regardless of the input used to create them, may be |
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151 converted to parse trees represented as list- or tuple- trees, or may |
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152 be compiled into executable code objects. Parse trees may be |
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153 extracted with or without line numbering information. |
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154 |
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155 \begin{funcdesc}{ast2list}{ast\optional{\, line_info\code{ = 0}}} |
| 4084 | 156 This function accepts an AST object from the caller in |
| 157 \code{\var{ast}} and returns a Python list representing the | |
| 158 equivelent parse tree. The resulting list representation can be used | |
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159 for inspection or the creation of a new parse tree in list form. This |
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160 function does not fail so long as memory is available to build the |
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161 list representation. If the parse tree will only be used for |
| 4084 | 162 inspection, \code{ast2tuple()} should be used instead to reduce memory |
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163 consumption and fragmentation. When the list representation is |
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164 required, this function is significantly faster than retrieving a |
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165 tuple representation and converting that to nested lists. |
| 4084 | 166 |
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167 If \code{\var{line_info}} is true, line number information will be |
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168 included for all terminal tokens as a third element of the list |
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169 representing the token. Note that the line number provided specifies |
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170 the line on which the token \emph{ends\/}. This information is |
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171 omitted if the flag is false or omitted. |
| 4084 | 172 \end{funcdesc} |
| 173 | |
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174 \begin{funcdesc}{ast2tuple}{ast\optional{\, line_info\code{ = 0}}} |
| 3198 | 175 This function accepts an AST object from the caller in |
| 176 \code{\var{ast}} and returns a Python tuple representing the | |
| 4084 | 177 equivelent parse tree. Other than returning a tuple instead of a |
| 178 list, this function is identical to \code{ast2list()}. | |
| 179 | |
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180 If \code{\var{line_info}} is true, line number information will be |
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181 included for all terminal tokens as a third element of the list |
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182 representing the token. This information is omitted if the flag is |
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183 false or omitted. |
| 3198 | 184 \end{funcdesc} |
| 185 | |
| 4084 | 186 \begin{funcdesc}{compileast}{ast\optional{\, filename\code{ = '<ast>'}}} |
| 3198 | 187 The Python byte compiler can be invoked on an AST object to produce |
| 188 code objects which can be used as part of an \code{exec} statement or | |
| 189 a call to the built-in \code{eval()} function. This function provides | |
| 190 the interface to the compiler, passing the internal parse tree from | |
| 191 \code{\var{ast}} to the parser, using the source file name specified | |
| 192 by the \code{\var{filename}} parameter. The default value supplied | |
| 193 for \code{\var{filename}} indicates that the source was an AST object. | |
| 4084 | 194 |
| 195 Compiling an AST object may result in exceptions related to | |
| 196 compilation; an example would be a \code{SyntaxError} caused by the | |
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197 parse tree for \code{del f(0)}: this statement is considered legal |
| 4084 | 198 within the formal grammar for Python but is not a legal language |
| 199 construct. The \code{SyntaxError} raised for this condition is | |
| 200 actually generated by the Python byte-compiler normally, which is why | |
| 201 it can be raised at this point by the \code{parser} module. Most | |
| 202 causes of compilation failure can be diagnosed programmatically by | |
| 203 inspection of the parse tree. | |
| 3198 | 204 \end{funcdesc} |
| 205 | |
| 206 | |
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207 \subsection{Queries on AST Objects} |
| 3198 | 208 |
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209 Two functions are provided which allow an application to determine if |
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210 an AST was create as an expression or a suite. Neither of these |
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211 functions can be used to determine if an AST was created from source |
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212 code via \code{expr()} or \code{suite()} or from a parse tree via |
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213 \code{sequence2ast()}. |
| 3198 | 214 |
| 215 \begin{funcdesc}{isexpr}{ast} | |
| 216 When \code{\var{ast}} represents an \code{'eval'} form, this function | |
| 217 returns a true value (\code{1}), otherwise it returns false | |
| 218 (\code{0}). This is useful, since code objects normally cannot be | |
| 219 queried for this information using existing built-in functions. Note | |
| 220 that the code objects created by \code{compileast()} cannot be queried | |
| 221 like this either, and are identical to those created by the built-in | |
| 222 \code{compile()} function. | |
| 223 \end{funcdesc} | |
| 224 | |
| 225 | |
| 226 \begin{funcdesc}{issuite}{ast} | |
| 227 This function mirrors \code{isexpr()} in that it reports whether an | |
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228 AST object represents an \code{'exec'} form, commonly known as a |
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229 ``suite.'' It is not safe to assume that this function is equivelent |
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230 to \code{not isexpr(\var{ast})}, as additional syntactic fragments may |
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231 be supported in the future. |
| 3198 | 232 \end{funcdesc} |
| 233 | |
| 234 | |
| 235 \subsection{Exceptions and Error Handling} | |
| 236 | |
| 237 The parser module defines a single exception, but may also pass other | |
| 238 built-in exceptions from other portions of the Python runtime | |
| 239 environment. See each function for information about the exceptions | |
| 240 it can raise. | |
| 241 | |
| 242 \begin{excdesc}{ParserError} | |
