2 :mod:`traceback` --- Print or retrieve a stack traceback
3 ========================================================
6 :synopsis: Print or retrieve a stack traceback.
9 This module provides a standard interface to extract, format and print stack
10 traces of Python programs. It exactly mimics the behavior of the Python
11 interpreter when it prints a stack trace. This is useful when you want to print
12 stack traces under program control, such as in a "wrapper" around the
15 .. index:: object: traceback
17 The module uses traceback objects --- this is the object type that is stored in
18 the variables ``sys.exc_traceback`` (deprecated) and ``sys.last_traceback`` and
19 returned as the third item from :func:`sys.exc_info`.
21 The module defines the following functions:
24 .. function:: print_tb(traceback[, limit[, file]])
26 Print up to *limit* stack trace entries from *traceback*. If *limit* is omitted
27 or ``None``, all entries are printed. If *file* is omitted or ``None``, the
28 output goes to ``sys.stderr``; otherwise it should be an open file or file-like
29 object to receive the output.
32 .. function:: print_exception(type, value, traceback[, limit[, file]])
34 Print exception information and up to *limit* stack trace entries from
35 *traceback* to *file*. This differs from :func:`print_tb` in the following ways:
36 (1) if *traceback* is not ``None``, it prints a header ``Traceback (most recent
37 call last):``; (2) it prints the exception *type* and *value* after the stack
38 trace; (3) if *type* is :exc:`SyntaxError` and *value* has the appropriate
39 format, it prints the line where the syntax error occurred with a caret
40 indicating the approximate position of the error.
43 .. function:: print_exc([limit[, file]])
45 This is a shorthand for ``print_exception(sys.exc_type, sys.exc_value,
46 sys.exc_traceback, limit, file)``. (In fact, it uses :func:`sys.exc_info` to
47 retrieve the same information in a thread-safe way instead of using the
48 deprecated variables.)
51 .. function:: format_exc([limit])
53 This is like ``print_exc(limit)`` but returns a string instead of printing to a
59 .. function:: print_last([limit[, file]])
61 This is a shorthand for ``print_exception(sys.last_type, sys.last_value,
62 sys.last_traceback, limit, file)``.
65 .. function:: print_stack([f[, limit[, file]]])
67 This function prints a stack trace from its invocation point. The optional *f*
68 argument can be used to specify an alternate stack frame to start. The optional
69 *limit* and *file* arguments have the same meaning as for
70 :func:`print_exception`.
73 .. function:: extract_tb(traceback[, limit])
75 Return a list of up to *limit* "pre-processed" stack trace entries extracted
76 from the traceback object *traceback*. It is useful for alternate formatting of
77 stack traces. If *limit* is omitted or ``None``, all entries are extracted. A
78 "pre-processed" stack trace entry is a quadruple (*filename*, *line number*,
79 *function name*, *text*) representing the information that is usually printed
80 for a stack trace. The *text* is a string with leading and trailing whitespace
81 stripped; if the source is not available it is ``None``.
84 .. function:: extract_stack([f[, limit]])
86 Extract the raw traceback from the current stack frame. The return value has
87 the same format as for :func:`extract_tb`. The optional *f* and *limit*
88 arguments have the same meaning as for :func:`print_stack`.
91 .. function:: format_list(list)
93 Given a list of tuples as returned by :func:`extract_tb` or
94 :func:`extract_stack`, return a list of strings ready for printing. Each string
95 in the resulting list corresponds to the item with the same index in the
96 argument list. Each string ends in a newline; the strings may contain internal
97 newlines as well, for those items whose source text line is not ``None``.
100 .. function:: format_exception_only(type, value)
102 Format the exception part of a traceback. The arguments are the exception type
103 and value such as given by ``sys.last_type`` and ``sys.last_value``. The return
104 value is a list of strings, each ending in a newline. Normally, the list
105 contains a single string; however, for :exc:`SyntaxError` exceptions, it
106 contains several lines that (when printed) display detailed information about
107 where the syntax error occurred. The message indicating which exception
108 occurred is the always last string in the list.
111 .. function:: format_exception(type, value, tb[, limit])
113 Format a stack trace and the exception information. The arguments have the
114 same meaning as the corresponding arguments to :func:`print_exception`. The
115 return value is a list of strings, each ending in a newline and some containing
116 internal newlines. When these lines are concatenated and printed, exactly the
117 same text is printed as does :func:`print_exception`.
120 .. function:: format_tb(tb[, limit])
122 A shorthand for ``format_list(extract_tb(tb, limit))``.
125 .. function:: format_stack([f[, limit]])
127 A shorthand for ``format_list(extract_stack(f, limit))``.
130 .. function:: tb_lineno(tb)
132 This function returns the current line number set in the traceback object. This
133 function was necessary because in versions of Python prior to 2.3 when the
134 :option:`-O` flag was passed to Python the ``tb.tb_lineno`` was not updated
135 correctly. This function has no use in versions past 2.3.
