Source code:
———
This module provides a standard interface to extract, format and print stack traces of Python programs. It is more flexible than the interpreter’s default traceback display, and therefore makes it possible to configure certain aspects of the output. Finally, it contains a utility for capturing enough information about an exception to print it later, without the need to save a reference to the actual exception. Since exceptions can be the roots of large objects graph, this utility can significantly improve memory management.
The module uses
— these are objects of type
, which are assigned to the
field of
instances.
See also
Module
Used to dump Python tracebacks explicitly, on a fault, after a timeout, or on a user signal.
Module
Interactive source code debugger for Python programs.
The module’s API can be divided into two parts:
Module-level functions offering basic functionality, which are useful for interactive inspection of exceptions and tracebacks.
class and its helper classes
and
. These offer both more flexibility in the output generated and the ability to store the information necessary for later formatting without holding references to actual exception and traceback objects.
Module-Level Functions
traceback.print_tb(tb, limit=None, file=None)
Print up to limit stack trace entries from
tb (starting from the caller’s frame) if limit is positive. Otherwise, print the last abs(limit) entries. If limit is omitted or None, all entries are printed. If file is omitted or None, the output goes to
; otherwise it should be an open
or
to receive the output.
Note
The meaning of the limit parameter is different than the meaning of
. A negative limit value corresponds to a positive value of sys.tracebacklimit, whereas the behaviour of a positive limit value cannot be achieved with sys.tracebacklimit.
Changed in version 3.5: Added negative limit support.
traceback.print_exception(exc, /, [value, tb, ]limit=None, file=None, chain=True)
Print exception information and stack trace entries from
tb to file. This differs from
in the following ways:
if tb is not None, it prints a header Traceback(mostrecentcalllast):
it prints the exception type and value after the stack trace
if type(value) is
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.
Since Python 3.10, instead of passing value and tb, an exception object can be passed as the first argument. If value and tb are provided, the first argument is ignored in order to provide backwards compatibility.
The optional limit argument has the same meaning as for
. If chain is true (the default), then chained exceptions (the
or
attributes of the exception) will be printed as well, like the interpreter itself does when printing an unhandled exception.
Changed in version 3.5: The etype argument is ignored and inferred from the type of value.
Changed in version 3.10: The etype parameter has been renamed to exc and is now positional-only.
traceback.print_exc(limit=None, file=None, chain=True)
This is a shorthand for print_exception(sys.exception(),limit=limit,file=file,chain=chain).
traceback.print_last(limit=None, file=None, chain=True)
This is a shorthand for print_exception(sys.last_exc,limit=limit,file=file,chain=chain). In general it will work only after an exception has reached an interactive prompt (see
).
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 or None, all entries are printed. The optional f argument can be used to specify an alternate
to start. The optional file argument has the same meaning as for
.
Changed in version 3.5: Added negative limit support.
Return a
object representing a list of “pre-processed” stack trace entries extracted from the
tb. It is useful for alternate formatting of stack traces. The optional limit argument has the same meaning as for
. A “pre-processed” stack trace entry is a
object with attributes representing the information that is usually printed for a stack trace.
Extract the raw traceback from the current
. The return value has the same format as for
. The optional f and limit arguments have the same meaning as for
.
traceback.print_list(extracted_list, file=None)
Print the list of tuples as returned by
or
as a formatted stack trace to the given file. If file is None, the output is written to
.
traceback.format_list(extracted_list)
Given a list of tuples or
objects as returned by
or
, 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(exc, /, [value, ]*, show_group=False)
Format the exception part of a traceback using an exception value such as given by
. The return value is a list of strings, each ending in a newline. The list contains the exception’s message, which is normally a single string; however, for
exceptions, it contains several lines that (when printed) display detailed information about where the syntax error occurred. Following the message, the list contains the exception’s
.
Since Python 3.10, instead of passing value, an exception object can be passed as the first argument. If value is provided, the first argument is ignored in order to provide backwards compatibility.
When show_group is True, and the exception is an instance of
, the nested exceptions are included as well, recursively, with indentation relative to their nesting depth.
Changed in version 3.10: The etype parameter has been renamed to exc and is now positional-only.
Changed in version 3.11: The returned list now includes any
attached to the exception.
Changed in version 3.13: show_group parameter was added.
traceback.format_exception(exc, /, [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
. 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().
