29.9. traceback
— Print or retrieve a stack traceback¶
Source code: Lib/traceback.py
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
(tb, limit=None, file=None)¶ Print up to limit stack trace entries from traceback object tb (starting from the caller’s frame) if limit is positive. Otherwise, print the last
abs(limit)
entries. If limit is omitted orNone
, all entries are printed. If file is omitted orNone
, the output goes tosys.stderr
; otherwise it should be an open file or file-like object to receive the output.Distinto en la versión 3.5: Added negative limit support.
-
traceback.
print_exception
(etype, value, tb, limit=None, file=None, chain=True)¶ Print exception information and stack trace entries from traceback object tb to file. This differs from
print_tb()
in the following ways:- if tb is not
None
, it prints a headerTraceback (most recent call last):
- it prints the exception etype and value after the stack trace
- if type(value) 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.
The optional limit argument has the same meaning as for
print_tb()
. 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.Distinto en la versión 3.5: The etype argument is ignored and inferred from the type of value.
- if tb is not
-
traceback.
print_exc
(limit=None, file=None, chain=True)¶ This is a shorthand for
print_exception(*sys.exc_info(), limit, file, chain)
.
-
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, chain)
. In general it will work only after an exception has reached an interactive prompt (seesys.last_type
).
-
traceback.
print_stack
(f=None, limit=None, file=None)¶ Print up to limit stack trace entries (starting from the invocation point) if limit is positive. Otherwise, print the last
abs(limit)
entries. If limit is omitted orNone
, all entries are printed. The optional f argument can be used to specify an alternate stack frame to start. The optional file argument has the same meaning as forprint_tb()
.Distinto en la versión 3.5: Added negative limit support.
-
traceback.
extract_tb
(tb, limit=None)¶ Return a list of «pre-processed» stack trace entries extracted from the traceback object tb. It is useful for alternate formatting of stack traces. The optional limit argument has the same meaning as for
print_tb()
. A «pre-processed» stack trace entry is a 4-tuple (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 isNone
.
-
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 forprint_stack()
.
-
traceback.
format_list
(extracted_list)¶ Given a list of tuples as returned by
extract_tb()
orextract_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 notNone
.
-
traceback.
format_exception_only
(etype, value)¶ Format the exception part of a traceback. The arguments are the exception type and value such as given by
sys.last_type
andsys.last_value
. The return value is a list of strings, each ending in a newline. Normally, the list contains a single string; however, forSyntaxError
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
(etype, 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 doesprint_exception()
.Distinto en la versión 3.5: The etype argument is ignored and inferred from the type of value.
-
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))
.
-
traceback.
clear_frames
(tb)¶ Clears the local variables of all the stack frames in a traceback tb by calling the
clear()
method of each frame object.Nuevo en la versión 3.4.
-
traceback.
walk_stack
(f)¶ Walk a stack following
f.f_back
from the given frame, yielding the frame and line number for each frame. If f isNone
, the current stack is used. This helper is used withStackSummary.extract()
.Nuevo en la versión 3.5.
-
traceback.
walk_tb
(tb)¶ Walk a traceback following
tb_next
yielding the frame and line number for each frame. This helper is used withStackSummary.extract()
.Nuevo en la versión 3.5.
The module also defines the following classes:
29.9.1. TracebackException
Objects¶
Nuevo en la versión 3.5.
TracebackException
objects are created from actual exceptions to
capture data for later printing in a lightweight fashion.
-
class
traceback.
TracebackException
(exc_type, exc_value, exc_traceback, *, limit=None, lookup_lines=True, capture_locals=False)¶ Capture an exception for later rendering. limit, lookup_lines and capture_locals are as for the
StackSummary
class.Note that when locals are captured, they are also shown in the traceback.
-
__cause__
¶ A
TracebackException
of the original__cause__
.
-
__context__
¶ A
TracebackException
of the original__context__
.
-
__suppress_context__
¶ The
__suppress_context__
value from the original exception.
-
stack
¶ A
StackSummary
representing the traceback.
-
exc_type
¶ The class of the original traceback.
-
filename
¶ For syntax errors - the file name where the error occurred.
-
lineno
¶ For syntax errors - the line number where the error occurred.
-
text
¶ For syntax errors - the text where the error occurred.
-
offset
¶ For syntax errors - the offset into the text where the error occurred.
-
msg
¶ For syntax errors - the compiler error message.
-
classmethod
from_exception
(exc, *, limit=None, lookup_lines=True, capture_locals=False)¶ Capture an exception for later rendering. limit, lookup_lines and capture_locals are as for the
StackSummary
class.Note that when locals are captured, they are also shown in the traceback.
-
format
(*, chain=True)¶ Format the exception.
If chain is not
True
,__cause__
and__context__
will not be formatted.The return value is a generator of strings, each ending in a newline and some containing internal newlines.
print_exception()
is a wrapper around this method which just prints the lines to a file.The message indicating which exception occurred is always the last string in the output.
-
format_exception_only
()¶ Format the exception part of the traceback.
The return value is a generator of strings, each ending in a newline.
Normally, the generator emits a single string; however, for
SyntaxError
exceptions, it emits several lines that (when printed) display detailed information about where the syntax error occurred.The message indicating which exception occurred is always the last string in the output.
-
29.9.2. StackSummary
Objects¶
Nuevo en la versión 3.5.
StackSummary
objects represent a call stack ready for formatting.
-
class
traceback.
StackSummary
¶ -
classmethod
extract
(frame_gen, *, limit=None, lookup_lines=True, capture_locals=False)¶ Construct a
StackSummary
object from a frame generator (such as is returned bywalk_stack()
orwalk_tb()
).If limit is supplied, only this many frames are taken from frame_gen. If lookup_lines is
False
, the returnedFrameSummary
objects will not have read their lines in yet, making the cost of creating theStackSummary
cheaper (which may be valuable if it may not actually get formatted). If capture_locals isTrue
the local variables in eachFrameSummary
are captured as object representations.
-
classmethod
from_list
(a_list)¶ Construct a
StackSummary
object from a supplied old-style list of tuples. Each tuple should be a 4-tuple with filename, lineno, name, line as the elements.
-
classmethod
29.9.3. FrameSummary
Objects¶
Nuevo en la versión 3.5.
FrameSummary
objects represent a single frame in a traceback.
-
class
traceback.
FrameSummary
(filename, lineno, name, lookup_line=True, locals=None, line=None)¶ Represent a single frame in the traceback or stack that is being formatted or printed. It may optionally have a stringified version of the frames locals included in it. If lookup_line is
False
, the source code is not looked up until theFrameSummary
has theline
attribute accessed (which also happens when casting it to a tuple).line
may be directly provided, and will prevent line lookups happening at all. locals is an optional local variable dictionary, and if supplied the variable representations are stored in the summary for later display.
29.9.4. 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 Exception:
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:")
# exc_type below is ignored on 3.5 and later
traceback.print_exception(exc_type, exc_value, exc_traceback,
limit=2, file=sys.stdout)
print("*** print_exc:")
traceback.print_exc(limit=2, file=sys.stdout)
print("*** format_exc, first and last line:")
formatted_lines = traceback.format_exc().splitlines()
print(formatted_lines[0])
print(formatted_lines[-1])
print("*** format_exception:")
# exc_type below is ignored on 3.5 and later
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:
[<FrameSummary file <doctest...>, line 10 in <module>>,
<FrameSummary file <doctest...>, line 4 in lumberjack>,
<FrameSummary file <doctest...>, line 7 in bright_side_of_death>]
*** 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']