138 .. _traceback-example:
143 This simple example implements a basic read-eval-print loop, similar to (but
144 less useful than) the standard Python interactive interpreter loop. For a more
145 complete implementation of the interpreter loop, refer to the :mod:`code`
148 import sys, traceback
150 def run_user_code(envdir):
151 source = raw_input(">>> ")
153 exec source in envdir
155 print "Exception in user code:"
157 traceback.print_exc(file=sys.stdout)
162 run_user_code(envdir)
165 The following example demonstrates the different ways to print and format the
166 exception and traceback::
168 import sys, traceback
171 bright_side_of_death()
173 def bright_side_of_death():
179 exceptionType, exceptionValue, exceptionTraceback = sys.exc_info()
180 print "*** print_tb:"
181 traceback.print_tb(exceptionTraceback, limit=1, file=sys.stdout)
182 print "*** print_exception:"
183 traceback.print_exception(exceptionType, exceptionValue, exceptionTraceback,
184 limit=2, file=sys.stdout)
185 print "*** print_exc:"
186 traceback.print_exc()
187 print "*** format_exc, first and last line:"
188 formatted_lines = traceback.format_exc().splitlines()
189 print formatted_lines[0]
190 print formatted_lines[-1]
191 print "*** format_exception:"
192 print repr(traceback.format_exception(exceptionType, exceptionValue,
194 print "*** extract_tb:"
195 print repr(traceback.extract_tb(exceptionTraceback))
196 print "*** format_tb:"
197 print repr(traceback.format_tb(exceptionTraceback))
198 print "*** tb_lineno:", traceback.tb_lineno(exceptionTraceback)
199 print "*** print_last:"
200 traceback.print_last()
203 The output for the example would look similar to this::
206 File "<doctest>", line 9, in <module>
209 Traceback (most recent call last):
210 File "<doctest>", line 9, in <module>
212 File "<doctest>", line 3, in lumberjack
213 bright_side_of_death()
214 IndexError: tuple index out of range
216 Traceback (most recent call last):
217 File "<doctest>", line 9, in <module>
219 File "<doctest>", line 3, in lumberjack
220 bright_side_of_death()
221 IndexError: tuple index out of range
222 *** format_exc, first and last line:
223 Traceback (most recent call last):
224 IndexError: tuple index out of range
225 *** format_exception:
226 ['Traceback (most recent call last):\n',
227 ' File "<doctest>", line 9, in <module>\n lumberjack()\n',
228 ' File "<doctest>", line 3, in lumberjack\n bright_side_of_death()\n',
229 ' File "<doctest>", line 6, in bright_side_of_death\n return tuple()[0]\n',
230 'IndexError: tuple index out of range\n']
232 [('<doctest>', 9, '<module>', 'lumberjack()'),
233 ('<doctest>', 3, 'lumberjack', 'bright_side_of_death()'),
234 ('<doctest>', 6, 'bright_side_of_death', 'return tuple()[0]')]
236 [' File "<doctest>", line 9, in <module>\n lumberjack()\n',
237 ' File "<doctest>", line 3, in lumberjack\n bright_side_of_death()\n',
238 ' File "<doctest>", line 6, in bright_side_of_death\n return tuple()[0]\n']
241 Traceback (most recent call last):
242 File "<doctest>", line 9, in <module>
244 File "<doctest>", line 3, in lumberjack
245 bright_side_of_death()
246 IndexError: tuple index out of range
249 The following example shows the different ways to print and format the stack::
252 >>> def another_function():
255 >>> def lumberstack():
256 ... traceback.print_stack()
257 ... print repr(traceback.extract_stack())
258 ... print repr(traceback.format_stack())
260 >>> another_function()
261 File "<doctest>", line 10, in <module>
263 File "<doctest>", line 3, in another_function
265 File "<doctest>", line 6, in lumberstack
266 traceback.print_stack()
267 [('<doctest>', 10, '<module>', 'another_function()'),
268 ('<doctest>', 3, 'another_function', 'lumberstack()'),
269 ('<doctest>', 7, 'lumberstack', 'print repr(traceback.extract_stack())')]
270 [' File "<doctest>", line 10, in <module>\n another_function()\n',
271 ' File "<doctest>", line 3, in another_function\n lumberstack()\n',
272 ' File "<doctest>", line 8, in lumberstack\n print repr(traceback.format_stack())\n']
275 This last example demonstrates the final few formatting functions::
278 >>> format_list([('spam.py', 3, '<module>', 'spam.eggs()'),
279 ... ('eggs.py', 42, 'eggs', 'return "bacon"')])
280 [' File "spam.py", line 3, in <module>\n spam.eggs()\n',
281 ' File "eggs.py", line 42, in eggs\n return "bacon"\n']
282 >>> theError = IndexError('tuple indx out of range')
283 >>> traceback.format_exception_only(type(theError), theError)
284 ['IndexError: tuple index out of range\n']