Changed in version 3.5: The etype argument is ignored and inferred from the type of value.
Changed in version 3.10: This function’s behavior and signature were modified to match
.
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
tb by calling the
method of each
.
Added in version 3.4.
traceback.walk_stack(f)
Walk a stack following
from the given frame, yielding the frame and line number for each frame. If f is None, the current stack is used. This helper is used with
.
Added in version 3.5.
Changed in version 3.14: This function previously returned a generator that would walk the stack when first iterated over. The generator returned now is the state of the stack when walk_stack is called.
traceback.walk_tb(tb)
Walk a traceback following
yielding the frame and line number for each frame. This helper is used with
.
Added in version 3.5.
TracebackException Objects
Added in version 3.5.
TracebackException objects are created from actual exceptions to capture data for later printing. They offer a more lightweight method of storing this information by avoiding holding references to
and
objects. In addition, they expose more options to configure the output compared to the module-level functions described above.
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. The meaning of limit, lookup_lines and capture_locals are as for the
class.
If compact is true, only data that is required by TracebackException’s
method is saved in the class attributes. In particular, the
field is calculated only if
is None and
is false.
Note that when locals are captured, they are also shown in the traceback.
max_group_width and max_group_depth control the formatting of exception groups (see
). The depth refers to the nesting level of the group, and the width refers to the size of a single exception group’s exceptions array. The formatted output is truncated when either limit is exceeded.
Changed in version 3.10: Added the compact parameter.
Changed in version 3.11: Added the max_group_width and max_group_depth parameters.
__cause__
A TracebackException of the original
.
__context__
A TracebackException of the original
.
exceptions
If self represents an
, this field holds a list of TracebackException instances representing the nested exceptions. Otherwise it is None.
Added in version 3.11.
__suppress_context__
The
value from the original exception.
__notes__
The
value from the original exception, or None if the exception does not have any notes. If it is not None is it formatted in the traceback after the exception string.
Added in version 3.11.
stack
A
representing the traceback.
exc_type
The class of the original exception.
Deprecated since version 3.13.
exc_type_str
String display of the class of the original exception.
Added in version 3.13.
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 be None if not present.
Added 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 be None if not present.
Added in version 3.10.
msg
For syntax errors - the compiler error message.
classmethodfrom_exception(exc, *, 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 and capture_locals are as for the
class.
Note that when locals are captured, they are also shown in the traceback.
print(*, file=None, chain=True)
Print to file (default sys.stderr) the exception information returned by
.
Added in version 3.11.
format(*, chain=True)
Format the exception.
If chain is not True,
and
will not be formatted.
The return value is a generator of strings, each ending in a newline and some containing internal newlines.
is a wrapper around this method which just prints the lines to a file.
format_exception_only(*, show_group=False)
Format the exception part of the traceback.
The return value is a generator of strings, each ending in a newline.
When show_group is False, the generator emits the exception’s message followed by its notes (if it has any). The exception message is normally a single string; however, for
exceptions, it consists of several lines that (when printed) display detailed information about where the syntax error occurred.
When show_group is True, and the exception is an instance of
, the nested exceptions are included as well, recursively, with indentation relative to their nesting depth.
Changed in version 3.11: The exception’s
are now included in the output.
Changed in version 3.13: Added the show_group parameter.
StackSummary Objects
Added in version 3.5.
StackSummary objects represent a call stack ready for formatting.
classtraceback.StackSummary
Construct a StackSummary object from a frame generator (such as is returned by
or
).
If limit is supplied, only this many frames are taken from frame_gen. If lookup_lines is False, the returned
objects will not have read their lines in yet, making the cost of creating the StackSummary cheaper (which may be valuable if it may not actually get formatted). If capture_locals is True the local variables in each FrameSummary are captured as object representations.
Changed in version 3.12: Exceptions raised from
on a local variable (when capture_locals is True) are no longer propagated to the caller.
classmethodfrom_list(a_list)
Construct a StackSummary object from a supplied list of
objects or old-style list of tuples. Each tuple should be a 4-tuple with filename, lineno, name, line as the elements.
format()
Returns a list of strings ready for printing. Each string in the resulting list corresponds to a single
from the stack. Each string ends in a newline; the strings may contain internal newlines as well, for those items with source text lines.
For long sequences of the same frame and line, the first few repetitions are shown, followed by a summary line stating the exact number 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 the
involved in the stack. This method is called for each
object to be printed by
. If it returns None, the frame is omitted from the output.
