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traceback.py
"""Extract, format and print information about Python stack traces.""" import collections import itertools import linecache import sys __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', 'walk_stack', 'walk_tb'] # # 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.""" if file is None: file = sys.stderr 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 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. """ 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() method. """ 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. # _cause_message = ( "\nThe above exception was the direct cause " "of the following exception:\n\n") _context_message = ( "\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 position of the error. """ # 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). if file is None: file = sys.stderr 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 error occurred. The message indicating which exception occurred is always the last string in the list. """ 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\n" % etype else: line = "%s: %s\n" % (etype, valuestr) return line def _some_str(value): try: return str(value) except: 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, limit, file, chain) # # 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(). """ if f is None: 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))'.""" if f is None: 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. """ if f is None: f = sys._getframe().f_back stack = StackSummary.extract(walk_stack(f), limit=limit) stack.reverse() return stack def clear_frames(tb): "Clear all references to local variables in the frames of a traceback." while tb is not None: try: tb.tb_frame.clear() except RuntimeError: # Ignore the exception raised if the frame is still executing. pass tb = tb.tb_next class FrameSummary: """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, locals=None, line=None): """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 object representations. :param line: If provided, use this instead of looking up the line in the linecache. """ self.filename = filename self.lineno = lineno self.name = name self._line = line if lookup_line: self.line self.locals = {k: repr(v) for k, v in locals.items()} if locals else None def __eq__(self, other): 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 return NotImplemented def __getitem__(self, pos): return (self.filename, self.lineno, self.name, self.line)[pos] def __iter__(self): return iter([self.filename, self.lineno, self.name, self.line]) def __repr__(self): return "<FrameSummary file {filename}, line {lineno} in {name}>".format( filename=self.filename, lineno=self.lineno, name=self.name) def __len__(self): return 4 @property def line(self): if self._line is None: self._line = linecache.getline(self.filename, self.lineno).strip() return self._line def walk_stack(f): """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. """ if f is None: f = sys._getframe().f_back.f_back while f is not None: yield f, f.f_lineno f = f.f_back def walk_tb(tb): """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. """ while tb is not None: yield tb.tb_frame, tb.tb_lineno tb = tb.tb_next _RECURSIVE_CUTOFF = 3 # Also hardcoded in traceback.c. class StackSummary(list): """A stack of frames.""" @classmethod def extract(klass, frame_gen, *, limit=None, lookup_lines=True, capture_locals=False): """Create a StackSummary from a traceback or stack object. :param frame_gen: A generator that yields (frame, lineno) tuples to include in the stack. :param limit: None to include all frames or the number of frames to include. :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. """ if limit is None: limit = getattr(sys, 'tracebacklimit', None) if limit is not None and limit < 0: limit = 0 if limit is not None: if limit >= 0: frame_gen = itertools.islice(frame_gen, limit) else: frame_gen = collections.deque(frame_gen, maxlen=-limit) result = klass() fnames = set() for f, lineno in frame_gen: co = f.f_code filename = co.co_filename name = co.co_name fnames.add(filename) linecache.lazycache(filename, f.f_globals) # Must defer line lookups until we have called checkcache. if capture_locals: f_locals = f.f_locals else: f_locals = None result.append(FrameSummary( filename, lineno, name, lookup_line=False, locals=f_locals)) for filename in fnames: linecache.checkcache(filename) # If immediate lookup was desired, trigger lookups now. if lookup_lines: for f in result: f.line return result @classmethod 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. result = StackSummary() for frame in a_list: if isinstance(frame, FrameSummary): result.append(frame) else: filename, lineno, name, line = frame result.append(FrameSummary(filename, lineno, name, line=line)) return