| 243 Exception raised when a failure occurs within the parser module. This | |
| 244 is generally produced for validation failures rather than the built in | |
| 245 \code{SyntaxError} thrown during normal parsing. | |
| 246 The exception argument is either a string describing the reason of the | |
| 4084 | 247 failure or a tuple containing a sequence causing the failure from a parse |
| 248 tree passed to \code{sequence2ast()} and an explanatory string. Calls to | |
| 249 \code{sequence2ast()} need to be able to handle either type of exception, | |
| 3198 | 250 while calls to other functions in the module will only need to be |
| 251 aware of the simple string values. | |
| 252 \end{excdesc} | |
| 253 | |
| 254 Note that the functions \code{compileast()}, \code{expr()}, and | |
| 255 \code{suite()} may throw exceptions which are normally thrown by the | |
| 256 parsing and compilation process. These include the built in | |
| 257 exceptions \code{MemoryError}, \code{OverflowError}, | |
| 258 \code{SyntaxError}, and \code{SystemError}. In these cases, these | |
| 259 exceptions carry all the meaning normally associated with them. Refer | |
| 260 to the descriptions of each function for detailed information. | |
| 261 | |
| 262 | |
| 4084 | 263 \subsection{AST Objects} |
| 264 | |
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265 AST objects returned by \code{expr()}, \code{suite()}, and |
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266 \code{sequence2ast()} have no methods of their own. |
| 4084 | 267 Some of the functions defined which accept an AST object as their |
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268 first argument may change to object methods in the future. The type |
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269 of these objects is available as \code{ASTType} in the module. |
| 4084 | 270 |
| 271 Ordered and equality comparisons are supported between AST objects. | |
| 272 | |
| 273 | |
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274 \subsection{Examples} |
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275 \nodename{AST Examples} |
| 3198 | 276 |
| 4084 | 277 The parser modules allows operations to be performed on the parse tree |
| 278 of Python source code before the bytecode is generated, and provides | |
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279 for inspection of the parse tree for information gathering purposes. |
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280 Two examples are presented. The simple example demonstrates emulation |
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281 of the \code{compile()} built-in function and the complex example |
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282 shows the use of a parse tree for information discovery. |
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283 |
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284 \subsubsection{Emulation of \sectcode{compile()}} |
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285 |
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286 While many useful operations may take place between parsing and |
| 4084 | 287 bytecode generation, the simplest operation is to do nothing. For |
| 288 this purpose, using the \code{parser} module to produce an | |
| 289 intermediate data structure is equivelent to the code | |
| 290 | |
| 291 \begin{verbatim} | |
| 292 >>> code = compile('a + 5', 'eval') | |
| 293 >>> a = 5 | |
| 294 >>> eval(code) | |
| 295 10 | |
| 296 \end{verbatim} | |
| 297 | |
| 298 The equivelent operation using the \code{parser} module is somewhat | |
| 299 longer, and allows the intermediate internal parse tree to be retained | |
| 300 as an AST object: | |
| 3198 | 301 |
| 302 \begin{verbatim} | |
| 303 >>> import parser | |
| 304 >>> ast = parser.expr('a + 5') | |
| 305 >>> code = parser.compileast(ast) | |
| 306 >>> a = 5 | |
| 307 >>> eval(code) | |
| 308 10 | |
| 309 \end{verbatim} | |
| 310 | |
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311 An application which needs both AST and code objects can package this |
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312 code into readily available functions: |
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313 |
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314 \begin{verbatim} |
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315 import parser |
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316 |
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317 def load_suite(source_string): |
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318 ast = parser.suite(source_string) |
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319 code = parser.compileast(ast) |
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320 return ast, code |
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321 |
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322 def load_expression(source_string): |
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323 ast = parser.expr(source_string) |
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324 code = parser.compileast(ast) |
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325 return ast, code |
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326 \end{verbatim} |
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327 |
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328 \subsubsection{Information Discovery} |
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329 |
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330 Some applications benefit from direct access to the parse tree. The |
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331 remainder of this section demonstrates how the parse tree provides |
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332 access to module documentation defined in docstrings without requiring |
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333 that the code being examined be loaded into a running interpreter via |
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334 \code{import}. This can be very useful for performing analyses of |
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335 untrusted code. |
| 3198 | 336 |
| 4084 | 337 Generally, the example will demonstrate how the parse tree may be |
| 338 traversed to distill interesting information. Two functions and a set | |
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339 of classes are developed which provide programmatic access to high |
| 4084 | 340 level function and class definitions provided by a module. The |
| 341 classes extract information from the parse tree and provide access to | |
| 342 the information at a useful semantic level, one function provides a | |
| 343 simple low-level pattern matching capability, and the other function | |
