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28.9. traceback — Print or retrieve a stack traceback
This module provides a standard interface to extract, format and print stack traces of Python programs. It exactly mimics the behavior of the Python interpreter when it prints a stack trace. This is useful when you want to print stack traces under program control, such as in a “wrapper” around the interpreter.
The module uses traceback objects — this is the object type that is stored in the sys.last_traceback variable and returned as the third item from sys.exc_info().
The module defines the following functions:
- traceback.print_tb(traceback, limit=None, file=None)
Print up to limit stack trace entries from traceback. If limit is omitted or None, all entries are printed. If file is omitted or None, the output goes to sys.stderr; otherwise it should be an open file or file-like object to receive the output.
- traceback.print_exception(type, value, traceback, limit=None, file=None, chain=True)
Print exception information and up to limit stack trace entries from traceback to file. This differs from print_tb() in the following ways:
- if traceback is not None, it prints a header Traceback (most recent call last):
- it prints the exception type and value after the stack trace
- if type is SyntaxError and value has the appropriate format, it prints the line where the syntax error occurred with a caret indicating the approximate position of the error.
If chain is true (the default), then chained exceptions (the __cause__ or __context__ attributes of the exception) will be printed as well, like the interpreter itself does when printing an unhandled exception.
- traceback.print_exc(limit=None, file=None, chain=True)
This is a shorthand for print_exception(*sys.exc_info()).
- traceback.print_last(limit=None, file=None, chain=True)
This is a shorthand for print_exception(sys.last_type, sys.last_value, sys.last_traceback, limit, file). In general it will work only after an exception has reached an interactive prompt (see sys.last_type).
- traceback.print_stack(f=None, limit=None, file=None)
This function prints a stack trace from its invocation point. The optional f argument can be used to specify an alternate stack frame to start. The optional limit and file arguments have the same meaning as for print_exception().
- traceback.extract_tb(traceback, limit=None)
Return a list of up to limit “pre-processed” stack trace entries extracted from the traceback object traceback. It is useful for alternate formatting of stack traces. If limit is omitted or None, all entries are extracted. A “pre-processed” stack trace entry is a quadruple (filename, line number, function name, text) representing the information that is usually printed for a stack trace. The text is a string with leading and trailing whitespace stripped; if the source is not available it is None.
- traceback.extract_stack(f=None, limit=None)
Extract the raw traceback from the current stack frame. The return value has the same format as for extract_tb(). The optional f and limit arguments have the same meaning as for print_stack().
- traceback.format_list(list)
Given a list of tuples as returned by extract_tb() or extract_stack(), return a list of strings ready for printing. Each string in the resulting list corresponds to the item with the same index in the argument list. Each string ends in a newline; the strings may contain internal newlines as well, for those items whose source text line is not None.
- traceback.format_exception_only(type, value)
Format the exception part of a traceback. The arguments are the exception type and value such as given by sys.last_type and sys.last_value. The return value is a list of strings, each ending in a newline. Normally, the list contains a single string; however, for SyntaxError exceptions, it contains several lines that (when printed) display detailed information about where the syntax error occurred. The message indicating which exception occurred is the always last string in the list.
- traceback.format_exception(type, value, tb, limit=None, chain=True)
Format a stack trace and the exception information. The arguments have the same meaning as the corresponding arguments to print_exception(). The return value is a list of strings, each ending in a newline and some containing internal newlines. When these lines are concatenated and printed, exactly the same text is printed as does print_exception().
- traceback.format_exc(limit=None, chain=True)
This is like print_exc(limit) but returns a string instead of printing to a file.
- traceback.format_tb(tb, limit=None)
A shorthand for format_list(extract_tb(tb, limit)).
- traceback.format_stack(f=None, limit=None)
A shorthand for format_list(extract_stack(f, limit)).
