Mercurial > cpython-lru_cache-weakref

annotate Doc/lib/libtraceback.tex @ 25838:8540887bcec6 2.2

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Backport of 2.183: Fix memory leak in add_subclass() found by NealN with valgrind.
author Guido van Rossum <guido@python.org>
date Fri, 18 Oct 2002 13:42:21 +0000
parents dbdada966858
children cf5928fab108
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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 variables \code{sys.exc_traceback} (deprecated) and
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16 \code{sys.last_traceback} and returned as the third item from
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17 \function{sys.exc_info()}.
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18 \obindex{traceback}
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19
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20 The module defines the following functions:
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21
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22 \begin{funcdesc}{print_tb}{traceback\optional{, limit\optional{, file}}}
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23 Print up to \var{limit} stack trace entries from \var{traceback}. If
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24 \var{limit} is omitted or \code{None}, all entries are printed.
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25 If \var{file} is omitted or \code{None}, the output goes to
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26 \code{sys.stderr}; otherwise it should be an open file or file-like
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27 object to receive the output.
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28 \end{funcdesc}
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29
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30 \begin{funcdesc}{print_exception}{type, value, traceback\optional{,
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31 limit\optional{, file}}}
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32 Print exception information and up to \var{limit} stack trace entries
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33 from \var{traceback} to \var{file}.
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34 This differs from \function{print_tb()} in the
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35 following ways: (1) if \var{traceback} is not \code{None}, it prints a
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36 header \samp{Traceback (most recent call last):}; (2) it prints the
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37 exception \var{type} and \var{value} after the stack trace; (3) if
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38 \var{type} is \exception{SyntaxError} and \var{value} has the
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39 appropriate format, it prints the line where the syntax error occurred
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40 with a caret indicating the approximate position of the error.
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41 \end{funcdesc}
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42
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43 \begin{funcdesc}{print_exc}{\optional{limit\optional{, file}}}
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44 This is a shorthand for \code{print_exception(sys.exc_type,
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45 sys.exc_value, sys.exc_traceback, \var{limit}, \var{file})}. (In
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46 fact, it uses \function{sys.exc_info()} to retrieve the same
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47 information in a thread-safe way instead of using the deprecated
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48 variables.)
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49 \end{funcdesc}
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50
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51 \begin{funcdesc}{print_last}{\optional{limit\optional{, file}}}
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52 This is a shorthand for \code{print_exception(sys.last_type,
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53 sys.last_value, sys.last_traceback, \var{limit}, \var{file})}.
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54 \end{funcdesc}
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55
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56 \begin{funcdesc}{print_stack}{\optional{f\optional{, limit\optional{, file}}}}
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57 This function prints a stack trace from its invocation point. The
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58 optional \var{f} argument can be used to specify an alternate stack
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59 frame to start. The optional \var{limit} and \var{file} arguments have the
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60 same meaning as for \function{print_exception()}.
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61 \end{funcdesc}
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62
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63 \begin{funcdesc}{extract_tb}{traceback\optional{, limit}}
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64 Return a list of up to \var{limit} ``pre-processed'' stack trace
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65 entries extracted from the traceback object \var{traceback}. It is
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66 useful for alternate formatting of stack traces. If \var{limit} is
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67 omitted or \code{None}, all entries are extracted. A
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68 ``pre-processed'' stack trace entry is a quadruple (\var{filename},
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69 \var{line number}, \var{function name}, \var{text}) representing
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70 the information that is usually printed for a stack trace. The
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71 \var{text} is a string with leading and trailing whitespace
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72 stripped; if the source is not available it is \code{None}.
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73 \end{funcdesc}
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74
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75 \begin{funcdesc}{extract_stack}{\optional{f\optional{, limit}}}
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76 Extract the raw traceback from the current stack frame. The return
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77 value has the same format as for \function{extract_tb()}. The
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78 optional \var{f} and \var{limit} arguments have the same meaning as
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79 for \function{print_stack()}.
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80 \end{funcdesc}
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81
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82 \begin{funcdesc}{format_list}{list}
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83 Given a list of tuples as returned by \function{extract_tb()} or
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84 \function{extract_stack()}, return a list of strings ready for
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85 printing. Each string in the resulting list corresponds to the item
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86 with the same index in the argument list. Each string ends in a
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87 newline; the strings may contain internal newlines as well, for those
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88 items whose source text line is not \code{None}.
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89 \end{funcdesc}
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90
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91 \begin{funcdesc}{format_exception_only}{type, value}
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92 Format the exception part of a traceback. The arguments are the
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93 exception type and value such as given by \code{sys.last_type} and
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94 \code{sys.last_value}. The return value is a list of strings, each
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95 ending in a newline. Normally, the list contains a single string;
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96 however, for \exception{SyntaxError} exceptions, it contains several
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97 lines that (when printed) display detailed information about where the
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98 syntax error occurred. The message indicating which exception
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99 occurred is the always last string in the list.
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100 \end{funcdesc}
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101
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102 \begin{funcdesc}{format_exception}{type, value, tb\optional{, limit}}
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103 Format a stack trace and the exception information. The arguments
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104 have the same meaning as the corresponding arguments to
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105 \function{print_exception()}. The return value is a list of strings,
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106 each ending in a newline and some containing internal newlines. When
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107 these lines are concatenated and printed, exactly the same text is
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108 printed as does \function{print_exception()}.
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109 \end{funcdesc}
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110
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111 \begin{funcdesc}{format_tb}{tb\optional{, limit}}
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112 A shorthand for \code{format_list(extract_tb(\var{tb}, \var{limit}))}.
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113 \end{funcdesc}
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114
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115 \begin{funcdesc}{format_stack}{\optional{f\optional{, limit}}}
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116 A shorthand for \code{format_list(extract_stack(\var{f}, \var{limit}))}.
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117 \end{funcdesc}
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118
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119 \begin{funcdesc}{tb_lineno}{tb}
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120 This function returns the current line number set in the traceback
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121 object. This is normally the same as the \code{\var{tb}.tb_lineno}
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122 field of the object, but when optimization is used (the -O flag) this
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123 field is not updated correctly; this function calculates the correct
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124 value.
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125 \end{funcdesc}
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126
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127
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128 \subsection{Traceback Example \label{traceback-example}}
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129
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130 This simple example implements a basic read-eval-print loop, similar
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131 to (but less useful than) the standard Python interactive interpreter
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132 loop. For a more complete implementation of the interpreter loop,
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133 refer to the \refmodule{code} module.
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134
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135 \begin{verbatim}
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136 import sys, traceback
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137
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138 def run_user_code(envdir):
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139 source = raw_input(">>> ")
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140 try:
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141 exec source in envdir
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142 except:
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143 print "Exception in user code:"
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144 print '-'*60
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145 traceback.print_exc(file=sys.stdout)
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146 print '-'*60
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147
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148 envdir = {}
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149 while 1:
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150 run_user_code(envdir)
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151 \end{verbatim}