| 344 defines a high-level interface to the classes by handling file | |
| 345 operations on behalf of the caller. All source files mentioned here | |
| 346 which are not part of the Python installation are located in the | |
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347 \file{Demo/parser/} directory of the distribution. |
| 4084 | 348 |
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349 The dynamic nature of Python allows the programmer a great deal of |
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350 flexibility, but most modules need only a limited measure of this when |
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351 defining classes, functions, and methods. In this example, the only |
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352 definitions that will be considered are those which are defined in the |
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353 top level of their context, e.g., a function defined by a \code{def} |
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354 statement at column zero of a module, but not a function defined |
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355 within a branch of an \code{if} ... \code{else} construct, though |
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356 there are some good reasons for doing so in some situations. Nesting |
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357 of definitions will be handled by the code developed in the example. |
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358 |
| 4084 | 359 To construct the upper-level extraction methods, we need to know what |
| 360 the parse tree structure looks like and how much of it we actually | |
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361 need to be concerned about. Python uses a moderately deep parse tree |
| 4084 | 362 so there are a large number of intermediate nodes. It is important to |
| 363 read and understand the formal grammar used by Python. This is | |
| 364 specified in the file \file{Grammar/Grammar} in the distribution. | |
| 365 Consider the simplest case of interest when searching for docstrings: | |
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366 a module consisting of a docstring and nothing else. (See file |
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367 \file{docstring.py}.) |
| 4084 | 368 |
| 369 \begin{verbatim} | |
| 370 """Some documentation. | |
| 371 """ | |
| 372 \end{verbatim} | |
| 373 | |
| 374 Using the interpreter to take a look at the parse tree, we find a | |
| 375 bewildering mass of numbers and parentheses, with the documentation | |
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376 buried deep in nested tuples. |
| 3198 | 377 |
| 4084 | 378 \begin{verbatim} |
| 379 >>> import parser | |
| 380 >>> import pprint | |
| 381 >>> ast = parser.suite(open('docstring.py').read()) | |
| 382 >>> tup = parser.ast2tuple(ast) | |
| 383 >>> pprint.pprint(tup) | |
| 384 (257, | |
| 385 (264, | |
| 386 (265, | |
| 387 (266, | |
| 388 (267, | |
| 389 (307, | |
| 390 (287, | |
| 391 (288, | |
| 392 (289, | |
| 393 (290, | |
| 394 (292, | |
| 395 (293, | |
| 396 (294, | |
| 397 (295, | |
| 398 (296, | |
| 399 (297, | |
| 400 (298, | |
| 401 (299, | |
| 402 (300, (3, '"""Some documentation.\012"""'))))))))))))))))), | |
| 403 (4, ''))), | |
| 404 (4, ''), | |
| 405 (0, '')) | |
| 406 \end{verbatim} | |
| 407 | |
| 408 The numbers at the first element of each node in the tree are the node | |
| 409 types; they map directly to terminal and non-terminal symbols in the | |
| 410 grammar. Unfortunately, they are represented as integers in the | |
| 411 internal representation, and the Python structures generated do not | |
| 412 change that. However, the \code{symbol} and \code{token} modules | |
| 413 provide symbolic names for the node types and dictionaries which map | |
| 414 from the integers to the symbolic names for the node types. | |
| 3198 | 415 |
| 4084 | 416 In the output presented above, the outermost tuple contains four |
| 417 elements: the integer \code{257} and three additional tuples. Node | |
| 418 type \code{257} has the symbolic name \code{file_input}. Each of | |
| 419 these inner tuples contains an integer as the first element; these | |
| 420 integers, \code{264}, \code{4}, and \code{0}, represent the node types | |
| 421 \code{stmt}, \code{NEWLINE}, and \code{ENDMARKER}, respectively. | |
| 422 Note that these values may change depending on the version of Python | |
| 423 you are using; consult \file{symbol.py} and \file{token.py} for | |
| 424 details of the mapping. It should be fairly clear that the outermost | |
| 425 node is related primarily to the input source rather than the contents | |
| 426 of the file, and may be disregarded for the moment. The \code{stmt} | |
| 427 node is much more interesting. In particular, all docstrings are | |
| 428 found in subtrees which are formed exactly as this node is formed, | |
| 429 with the only difference being the string itself. The association | |
| 430 between the docstring in a similar tree and the defined entity (class, | |
| 431 function, or module) which it describes is given by the position of | |
| 432 the docstring subtree within the tree defining the described | |
| 433 structure. | |
| 3198 | 434 |
| 4084 | 435 By replacing the actual docstring with something to signify a variable |
|
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436 component of the tree, we allow a simple pattern matching approach to |
|
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437 check any given subtree for equivelence to the general pattern for |
|
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438 docstrings. Since the example demonstrates information extraction, we |
|
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439 can safely require that the tree be in tuple form rather than list |
|
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440 form, allowing a simple variable representation to be |
|
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441 \code{['variable_name']}. A simple recursive function can implement |
| 4084 | 442 the pattern matching, returning a boolean and a dictionary of variable |
|
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443 name to value mappings. (See file \file{example.py}.) |
| 4084 | 444 |
| 445 \begin{verbatim} | |
| 446 from types import ListType, TupleType | |
| 447 | |
| 448 def match(pattern, data, vars=None): | |
| 449 if vars is None: | |
| 450 vars = {} | |
| 451 if type(pattern) is ListType: | |
| 452 vars[pattern[0]] = data | |
| 453 return 1, vars | |
| 454 if type(pattern) is not TupleType: | |
| 455 return (pattern == data), vars | |
| 456 if len(data) != len(pattern): | |
| 457 return 0, vars | |
| 458 for pattern, data in map(None, pattern, data): | |
| 459 same, vars = match(pattern, data, vars) | |
| 460 if not same: | |