Added in version 3.11.
FrameSummary Objects
Added in version 3.5.
A FrameSummary object represents a single
in a
.
classtraceback.FrameSummary(filename, lineno, name, *, lookup_line=True, locals=None, line=None, end_lineno=None, colno=None, end_colno=None)
Represents a single
in the
or stack that is being formatted or printed. It may optionally have a stringified version of the frame’s locals included in it. If lookup_line is False, the source code is not looked up until the FrameSummary has the
attribute accessed (which also happens when casting it to a
). line may be directly provided, and will prevent line lookups happening at all. locals is an optional local variable mapping, and if supplied the variable representations are stored in the summary for later display.
FrameSummary instances have the following attributes:
filename
The filename of the source code for this frame. Equivalent to accessing
on a
f.
lineno
The line number of the source code for this frame.
name
Equivalent to accessing
on a
f.
line
A string representing the source code for this frame, with leading and trailing whitespace stripped. If the source is not available, it is None.
end_lineno
The last line number of the source code for this frame. By default, it is set to lineno and indexation starts from 1.
Changed in version 3.13: The default value changed from None to lineno.
colno
The column number of the source code for this frame. By default, it is None and indexation starts from 0.
end_colno
The last column number of the source code for this frame. By default, it is None and indexation starts from 0.
Examples of Using the Module-Level Functions
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
module.
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 the exception and traceback:
importsys,tracebackdeflumberjack():bright_side_of_life()defbright_side_of_life():returntuple()[0]try:lumberjack()exceptIndexErrorasexc: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 ~~~~~~~~~~^^\n', ' File "<doctest default[0]>", line 4, in lumberjack\n bright_side_of_life()\n ~~~~~~~~~~~~~~~~~~~^^\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 ~~~~~~~~~~^^\n', ' File "<doctest default[0]>", line 4, in lumberjack\n bright_side_of_life()\n ~~~~~~~~~~~~~~~~~~~^^\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>", line 10, in <module>another_function() File "<doctest>", line 3, in another_functionlumberstack() File "<doctest>", line 6, in lumberstacktraceback.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(an_error)['IndexError: tuple index out of range\n']Examples of Using
With the helper class, we have more options:
>>> importsys>>> fromtracebackimportTracebackException>>>>>> deflumberjack():... bright_side_of_life()...>>> defbright_side_of_life():... t="bright","side","of","life"... returnt[5]...>>> try:... lumberjack()... exceptIndexErrorase:... exc=e...>>> try:... try:... lumberjack()... except:... 1/0... exceptExceptionase:... chained_exc=e...>>> # limit works as with the module-level functions>>> TracebackException.from_exception(exc,limit=-2).print()Traceback (most recent call last): File "<python-input-1>", line 6, in lumberjackbright_side_of_life()~~~~~~~~~~~~~~~~~~~^^ File "<python-input-1>", line 10, in bright_side_of_lifereturnt[5]~^^^IndexError: tuple index out of range>>> # capture_locals adds local variables in frames>>> TracebackException.from_exception(exc,limit=-2,capture_locals=True).print()Traceback (most recent call last): File "<python-input-1>", line 6, in lumberjackbright_side_of_life()~~~~~~~~~~~~~~~~~~~^^ File "<python-input-1>", line 10, in bright_side_of_lifereturnt[5]~^^^t=("bright","side","of","life")IndexError: tuple index out of range>>> # The *chain* kwarg to print() controls whether chained>>> # exceptions are displayed>>> TracebackException.from_exception(chained_exc).print()Traceback (most recent call last): File "<python-input-19>", line 4, in <module>lumberjack()~~~~~~~~~~^^ File "<python-input-8>", line 7, in lumberjackbright_side_of_life()~~~~~~~~~~~~~~~~~~~^^ File "<python-input-8>", line 11, in bright_side_of_lifereturnt[5]~^^^IndexError: tuple index out of rangeDuring handling of the above exception, another exception occurred:Traceback (most recent call last): File "<python-input-19>", line 6, in <module>1/0~^~ZeroDivisionError: division by zero>>> TracebackException.from_exception(chained_exc).print(chain=False)Traceback (most recent call last): File "<python-input-19>", line 6, in <module>1/0~^~ZeroDivisionError: division by zero