traceback — Print or retrieve a stack traceback

Source code:Lib/traceback.py

This module provides a standard interface to extract, format and print stacktraces of Python programs. It exactly mimics the behavior of the Pythoninterpreter when it prints a stack trace. This is useful when you want to printstack traces under program control, such as in a “wrapper” around theinterpreter.

The module uses traceback objects — this is the object type that is stored inthesys.last_traceback variable and returned as the third item fromsys.exc_info().

The module defines the following functions:

traceback.print_tb(tb,limit=None,file=None)

Print up tolimit stack trace entries from traceback objecttb (startingfrom the caller’s frame) iflimit is positive. Otherwise, print the lastabs(limit) entries. Iflimit is omitted orNone, all entries areprinted. Iffile is omitted orNone, the output goes tosys.stderr; otherwise it should be an open file or file-like object toreceive the output.

Changed in version 3.5:Added negativelimit support.

traceback.print_exception(etype,value,tb,limit=None,file=None,chain=True)

Print exception information and stack trace entries from traceback objecttb tofile. This differs fromprint_tb() in the followingways:

  • iftb is notNone, it prints a headerTraceback(mostrecentcalllast):

  • it prints the exceptionetype andvalue after the stack trace

  • iftype(value) isSyntaxError andvalue has the appropriateformat, it prints the line where the syntax error occurred with a caretindicating the approximate position of the error.

The optionallimit argument has the same meaning as forprint_tb().Ifchain is true (the default), then chained exceptions (the__cause__ or__context__ attributes of the exception) will beprinted as well, like the interpreter itself does when printing an unhandledexception.

Changed in version 3.5:Theetype argument is ignored and inferred from the type ofvalue.

traceback.print_exc(limit=None,file=None,chain=True)

This is a shorthand forprint_exception(*sys.exc_info(),limit,file,chain).

traceback.print_last(limit=None,file=None,chain=True)

This is a shorthand forprint_exception(sys.last_type,sys.last_value,sys.last_traceback,limit,file,chain). In general it will work onlyafter an exception has reached an interactive prompt (seesys.last_type).

traceback.print_stack(f=None,limit=None,file=None)

Print up tolimit stack trace entries (starting from the invocationpoint) iflimit is positive. Otherwise, print the lastabs(limit)entries. Iflimit is omitted orNone, all entries are printed.The optionalf argument can be used to specify an alternate stack frameto start. The optionalfile argument has the same meaning as forprint_tb().

Changed in version 3.5:Added negativelimit support.

traceback.extract_tb(tb,limit=None)

Return aStackSummary object representing a list of “pre-processed”stack trace entries extracted from the traceback objecttb. It is usefulfor alternate formatting of stack traces. The optionallimit argument hasthe same meaning as forprint_tb(). A “pre-processed” stack traceentry is aFrameSummary object containing attributesfilename,lineno,name, andline representing theinformation that is usually printed for a stack trace. Theline is a string with leading and trailingwhitespace 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 hasthe same format as forextract_tb(). The optionalf andlimitarguments have the same meaning as forprint_stack().

traceback.format_list(extracted_list)

Given a list of tuples orFrameSummary objects as returned byextract_tb() orextract_stack(), return a list of strings readyfor printing. Each string in the resulting list corresponds to the item withthe same index in the argument list. Each string ends in a newline; thestrings may contain internal newlines as well, for those items whose sourcetext line is notNone.

traceback.format_exception_only(etype,value)

Format the exception part of a traceback. The arguments are the exceptiontype and value such as given bysys.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, forSyntaxErrorexceptions, it contains several lines that (when printed) display detailedinformation about where the syntax error occurred. The message indicatingwhich 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 thesame meaning as the corresponding arguments toprint_exception(). Thereturn value is a list of strings, each ending in a newline and somecontaining internal newlines. When these lines are concatenated and printed,exactly the same text is printed as doesprint_exception().

Changed in version 3.5:Theetype argument is ignored and inferred from the type ofvalue.

traceback.format_exc(limit=None,chain=True)

This is likeprint_exc(limit) but returns a string instead of printing toa file.

traceback.format_tb(tb,limit=None)

A shorthand forformat_list(extract_tb(tb,limit)).

traceback.format_stack(f=None,limit=None)

A shorthand forformat_list(extract_stack(f,limit)).

traceback.clear_frames(tb)

Clears the local variables of all the stack frames in a tracebacktbby calling theclear() method of each frame object.

New in version 3.4.

traceback.walk_stack(f)

Walk a stack followingf.f_back from the given frame, yielding the frameand line number for each frame. Iff isNone, the current stack isused. This helper is used withStackSummary.extract().

New in version 3.5.

traceback.walk_tb(tb)

Walk a traceback followingtb_next yielding the frame and line numberfor each frame. This helper is used withStackSummary.extract().

New in version 3.5.

The module also defines the following classes:

TracebackException Objects

New in version 3.5.

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)

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.

__cause__

ATracebackException of the original__cause__.

__context__

ATracebackException of the original__context__.

__suppress_context__

The__suppress_context__ value from the original exception.

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.

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.

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.

format(*,chain=True)

Format the exception.

Ifchain is notTrue,__cause__ and__context__ will notbe 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.

The message indicating which exception occurred is always the laststring 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, forSyntaxError exceptions, it emits several lines that (whenprinted) display detailed information about where the syntaxerror occurred.

The message indicating which exception occurred is always the laststring in the output.

StackSummary Objects

New in version 3.5.

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 with filename, lineno, name, line as the elements.

format()

Returns a list of strings ready for printing. Each string in theresulting list corresponds to a single frame from the 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.

FrameSummary Objects

New in version 3.5.

FrameSummary objects represent a single frame in a traceback.

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

Represent a single frame in the traceback or stack that is being formattedor printed. It may optionally have a stringified version of the frameslocals included in it. Iflookup_line isFalse, the source code is notlooked up until theFrameSummary has thelineattribute accessed (which also happens when casting it to a tuple).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.

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_death()defbright_side_of_death():returntuple()[0]try:lumberjack()exceptIndexError: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 latertraceback.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 laterprint(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:

>>>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']