| 461 break | |
| 462 return same, vars | |
| 463 \end{verbatim} | |
| 464 | |
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465 Using this simple representation for syntactic variables and the symbolic |
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466 node types, the pattern for the candidate docstring subtrees becomes |
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467 fairly readable. (See file \file{example.py}.) |
| 3198 | 468 |
| 4084 | 469 \begin{verbatim} |
|
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470 import symbol |
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471 import token |
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472 |
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473 DOCSTRING_STMT_PATTERN = ( |
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474 symbol.stmt, |
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475 (symbol.simple_stmt, |
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476 (symbol.small_stmt, |
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477 (symbol.expr_stmt, |
|
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478 (symbol.testlist, |
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479 (symbol.test, |
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480 (symbol.and_test, |
|
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481 (symbol.not_test, |
|
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482 (symbol.comparison, |
|
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|
483 (symbol.expr, |
|
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|
484 (symbol.xor_expr, |
|
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|
485 (symbol.and_expr, |
|
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486 (symbol.shift_expr, |
|
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487 (symbol.arith_expr, |
|
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|
488 (symbol.term, |
|
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|
489 (symbol.factor, |
|
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|
490 (symbol.power, |
|
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|
491 (symbol.atom, |
|
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|
492 (token.STRING, ['docstring']) |
|
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|
493 )))))))))))))))), |
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494 (token.NEWLINE, '') |
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|
495 )) |
| 4084 | 496 \end{verbatim} |
| 497 | |
| 498 Using the \code{match()} function with this pattern, extracting the | |
| 499 module docstring from the parse tree created previously is easy: | |
| 500 | |
| 501 \begin{verbatim} | |
| 502 >>> found, vars = match(DOCSTRING_STMT_PATTERN, tup[1]) | |
| 503 >>> found | |
| 504 1 | |
| 505 >>> vars | |
| 506 {'docstring': '"""Some documentation.\012"""'} | |
| 507 \end{verbatim} | |
| 508 | |
| 509 Once specific data can be extracted from a location where it is | |
| 510 expected, the question of where information can be expected | |
| 511 needs to be answered. When dealing with docstrings, the answer is | |
| 512 fairly simple: the docstring is the first \code{stmt} node in a code | |
| 513 block (\code{file_input} or \code{suite} node types). A module | |
| 514 consists of a single \code{file_input} node, and class and function | |
| 515 definitions each contain exactly one \code{suite} node. Classes and | |
| 516 functions are readily identified as subtrees of code block nodes which | |
| 517 start with \code{(stmt, (compound_stmt, (classdef, ...} or | |
| 518 \code{(stmt, (compound_stmt, (funcdef, ...}. Note that these subtrees | |
| 519 cannot be matched by \code{match()} since it does not support multiple | |
| 520 sibling nodes to match without regard to number. A more elaborate | |
| 521 matching function could be used to overcome this limitation, but this | |
| 522 is sufficient for the example. | |
| 523 | |
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|
524 Given the ability to determine whether a statement might be a |
|
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|
525 docstring and extract the actual string from the statement, some work |
|
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|
526 needs to be performed to walk the parse tree for an entire module and |
|
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|
527 extract information about the names defined in each context of the |
|
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|
528 module and associate any docstrings with the names. The code to |
|
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|
529 perform this work is not complicated, but bears some explanation. |
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|
530 |
|
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|
531 The public interface to the classes is straightforward and should |
|
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|
532 probably be somewhat more flexible. Each ``major'' block of the |
|
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|
533 module is described by an object providing several methods for inquiry |
|
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|
534 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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|
535 parse tree which it represents. The \code{ModuleInfo} constructor |
|
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|
536 accepts an optional \code{\var{name}} parameter since it cannot |
|
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|
537 otherwise determine the name of the module. |
|
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|
538 |
|
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|
539 The public classes include \code{ClassInfo}, \code{FunctionInfo}, |
|
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Final set of changes by Fred before 1.4beta3
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|
540 and \code{ModuleInfo}. All objects provide the |
|
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Final set of changes by Fred before 1.4beta3
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|
541 methods \code{get_name()}, \code{get_docstring()}, |
|
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Final set of changes by Fred before 1.4beta3
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|
542 \code{get_class_names()}, and \code{get_class_info()}. The |
|
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Final set of changes by Fred before 1.4beta3
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|
543 \code{ClassInfo} objects support \code{get_method_names()} and |
|
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|
544 \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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|
