"""Extract, format and print information about Python stack traces."""
__all__ = ['extract_stack', 'extract_tb', 'format_exception',
'format_exception_only', 'format_list', 'format_stack',
'format_tb', 'print_exc', 'format_exc', 'print_exception',
'print_last', 'print_stack', 'print_tb', 'clear_frames',
'FrameSummary', 'StackSummary', 'TracebackException',
# Formatting and printing lists of traceback lines.
def print_list(extracted_list, file=None):
"""Print the list of tuples as returned by extract_tb() or
extract_stack() as a formatted stack trace to the given file."""
for item in StackSummary.from_list(extracted_list).format():
print(item, file=file, end="")
def format_list(extracted_list):
"""Format a list of tuples or FrameSummary objects for printing.
Given a list of tuples or FrameSummary objects as returned by
extract_tb() or extract_stack(), return a list of strings ready
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.
return StackSummary.from_list(extracted_list).format()
# Printing and Extracting Tracebacks.
def print_tb(tb, limit=None, file=None):
"""Print up to 'limit' stack trace entries from the traceback 'tb'.
If 'limit' is omitted or None, all entries are printed. If 'file'
is omitted or None, the output goes to sys.stderr; otherwise
'file' should be an open file or file-like object with a write()
print_list(extract_tb(tb, limit=limit), file=file)
def format_tb(tb, limit=None):
"""A shorthand for 'format_list(extract_tb(tb, limit))'."""
return extract_tb(tb, limit=limit).format()
def extract_tb(tb, limit=None):
Return a StackSummary object representing a list of
pre-processed entries from traceback.
This 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 FrameSummary object
containing attributes filename, lineno, name, and line
representing the information that is usually printed for a stack
trace. The line is a string with leading and trailing
whitespace stripped; if the source is not available it is None.
return StackSummary.extract(walk_tb(tb), limit=limit)
# Exception formatting and output.
"\nThe above exception was the direct cause "
"of the following exception:\n\n")
"\nDuring handling of the above exception, "
"another exception occurred:\n\n")
def print_exception(etype, value, tb, limit=None, file=None, chain=True):
"""Print exception up to 'limit' stack trace entries from 'tb' to 'file'.
This differs from print_tb() in the following ways: (1) if
traceback is not None, it prints a header "Traceback (most recent
call last):"; (2) it prints the exception type and value after the
stack trace; (3) if type is SyntaxError and value has the
appropriate format, it prints the line where the syntax error
occurred with a caret on the next line indicating the approximate
# format_exception has ignored etype for some time, and code such as cgitb
# passes in bogus values as a result. For compatibility with such code we
# ignore it here (rather than in the new TracebackException API).
for line in TracebackException(
type(value), value, tb, limit=limit).format(chain=chain):
print(line, file=file, end="")
def 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 does print_exception().
# format_exception has ignored etype for some time, and code such as cgitb
# passes in bogus values as a result. For compatibility with such code we
# ignore it here (rather than in the new TracebackException API).
return list(TracebackException(
type(value), value, tb, limit=limit).format(chain=chain))
def 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 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
The message indicating which exception occurred is always the last
return list(TracebackException(etype, value, None).format_exception_only())
# -- not official API but folk probably use these two functions.
def _format_final_exc_line(etype, value):
valuestr = _some_str(value)
if value is None or not valuestr:
line = "%s: %s\n" % (etype, valuestr)
return '<unprintable %s object>' % type(value).__name__
def print_exc(limit=None, file=None, chain=True):
"""Shorthand for 'print_exception(*sys.exc_info(), limit, file)'."""
print_exception(*sys.exc_info(), limit=limit, file=file, chain=chain)
def format_exc(limit=None, chain=True):
"""Like print_exc() but return a string."""
return "".join(format_exception(*sys.exc_info(), limit=limit, chain=chain))
def 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)'."""
if not hasattr(sys, "last_type"):
raise ValueError("no last exception")
print_exception(sys.last_type, sys.last_value, sys.last_traceback,
# Printing and Extracting Stacks.
def print_stack(f=None, limit=None, file=None):
"""Print a stack trace from its invocation point.
The optional 'f' argument can be used to specify an alternate
stack frame at which to start. The optional 'limit' and 'file'
arguments have the same meaning as for print_exception().
f = sys._getframe().f_back
print_list(extract_stack(f, limit=limit), file=file)
def format_stack(f=None, limit=None):
"""Shorthand for 'format_list(extract_stack(f, limit))'."""
f = sys._getframe().f_back
return format_list(extract_stack(f, limit=limit))
def 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(). Each item in the list is a quadruple (filename,
line number, function name, text), and the entries are in order
from oldest to newest stack frame.
f = sys._getframe().f_back
stack = StackSummary.extract(walk_stack(f), limit=limit)
"Clear all references to local variables in the frames of a traceback."
# Ignore the exception raised if the frame is still executing.
"""A single frame from a traceback.
- :attr:`filename` The filename for the frame.
- :attr:`lineno` The line within filename for the frame that was
active when the frame was captured.
- :attr:`name` The name of the function or method that was executing
when the frame was captured.
- :attr:`line` The text from the linecache module for the
of code that was running when the frame was captured.
- :attr:`locals` Either None if locals were not supplied, or a dict
mapping the name to the repr() of the variable.
__slots__ = ('filename', 'lineno', 'name', '_line', 'locals')
def __init__(self, filename, lineno, name, *, lookup_line=True,
"""Construct a FrameSummary.
:param lookup_line: If True, `linecache` is consulted for the source
code line. Otherwise, the line will be looked up when first needed.