28.9.1. Traceback Examples
This simple example implements a basic read-eval-print loop, similar to (but less useful than) the standard Python interactive interpreter loop. For a more complete implementation of the interpreter loop, refer to the code module.
import sys, traceback
def run_user_code(envdir):
source = input(">>> ")
try:
exec(source, envdir)
except:
print("Exception in user code:")
print("-"*60)
traceback.print_exc(file=sys.stdout)
print("-"*60)
envdir = {}
while True:
run_user_code(envdir)
The following example demonstrates the different ways to print and format the exception and traceback:
import sys, traceback
def lumberjack():
bright_side_of_death()
def bright_side_of_death():
return tuple()[0]
try:
lumberjack()
except IndexError:
exc_type, exc_value, exc_traceback = sys.exc_info()
print("*** print_tb:")
traceback.print_tb(exc_traceback, limit=1, file=sys.stdout)
print("*** print_exception:")
traceback.print_exception(exc_type, exc_value, exc_traceback,
limit=2, file=sys.stdout)
print("*** print_exc:")
traceback.print_exc()
print("*** format_exc, first and last line:")
formatted_lines = traceback.format_exc().splitlines()
print(formatted_lines[0])
print(formatted_lines[-1])
print("*** format_exception:")
print(repr(traceback.format_exception(exc_type, exc_value,
exc_traceback)))
print("*** extract_tb:")
print(repr(traceback.extract_tb(exc_traceback)))
print("*** format_tb:")
print(repr(traceback.format_tb(exc_traceback)))
print("*** tb_lineno:", exc_traceback.tb_lineno)
The output for the example would look similar to this:
*** print_tb:
File "<doctest...>", line 10, in <module>
lumberjack()
*** print_exception:
Traceback (most recent call last):
File "<doctest...>", line 10, in <module>
lumberjack()
File "<doctest...>", line 4, in lumberjack
bright_side_of_death()
IndexError: tuple index out of range
*** print_exc:
Traceback (most recent call last):
File "<doctest...>", line 10, in <module>
lumberjack()
File "<doctest...>", line 4, in lumberjack
bright_side_of_death()
IndexError: tuple index out of range
*** format_exc, first and last line:
Traceback (most recent call last):
IndexError: tuple index out of range
*** format_exception:
['Traceback (most recent call last):\n',
' File "<doctest...>", line 10, in <module>\n lumberjack()\n',
' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n',
' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n',
'IndexError: tuple index out of range\n']
*** extract_tb:
[('<doctest...>', 10, '<module>', 'lumberjack()'),
('<doctest...>', 4, 'lumberjack', 'bright_side_of_death()'),
('<doctest...>', 7, 'bright_side_of_death', 'return tuple()[0]')]
*** format_tb:
[' File "<doctest...>", line 10, in <module>\n lumberjack()\n',
' File "<doctest...>", line 4, in lumberjack\n bright_side_of_death()\n',
' File "<doctest...>", line 7, in bright_side_of_death\n return tuple()[0]\n']
*** tb_lineno: 10
The following example shows the different ways to print and format the stack:
>>> import traceback
>>> def another_function():
... lumberstack()
...
>>> def lumberstack():
... traceback.print_stack()
... print(repr(traceback.extract_stack()))
... print(repr(traceback.format_stack()))
...
>>> another_function()
File "<doctest>", line 10, in <module>
another_function()
File "<doctest>", line 3, in another_function
lumberstack()
File "<doctest>", line 6, in lumberstack
traceback.print_stack()
[('<doctest>', 10, '<module>', 'another_function()'),
('<doctest>', 3, 'another_function', 'lumberstack()'),
('<doctest>', 7, 'lumberstack', 'print(repr(traceback.extract_stack()))')]
[' File "<doctest>", line 10, in <module>\n another_function()\n',
' File "<doctest>", line 3, in another_function\n lumberstack()\n',
' File "<doctest>", line 8, in lumberstack\n print(repr(traceback.format_stack()))\n']
This last example demonstrates the final few formatting functions:
>>> import traceback
>>> traceback.format_list([('spam.py', 3, '<module>', 'spam.eggs()'),
... ('eggs.py', 42, 'eggs', 'return "bacon"')])
[' File "spam.py", line 3, in <module>\n spam.eggs()\n',
' File "eggs.py", line 42, in eggs\n return "bacon"\n']
>>> an_error = IndexError('tuple index out of range')
>>> traceback.format_exception_only(type(an_error), an_error)
['IndexError: tuple index out of range\n']