TracebackException Objects¶

TracebackException objects are created from actual exceptions tocapture data for later printing in a lightweight fashion.

classtraceback.TracebackException(exc_type,exc_value,exc_traceback,*,limit=None,lookup_lines=True,capture_locals=False,compact=False,max_group_width=15,max_group_depth=10)¶

Capture an exception for later rendering.limit,lookup_lines andcapture_locals are as for theStackSummary class.

Ifcompact is true, only data that is required byTracebackException’sformat() methodis saved in the class attributes. In particular, the__context__ field is calculated only if__cause__ isNone and__suppress_context__ is false.

Note that when locals are captured, they are also shown in the traceback.

max_group_width andmax_group_depth control the formatting of exceptiongroups (seeBaseExceptionGroup). The depth refers to the nestinglevel of the group, and the width refers to the size of a single exceptiongroup’s exceptions array. The formatted output is truncated when eitherlimit is exceeded.

Changed in version 3.10:Added thecompact parameter.

Changed in version 3.11:Added themax_group_width andmax_group_depth parameters.

__cause__¶

ATracebackException of the original__cause__.

__context__¶

ATracebackException of the original__context__.

exceptions¶

Ifself represents anExceptionGroup, this field holds a list ofTracebackException instances representing the nested exceptions.Otherwise it isNone.

New in version 3.11.

__suppress_context__¶

The__suppress_context__ value from the originalexception.

__notes__¶

The__notes__ value from the original exception,orNoneif the exception does not have any notes. If it is notNoneis it formatted in the traceback after the exception string.

New in version 3.11.

stack¶

AStackSummary 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.

end_lineno¶

For syntax errors - the end line number where the error occurred.Can beNone if not present.

New in version 3.10.

text¶

For syntax errors - the text where the error occurred.

offset¶

For syntax errors - the offset into the text where the error occurred.

end_offset¶

For syntax errors - the end offset into the text where the error occurred.Can beNone if not present.

New in version 3.10.

msg¶

For syntax errors - the compiler error message.

classmethodfrom_exception(exc,*,limit=None,lookup_lines=True,capture_locals=False)¶

Capture an exception for later rendering.limit,lookup_lines andcapture_locals are as for theStackSummary class.

Note that when locals are captured, they are also shown in the traceback.

print(*,file=None,chain=True)¶

Print tofile (defaultsys.stderr) the exception information returned byformat().

New in version 3.11.

format(*,chain=True)¶

Format the exception.

Ifchain is notTrue,__cause__ and__context__will not be formatted.

The return value is a generator of strings, each ending in a newline andsome containing internal newlines.print_exception()is a wrapper around this method which just prints the lines to a file.

format_exception_only()¶

Format the exception part of the traceback.

The return value is a generator of strings, each ending in a newline.

The generator emits the exception’s message followed by its notes(if it has any). The exception message is normally a single string;however, forSyntaxError exceptions, it consists of severallines that (when printed) display detailed information about wherethe syntax error occurred.

Changed in version 3.11:The exception’snotes are nowincluded in the output.

StackSummary Objects¶

StackSummary objects represent a call stack ready for formatting.

classtraceback.StackSummary¶

classmethodextract(frame_gen,*,limit=None,lookup_lines=True,capture_locals=False)¶

Construct aStackSummary object from a frame generator (such asis returned bywalk_stack() orwalk_tb()).

Iflimit is supplied, only this many frames are taken fromframe_gen.Iflookup_lines isFalse, the returnedFrameSummaryobjects will not have read their lines in yet, making the cost ofcreating theStackSummary cheaper (which may be valuable if itmay not actually get formatted). Ifcapture_locals isTrue thelocal variables in eachFrameSummary are captured as objectrepresentations.

classmethodfrom_list(a_list)¶

Construct aStackSummary object from a supplied list ofFrameSummary objects or old-style list of tuples. Each tupleshould be a 4-tuple withfilename,lineno,name,line as theelements.

format()¶

Returns a list of strings ready for printing. Each string in theresulting list corresponds to a singleframe fromthe stack.Each string ends in a newline; the strings may contain internalnewlines as well, for those items with source text lines.

For long sequences of the same frame and line, the first fewrepetitions are shown, followed by a summary line stating the exactnumber of further repetitions.

Changed in version 3.6:Long sequences of repeated frames are now abbreviated.

format_frame_summary(frame_summary)¶

Returns a string for printing one of theframesinvolved in the stack.This method is called for eachFrameSummary object to beprinted byStackSummary.format(). If it returnsNone, theframe is omitted from the output.

New in version 3.11.

FrameSummary Objects¶

AFrameSummary object represents a singleframein atraceback.

classtraceback.FrameSummary(filename,lineno,name,lookup_line=True,locals=None,line=None)¶

Represents a singleframe in thetraceback or stack that is being formattedor printed. It may optionally have a stringified version of the frame’slocals included in it. Iflookup_line isFalse, the source code is notlooked up until theFrameSummary has thelineattribute accessed (which also happens when casting it to atuple).line may be directly provided, and will prevent linelookups happening at all.locals is an optional local variabledictionary, and if supplied the variable representations are stored in thesummary for later display.

FrameSummary instances have the following attributes:

filename¶

The filename of the source code for this frame. Equivalent to accessingf.f_code.co_filename on aframe objectf.

lineno¶

The line number of the source code for this frame.

name¶

Equivalent to accessingf.f_code.co_name onaframe objectf.

line¶

A string representing the source code for this frame, with leading andtrailing whitespace stripped.If the source is not available, it isNone.

Traceback Examples¶

This simple example implements a basic read-eval-print loop, similar to (butless useful than) the standard Python interactive interpreter loop. For a morecomplete implementation of the interpreter loop, refer to thecodemodule.

importsys,tracebackdefrun_user_code(envdir):source=input(">>> ")try:exec(source,envdir)exceptException:print("Exception in user code:")print("-"*60)traceback.print_exc(file=sys.stdout)print("-"*60)envdir={}whileTrue:run_user_code(envdir)

The following example demonstrates the different ways to print and format theexception and traceback:

importsys,tracebackdeflumberjack():bright_side_of_life()defbright_side_of_life():returntuple()[0]try:lumberjack()exceptIndexError:exc=sys.exception()print("*** print_tb:")traceback.print_tb(exc.__traceback__,limit=1,file=sys.stdout)print("*** print_exception:")traceback.print_exception(exc,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:")print(repr(traceback.format_exception(exc)))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_life()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_life()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 default[0]>", line 10, in <module>\n    lumberjack()\n', '  File "<doctest default[0]>", line 4, in lumberjack\n    bright_side_of_life()\n', '  File "<doctest default[0]>", line 7, in bright_side_of_life\n    return tuple()[0]\n           ~~~~~~~^^^\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_life>]*** format_tb:['  File "<doctest default[0]>", line 10, in <module>\n    lumberjack()\n', '  File "<doctest default[0]>", line 4, in lumberjack\n    bright_side_of_life()\n', '  File "<doctest default[0]>", line 7, in bright_side_of_life\n    return tuple()[0]\n           ~~~~~~~^^^\n']*** tb_lineno: 10

The following example shows the different ways to print and format the stack:

>>>importtraceback>>>defanother_function():...lumberstack()...>>>deflumberstack():...traceback.print_stack()...print(repr(traceback.extract_stack()))...print(repr(traceback.format_stack()))...>>>another_function()  File"<doctest>", line10, in<module>another_function()  File"<doctest>", line3, inanother_functionlumberstack()  File"<doctest>", line6, inlumberstacktraceback.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:

>>>importtraceback>>>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']