:param locals: If supplied the frame locals, which will be captured as
:param line: If provided, use this instead of looking up the line in
self.locals = {k: repr(v) for k, v in locals.items()} if locals else None
if isinstance(other, FrameSummary):
return (self.filename == other.filename and
self.lineno == other.lineno and
self.name == other.name and
self.locals == other.locals)
if isinstance(other, tuple):
return (self.filename, self.lineno, self.name, self.line) == other
def __getitem__(self, pos):
return (self.filename, self.lineno, self.name, self.line)[pos]
return iter([self.filename, self.lineno, self.name, self.line])
return "<FrameSummary file {filename}, line {lineno} in {name}>".format(
filename=self.filename, lineno=self.lineno, name=self.name)
self._line = linecache.getline(self.filename, self.lineno).strip()
"""Walk a stack yielding the frame and line number for each frame.
This will follow f.f_back from the given frame. If no frame is given, the
current stack is used. Usually used with StackSummary.extract.
f = sys._getframe().f_back.f_back
"""Walk a traceback yielding the frame and line number for each frame.
This will follow tb.tb_next (and thus is in the opposite order to
walk_stack). Usually used with StackSummary.extract.
yield tb.tb_frame, tb.tb_lineno
_RECURSIVE_CUTOFF = 3 # Also hardcoded in traceback.c.
class StackSummary(list):
def extract(klass, frame_gen, *, limit=None, lookup_lines=True,
"""Create a StackSummary from a traceback or stack object.
:param frame_gen: A generator that yields (frame, lineno) tuples to
:param limit: None to include all frames or the number of frames to
:param lookup_lines: If True, lookup lines for each frame immediately,
otherwise lookup is deferred until the frame is rendered.
:param capture_locals: If True, the local variables from each frame will
be captured as object representations into the FrameSummary.
limit = getattr(sys, 'tracebacklimit', None)
if limit is not None and limit < 0:
frame_gen = itertools.islice(frame_gen, limit)
frame_gen = collections.deque(frame_gen, maxlen=-limit)
for f, lineno in frame_gen:
filename = co.co_filename
linecache.lazycache(filename, f.f_globals)
# Must defer line lookups until we have called checkcache.
result.append(FrameSummary(
filename, lineno, name, lookup_line=False, locals=f_locals))
linecache.checkcache(filename)
# If immediate lookup was desired, trigger lookups now.
def from_list(klass, a_list):
Create a StackSummary object from a supplied list of
FrameSummary objects or old-style list of tuples.
# While doing a fast-path check for isinstance(a_list, StackSummary) is
# appealing, idlelib.run.cleanup_traceback and other similar code may
# break this by making arbitrary frames plain tuples, so we need to
# check on a frame by frame basis.
if isinstance(frame, FrameSummary):
filename, lineno, name, line = frame
result.append(FrameSummary(filename, lineno, name, line=line))
"""Format the stack ready for printing.
Returns a list of strings ready for printing. Each string in the
resulting list corresponds to a single frame 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.
if (last_file is None or last_file != frame.filename or
last_line is None or last_line != frame.lineno or
last_name is None or last_name != frame.name):
if count > _RECURSIVE_CUTOFF:
count -= _RECURSIVE_CUTOFF
f' [Previous line repeated {count} more '
f'time{"s" if count > 1 else ""}]\n'
last_file = frame.filename
if count > _RECURSIVE_CUTOFF:
row.append(' File "{}", line {}, in {}\n'.format(
frame.filename, frame.lineno, frame.name))
row.append(' {}\n'.format(frame.line.strip()))
for name, value in sorted(frame.locals.items()):
row.append(' {name} = {value}\n'.format(name=name, value=value))
result.append(''.join(row))
if count > _RECURSIVE_CUTOFF:
count -= _RECURSIVE_CUTOFF
f' [Previous line repeated {count} more '
f'time{"s" if count > 1 else ""}]\n'
class TracebackException:
"""An exception ready for rendering.
The traceback module captures enough attributes from the original exception
to this intermediary form to ensure that no references are held, while
still being able to fully print or format it.
Use `from_exception` to create TracebackException instances from exception
objects, or the constructor to create TracebackException instances from
- :attr:`__cause__` A TracebackException of the original *__cause__*.
- :attr:`__context__` A TracebackException of the original *__context__*.
- :attr:`__suppress_context__` The *__suppress_context__* value from the
- :attr:`stack` A `StackSummary` representing the traceback.
- :attr:`exc_type` The class of the original traceback.
- :attr:`filename` For syntax errors - the filename where the error
- :attr:`lineno` For syntax errors - the linenumber where the error
- :attr:`text` For syntax errors - the text where the error
- :attr:`offset` For syntax errors - the offset into the text where the
- :attr:`msg` For syntax errors - the compiler error message.
def __init__(self, exc_type, exc_value, exc_traceback, *, limit=None,
lookup_lines=True, capture_locals=False, _seen=None):
# NB: we need to accept exc_traceback, exc_value, exc_traceback to
# permit backwards compat with the existing API, otherwise we
# need stub thunk objects just to glue it together.
# Handle loops in __cause__ or __context__.
# Gracefully handle (the way Python 2.4 and earlier did) the case of
# being called with no type or value (None, None, None).
if (exc_value and exc_value.__cause__ is not None
and id(exc_value.__cause__) not in _seen):
cause = TracebackException(
type(exc_value.__cause__),
exc_value.__cause__.__traceback__,
capture_locals=capture_locals,
if (exc_value and exc_value.__context__ is not None
and id(exc_value.__context__) not in _seen):
context = TracebackException(
type(exc_value.__context__),
exc_value.__context__.__traceback__,
capture_locals=capture_locals,
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