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e02a3017ca
it yet since sphinx can't run the doctests using python3. Merged revisions 72038 via svnmerge from svn+ssh://pythondev@svn.python.org/python/trunk ........ r72038 | r.david.murray | 2009-04-27 13:22:36 -0400 (Mon, 27 Apr 2009) | 8 lines Make sys.xxx variable references into links, note that print_last only works when an exception gets to the interactive prompt, and update the examples after testing. The last one is now a valid Sphinx doctest, but of the preceding two one can't be made a doctest and the other one I'm postponing to 3.x because sphinx handles doctests as Unicode strings and that makes the 2.x output confusing. ........
277 lines
10 KiB
ReStructuredText
277 lines
10 KiB
ReStructuredText
:mod:`traceback` --- Print or retrieve a stack traceback
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========================================================
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.. module:: traceback
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:synopsis: Print or retrieve a stack traceback.
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This module provides a standard interface to extract, format and print stack
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traces of Python programs. It exactly mimics the behavior of the Python
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interpreter when it prints a stack trace. This is useful when you want to print
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stack traces under program control, such as in a "wrapper" around the
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interpreter.
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.. index:: object: traceback
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The module uses traceback objects --- this is the object type that is stored in
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the :data:`sys.last_traceback` variable and returned as the third item from
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:func:`sys.exc_info`.
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The module defines the following functions:
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.. function:: print_tb(traceback[, limit[, file]])
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Print up to *limit* stack trace entries from *traceback*. If *limit* is omitted
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or ``None``, all entries are printed. If *file* is omitted or ``None``, the
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output goes to ``sys.stderr``; otherwise it should be an open file or file-like
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object to receive the output.
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.. function:: print_exception(type, value, traceback[, limit[, file[, chain]]])
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Print exception information and up to *limit* stack trace entries from
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*traceback* to *file*. This differs from :func:`print_tb` in the following
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ways:
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* if *traceback* is not ``None``, it prints a header ``Traceback (most recent
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call last):``
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* it prints the exception *type* and *value* after the stack trace
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* if *type* is :exc:`SyntaxError` and *value* has the appropriate format, it
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prints the line where the syntax error occurred with a caret indicating the
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approximate position of the error.
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If *chain* is true (the default), then chained exceptions (the
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:attr:`__cause__` or :attr:`__context__` attributes of the exception) will be
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printed as well, like the interpreter itself does when printing an unhandled
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exception.
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.. function:: print_exc([limit[, file[, chain]]])
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This is a shorthand for ``print_exception(*sys.exc_info())``.
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.. function:: print_last([limit[, file[, chain]]])
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This is a shorthand for ``print_exception(sys.last_type, sys.last_value,
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sys.last_traceback, limit, file)``. In general it will work only after
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an exception has reached an interactive prompt (see :data:`sys.last_type`).
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.. function:: print_stack([f[, limit[, file]]])
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This function prints a stack trace from its invocation point. The optional *f*
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argument can be used to specify an alternate stack frame to start. The optional
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*limit* and *file* arguments have the same meaning as for
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:func:`print_exception`.
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.. function:: extract_tb(traceback[, limit])
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Return a list of up to *limit* "pre-processed" stack trace entries extracted
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from the traceback object *traceback*. It is useful for alternate formatting of
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stack traces. If *limit* is omitted or ``None``, all entries are extracted. A
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"pre-processed" stack trace entry is a quadruple (*filename*, *line number*,
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*function name*, *text*) representing the information that is usually printed
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for a stack trace. The *text* is a string with leading and trailing whitespace
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stripped; if the source is not available it is ``None``.
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.. function:: extract_stack([f[, limit]])
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Extract the raw traceback from the current stack frame. The return value has
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the same format as for :func:`extract_tb`. The optional *f* and *limit*
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arguments have the same meaning as for :func:`print_stack`.
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.. function:: format_list(list)
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Given a list of tuples as returned by :func:`extract_tb` or
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:func:`extract_stack`, return a list of strings ready for printing. Each string
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in the resulting list corresponds to the item with the same index in the
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argument list. Each string ends in a newline; the strings may contain internal
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newlines as well, for those items whose source text line is not ``None``.
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.. function:: format_exception_only(type, value)
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Format the exception part of a traceback. The arguments are the exception type
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and value such as given by ``sys.last_type`` and ``sys.last_value``. The return
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value is a list of strings, each ending in a newline. Normally, the list
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contains a single string; however, for :exc:`SyntaxError` exceptions, it
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contains several lines that (when printed) display detailed information about
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where the syntax error occurred. The message indicating which exception
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occurred is the always last string in the list.
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.. function:: format_exception(type, value, tb[, limit[, chain]])
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Format a stack trace and the exception information. The arguments have the
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same meaning as the corresponding arguments to :func:`print_exception`. The
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return value is a list of strings, each ending in a newline and some containing
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internal newlines. When these lines are concatenated and printed, exactly the
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same text is printed as does :func:`print_exception`.
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.. function:: format_exc([limit[, chain]])
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This is like ``print_exc(limit)`` but returns a string instead of printing to a
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file.
