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annotate Doc/lib/libtraceback.tex @ 37850:ebe59cd18273

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Use explicit relative import for an, ehm, relative import.
author Thomas Wouters <thomas@python.org>
date Tue, 18 Apr 2006 21:41:36 +0000
parents 7ada16a14968
children c94f37054f19
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1 \section{\module{traceback} ---
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2 Print or retrieve a stack traceback}
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3
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4 \declaremodule{standard}{traceback}
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5 \modulesynopsis{Print or retrieve a stack traceback.}
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6
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7
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8 This module provides a standard interface to extract, format and print
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9 stack traces of Python programs. It exactly mimics the behavior of
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10 the Python interpreter when it prints a stack trace. This is useful
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11 when you want to print stack traces under program control, such as in a
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12 ``wrapper'' around the interpreter.
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13
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14 The module uses traceback objects --- this is the object type that is
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15 stored in the \code{sys.last_traceback} variable and returned
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16 as the third item from \function{sys.exc_info()}.
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17 \obindex{traceback}
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18
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19 The module defines the following functions:
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20
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21 \begin{funcdesc}{print_tb}{traceback\optional{, limit\optional{, file}}}
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22 Print up to \var{limit} stack trace entries from \var{traceback}. If
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23 \var{limit} is omitted or \code{None}, all entries are printed.
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24 If \var{file} is omitted or \code{None}, the output goes to
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25 \code{sys.stderr}; otherwise it should be an open file or file-like
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26 object to receive the output.
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27 \end{funcdesc}
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28
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29 \begin{funcdesc}{print_exception}{type, value, traceback\optional{,
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30 limit\optional{, file}}}
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31 Print exception information and up to \var{limit} stack trace entries
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32 from \var{traceback} to \var{file}.
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33 This differs from \function{print_tb()} in the
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34 following ways: (1) if \var{traceback} is not \code{None}, it prints a
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35 header \samp{Traceback (most recent call last):}; (2) it prints the
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36 exception \var{type} and \var{value} after the stack trace; (3) if
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37 \var{type} is \exception{SyntaxError} and \var{value} has the
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38 appropriate format, it prints the line where the syntax error occurred
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39 with a caret indicating the approximate position of the error.
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40 \end{funcdesc}
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41
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42 \begin{funcdesc}{print_exc}{\optional{limit\optional{, file}}}
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43 This is a shorthand for \code{print_exception(*\function{sys.exc_info()}}.
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44 \end{funcdesc}
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45
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46 \begin{funcdesc}{format_exc}{\optional{limit}}
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47 This is like \code{print_exc(\var{limit})} but returns a string
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48 instead of printing to a file.
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49 \versionadded{2.4}
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50 \end{funcdesc}
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51
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52 \begin{funcdesc}{print_last}{\optional{limit\optional{, file}}}
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53 This is a shorthand for \code{print_exception(sys.last_type,
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54 sys.last_value, sys.last_traceback, \var{limit}, \var{file})}.
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55 \end{funcdesc}
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56
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57 \begin{funcdesc}{print_stack}{\optional{f\optional{, limit\optional{, file}}}}
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58 This function prints a stack trace from its invocation point. The
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59 optional \var{f} argument can be used to specify an alternate stack
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60 frame to start. The optional \var{limit} and \var{file} arguments have the
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61 same meaning as for \function{print_exception()}.
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62 \end{funcdesc}
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63
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64 \begin{funcdesc}{extract_tb}{traceback\optional{, limit}}
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65 Return a list of up to \var{limit} ``pre-processed'' stack trace
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66 entries extracted from the traceback object \var{traceback}. It is
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67 useful for alternate formatting of stack traces. If \var{limit} is
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68 omitted or \code{None}, all entries are extracted. A
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69 ``pre-processed'' stack trace entry is a quadruple (\var{filename},
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70 \var{line number}, \var{function name}, \var{text}) representing
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71 the information that is usually printed for a stack trace. The
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72 \var{text} is a string with leading and trailing whitespace
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73 stripped; if the source is not available it is \code{None}.
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74 \end{funcdesc}
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75
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76 \begin{funcdesc}{extract_stack}{\optional{f\optional{, limit}}}
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77 Extract the raw traceback from the current stack frame. The return
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78 value has the same format as for \function{extract_tb()}. The
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79 optional \var{f} and \var{limit} arguments have the same meaning as
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80 for \function{print_stack()}.
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81 \end{funcdesc}
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82
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83 \begin{funcdesc}{format_list}{list}
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84 Given a list of tuples as returned by \function{extract_tb()} or
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85 \function{extract_stack()}, return a list of strings ready for
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86 printing. Each string in the resulting list corresponds to the item
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87 with the same index in the argument list. Each string ends in a
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88 newline; the strings may contain internal newlines as well, for those
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89 items whose source text line is not \code{None}.
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90 \end{funcdesc}
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91
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92 \begin{funcdesc}{format_exception_only}{type, value}
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93 Format the exception part of a traceback. The arguments are the
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94 exception type and value such as given by \code{sys.last_type} and
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95 \code{sys.last_value}. The return value is a list of strings, each
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96 ending in a newline. Normally, the list contains a single string;
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97 however, for \exception{SyntaxError} exceptions, it contains several
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98 lines that (when printed) display detailed information about where the
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99 syntax error occurred. The message indicating which exception
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100 occurred is the always last string in the list.
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101 \end{funcdesc}
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102
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103 \begin{funcdesc}{format_exception}{type, value, tb\optional{, limit}}
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104 Format a stack trace and the exception information. The arguments
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105 have the same meaning as the corresponding arguments to
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106 \function{print_exception()}. The return value is a list of strings,
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107 each ending in a newline and some containing internal newlines. When
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108 these lines are concatenated and printed, exactly the same text is
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109 printed as does \function{print_exception()}.
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110 \end{funcdesc}
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111
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112 \begin{funcdesc}{format_tb}{tb\optional{, limit}}
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113 A shorthand for \code{format_list(extract_tb(\var{tb}, \var{limit}))}.
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114 \end{funcdesc}
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115
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116 \begin{funcdesc}{format_stack}{\optional{f\optional{, limit}}}
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117 A shorthand for \code{format_list(extract_stack(\var{f}, \var{limit}))}.
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118 \end{funcdesc}
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119
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120 \begin{funcdesc}{tb_lineno}{tb}
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121 This function returns the current line number set in the traceback
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122 object. This function was necessary because in versions of Python
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123 prior to 2.3 when the \programopt{-O} flag was passed to Python the
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124 \code{\var{tb}.tb_lineno} was not updated correctly. This function
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125 has no use in versions past 2.3.
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126 \end{funcdesc}
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127
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128
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129 \subsection{Traceback Example \label{traceback-example}}
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130
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131 This simple example implements a basic read-eval-print loop, similar
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Fred Drake <fdrake@acm.org>
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132 to (but less useful than) the standard Python interactive interpreter
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Fred Drake <fdrake@acm.org>
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133 loop. For a more complete implementation of the interpreter loop,
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Fred Drake <fdrake@acm.org>
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134 refer to the \refmodule{code} module.
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135
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136 \begin{verbatim}
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137 import sys, traceback
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138
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139 def run_user_code(envdir):
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140 source = raw_input(">>> ")
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141 try:
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Guido van Rossum <guido@python.org>
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142 exec source in envdir
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143 except:
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144 print "Exception in user code:"
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145 print '-'*60
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146 traceback.print_exc(file=sys.stdout)
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147 print '-'*60
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148
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149 envdir = {}
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150 while 1:
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151 run_user_code(envdir)
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152 \end{verbatim}