545 \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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|
546 |
|
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|
547 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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|
548 represent, most of the required information is in the same form and is |
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549 accessed in the same way, with classes having the distinction that |
|
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|
550 functions defined at the top level are referred to as ``methods.'' |
|
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|
551 Since the difference in nomenclature reflects a real semantic |
|
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552 distinction from functions defined outside of a class, the |
|
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553 implementation needs to maintain the distinction. |
|
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|
554 Hence, most of the functionality of the public classes can be |
|
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|
555 implemented in a common base class, \code{SuiteInfoBase}, with the |
|
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|
556 accessors for function and method information provided elsewhere. |
|
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|
557 Note that there is only one class which represents function and method |
|
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558 information; this parallels the use of the \code{def} statement to |
|
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559 define both types of elements. |
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|
560 |
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|
561 Most of the accessor functions are declared in \code{SuiteInfoBase} |
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562 and do not need to be overriden by subclasses. More importantly, the |
|
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|
563 extraction of most information from a parse tree is handled through a |
|
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|
564 method called by the \code{SuiteInfoBase} constructor. The example |
|
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|
565 code for most of the classes is clear when read alongside the formal |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
566 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
|
567 objects requires further examination. Here is the relevant part of |
|
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Final set of changes by Fred before 1.4beta3
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|
568 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
|
569 |
|
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changeset
|
570 \begin{verbatim} |
|
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Final set of changes by Fred before 1.4beta3
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|
571 class SuiteInfoBase: |
|
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Final set of changes by Fred before 1.4beta3
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|
572 _docstring = '' |
|
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573 _name = '' |
|
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|
574 |
|
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|
575 def __init__(self, tree = None): |
|
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576 self._class_info = {} |
|
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|
577 self._function_info = {} |
|
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|
578 if tree: |
|
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|
579 self._extract_info(tree) |
|
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Final set of changes by Fred before 1.4beta3
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changeset
|
580 |
|
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Final set of changes by Fred before 1.4beta3
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|
581 def _extract_info(self, tree): |
|
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|
582 # extract docstring |
|
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changeset
|
583 if len(tree) == 2: |
|
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584 found, vars = match(DOCSTRING_STMT_PATTERN[1], tree[1]) |
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585 else: |
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586 found, vars = match(DOCSTRING_STMT_PATTERN, tree[3]) |
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587 if found: |
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588 self._docstring = eval(vars['docstring']) |
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589 # discover inner definitions |
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590 for node in tree[1:]: |
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591 found, vars = match(COMPOUND_STMT_PATTERN, node) |
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592 if found: |
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593 cstmt = vars['compound'] |
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594 if cstmt[0] == symbol.funcdef: |
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595 name = cstmt[2][1] |
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596 self._function_info[name] = FunctionInfo(cstmt) |
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597 elif cstmt[0] == symbol.classdef: |
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598 name = cstmt[2][1] |
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599 self._class_info[name] = ClassInfo(cstmt) |
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600 \end{verbatim} |
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601 |
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602 After initializing some internal state, the constructor calls the |
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603 \code{_extract_info()} method. This method performs the bulk of the |
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604 information extraction which takes place in the entire example. The |
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605 extraction has two distinct phases: the location of the docstring for |
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606 the parse tree passed in, and the discovery of additional definitions |
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607 within the code block represented by the parse tree. |
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608 |
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609 The initial \code{if} test determines whether the nested suite is of |