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.. function:: format_tb(tb[, limit])
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A shorthand for ``format_list(extract_tb(tb, limit))``.
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.. function:: format_stack([f[, limit]])
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A shorthand for ``format_list(extract_stack(f, limit))``.
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.. _traceback-example:
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Traceback Examples
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------------------
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This simple example implements a basic read-eval-print loop, similar to (but
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less useful than) the standard Python interactive interpreter loop. For a more
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complete implementation of the interpreter loop, refer to the :mod:`code`
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module. ::
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import sys, traceback
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def run_user_code(envdir):
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source = input(">>> ")
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try:
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exec(source, envdir)
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except:
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print("Exception in user code:")
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print("-"*60)
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traceback.print_exc(file=sys.stdout)
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print("-"*60)
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envdir = {}
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while True:
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run_user_code(envdir)
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The following example demonstrates the different ways to print and format the
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exception and traceback:
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.. testcode::
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import sys, traceback
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def lumberjack():
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bright_side_of_death()
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def bright_side_of_death():
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return tuple()[0]
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try:
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lumberjack()
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except:
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exceptionType, exceptionValue, exceptionTraceback = sys.exc_info()
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print("*** print_tb:")
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traceback.print_tb(exceptionTraceback, limit=1, file=sys.stdout)
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print("*** print_exception:")
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traceback.print_exception(exceptionType, exceptionValue, exceptionTraceback,
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limit=2, file=sys.stdout)
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print("*** print_exc:")
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traceback.print_exc()
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print("*** format_exc, first and last line:")
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formatted_lines = traceback.format_exc().splitlines()
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print(formatted_lines[0])
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print(formatted_lines[-1])
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print("*** format_exception:")
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print(repr(traceback.format_exception(exceptionType, exceptionValue,
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exceptionTraceback)))
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print("*** extract_tb:")
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print(repr(traceback.extract_tb(exceptionTraceback)))
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print("*** format_tb:")
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print(repr(traceback.format_tb(exceptionTraceback)))
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print("*** tb_lineno:", traceback.tb_lineno(exceptionTraceback))
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The output for the example would look similar to this:
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.. testoutput::
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:options: +NORMALIZE_WHITESPACE
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*** print_tb:
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File "<doctest...>", line 10, in <module>
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lumberjack()
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*** print_exception:
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Traceback (most recent call last):
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File "<doctest...>", line 10, in <module>
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lumberjack()
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File "<doctest...>", line 4, in lumberjack
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bright_side_of_death()
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IndexError: tuple index out of range
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*** print_exc:
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Traceback (most recent call last):
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File "<doctest...>", line 10, in <module>
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lumberjack()
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File "<doctest...>", line 4, in lumberjack
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bright_side_of_death()
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IndexError: tuple index out of range
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*** format_exc, first and last line:
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Traceback (most recent call last):
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IndexError: tuple index out of range
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*** format_exception:
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['Traceback (most recent call last):\n',
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' File "<doctest...>", line 10, in <module>\n lumberjack()\n',
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' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n',
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' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n',
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'IndexError: tuple index out of range\n']
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*** extract_tb:
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[('<doctest...>', 10, '<module>', 'lumberjack()'),
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('<doctest...>', 4, 'lumberjack', 'bright_side_of_death()'),
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(u'<doctest...>', 7, 'bright_side_of_death', 'return tuple()[0]')]
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*** format_tb:
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[' File "<doctest...>", line 10, in <module>\n lumberjack()\n',
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' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n',
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' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n']
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*** tb_lineno: 10
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The following example shows the different ways to print and format the stack::
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>>> import traceback
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>>> def another_function():
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... lumberstack()
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...
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>>> def lumberstack():
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... traceback.print_stack()
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... print(repr(traceback.extract_stack()))
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... print(repr(traceback.format_stack()))
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...
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>>> another_function()
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File "<doctest>", line 10, in <module>
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another_function()
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File "<doctest>", line 3, in another_function
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lumberstack()
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File "<doctest>", line 6, in lumberstack
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traceback.print_stack()
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[('<doctest>', 10, '<module>', 'another_function()'),
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('<doctest>', 3, 'another_function', 'lumberstack()'),
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('<doctest>', 7, 'lumberstack', 'print(repr(traceback.extract_stack()))')]
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[' File "<doctest>", line 10, in <module>\n another_function()\n',
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' File "<doctest>", line 3, in another_function\n lumberstack()\n',
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' File "<doctest>", line 8, in lumberstack\n print(repr(traceback.format_stack()))\n']
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This last example demonstrates the final few formatting functions:
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.. doctest::
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:options: +NORMALIZE_WHITESPACE
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>>> import traceback
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>>> traceback.format_list([('spam.py', 3, '<module>', 'spam.eggs()'),
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... ('eggs.py', 42, 'eggs', 'return "bacon"')])
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[' File "spam.py", line 3, in <module>\n spam.eggs()\n',
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' File "eggs.py", line 42, in eggs\n return "bacon"\n']
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>>> an_error = IndexError('tuple index out of range')
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>>> traceback.format_exception_only(type(an_error), an_error)
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['IndexError: tuple index out of range\n']
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