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610 the ``short form'' or the ``long form.'' The short form is used when |
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611 the code block is on the same line as the definition of the code |
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612 block, as in |
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613 |
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614 \begin{verbatim} |
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615 def square(x): "Square an argument."; return x ** 2 |
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616 \end{verbatim} |
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617 |
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618 while the long form uses an indented block and allows nested |
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619 definitions: |
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620 |
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621 \begin{verbatim} |
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622 def make_power(exp): |
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623 "Make a function that raises an argument to the exponent `exp'." |
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624 def raiser(x, y=exp): |
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625 return x ** y |
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626 return raiser |
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627 \end{verbatim} |
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628 |
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629 When the short form is used, the code block may contain a docstring as |
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630 the first, and possibly only, \code{small_stmt} element. The |
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631 extraction of such a docstring is slightly different and requires only |
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632 a portion of the complete pattern used in the more common case. As |
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633 implemented, the docstring will only be found if there is only |
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634 one \code{small_stmt} node in the \code{simple_stmt} node. Since most |
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635 functions and methods which use the short form do not provide a |
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636 docstring, this may be considered sufficient. The extraction of the |
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637 docstring proceeds using the \code{match()} function as described |
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638 above, and the value of the docstring is stored as an attribute of the |
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639 \code{SuiteInfoBase} object. |
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640 |
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641 After docstring extraction, a simple definition discovery |
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642 algorithm operates on the \code{stmt} nodes of the \code{suite} node. The |
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643 special case of the short form is not tested; since there are no |
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644 \code{stmt} nodes in the short form, the algorithm will silently skip |
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645 the single \code{simple_stmt} node and correctly not discover any |
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646 nested definitions. |
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647 |
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648 Each statement in the code block is categorized as |
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649 a class definition, function or method definition, or |
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650 something else. For the definition statements, the name of the |
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651 element defined is extracted and a representation object |
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652 appropriate to the definition is created with the defining subtree |
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653 passed as an argument to the constructor. The repesentation objects |
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654 are stored in instance variables and may be retrieved by name using |
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655 the appropriate accessor methods. |
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|
656 |
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657 The public classes provide any accessors required which are more |
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658 specific than those provided by the \code{SuiteInfoBase} class, but |
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659 the real extraction algorithm remains common to all forms of code |
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660 blocks. A high-level function can be used to extract the complete set |
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661 of information from a source file. (See file \file{example.py}.) |
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|
662 |
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|
663 \begin{verbatim} |
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664 def get_docs(fileName): |
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665 source = open(fileName).read() |
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666 import os |
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667 basename = os.path.basename(os.path.splitext(fileName)[0]) |
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668 import parser |
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669 ast = parser.suite(source) |
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670 tup = parser.ast2tuple(ast) |
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671 return ModuleInfo(tup, basename) |
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672 \end{verbatim} |
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|
673 |
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|
674 This provides an easy-to-use interface to the documentation of a |
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675 module. If information is required which is not extracted by the code |
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676 of this example, the code may be extended at clearly defined points to |
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677 provide additional capabilities. |
| 4084 | 678 |
| 679 | |
| 680 %% | |
| 681 %% end of file |