result def format(self): """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. """ result = [] last_file = None last_line = None last_name = None count = 0 for frame in self: 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 result.append( f' [Previous line repeated {count} more ' f'time{"s" if count > 1 else ""}]\n' ) last_file = frame.filename last_line = frame.lineno last_name = frame.name count = 0 count += 1 if count > _RECURSIVE_CUTOFF: continue row = [] row.append(' File "{}", line {}, in {}\n'.format( frame.filename, frame.lineno, frame.name)) if frame.line: row.append(' {}\n'.format(frame.line.strip())) if frame.locals: 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 result.append( f' [Previous line repeated {count} more ' f'time{"s" if count > 1 else ""}]\n' ) return result 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 individual components. - :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 original exception. - :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 occurred. - :attr:`lineno` For syntax errors - the linenumber where the error occurred. - :attr:`text` For syntax errors - the text where the error occurred. - :attr:`offset` For syntax errors - the offset into the text where the error occurred. - :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__. if _seen is None: _seen = set() _seen.add(id(exc_value)) # Gracefully handle (the way Python 2.4 and earlier did) the case of # being called with no type or value (None, None, None). self._truncated = False try: 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__, exc_value.__cause__.__traceback__, limit=limit, lookup_lines=False, capture_locals=capture_locals, _seen=_seen) else: cause = None 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__, exc_value.__context__.__traceback__, limit=limit, lookup_lines=False, capture_locals=capture_locals, _seen=_seen) else: context = None except RecursionError: # The recursive call to the constructors above # may result in a stack overflow for long exception chains, # so we must truncate. self._truncated = True cause = None context = None self.__cause__ = cause self.__context__ = context self.__suppress_context__ = \ exc_value.__suppress_context__ if exc_value else False # TODO: locals. self.stack = StackSummary.extract( walk_tb(exc_traceback), limit=limit, lookup_lines=lookup_lines, capture_locals=capture_locals) self.exc_type = exc_type # Capture now to permit freeing resources: only complication is in the # unofficial API _format_final_exc_line self._str = _some_str(exc_value) if exc_type and issubclass(exc_type, SyntaxError): # Handle SyntaxError's specially self.filename = exc_value.filename lno = exc_value.lineno self.lineno = str(lno) if lno is not None else None self.text = exc_value.text self.offset = exc_value.offset self.msg = exc_value.msg if lookup_lines: self._load_lines() @classmethod def from_exception(cls, exc, *args, **kwargs): """Create a TracebackException from an exception.""" return cls(type(exc), exc, exc.__traceback__, *args, **kwargs) def _load_lines(self): """Private API. force all lines in the stack to be loaded.""" for frame in self.stack: frame.line if self.__context__: self.__context__._load_lines() if self.__cause__: self.__cause__._load_lines() def __eq__(self, other): if isinstance(other, TracebackException): return self.__dict__ == other.__dict__ return NotImplemented def __str__(self): return self._str def format_exception_only(self): """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. """ if self.exc_type is None: yield _format_final_exc_line(None, self._str) return stype = self.exc_type.__qualname__ smod = self.exc_type.__module__ if smod not in ("__main__", "builtins"): if not isinstance(smod, str): smod = "<unknown>" stype = smod + '.' + stype if not issubclass(self.exc_type, SyntaxError): yield _format_final_exc_line(stype, self._str) else: yield from self._format_syntax_error(stype) def _format_syntax_error(self, stype): """Format SyntaxError exceptions (internal helper).""" # Show exactly where the problem was found. filename_suffix = '' if self.lineno is not None: yield ' File "{}", line {}\n'.format( self.filename or "<string>", self.lineno) elif self.filename is not None: filename_suffix = ' ({})'.format(self.filename) text = self.text if text is not None: # text = " foo\n" # rtext = " foo" # ltext = "foo" rtext = text.rstrip('\n') ltext = rtext.lstrip(' \n\f') spaces = len(rtext) - len(ltext) yield ' {}\n'.format(ltext) # Convert 1-based column offset to 0-based index into stripped text caret = (self.offset or 0) - 1 - spaces if caret >= 0: # non-space whitespace (likes tabs) must be kept for alignment caretspace = ((c if c.isspace() else ' ') for c in ltext[:caret]) yield ' {}^\n'.format(''.join(caretspace)) msg = self.msg or "<no detail available>" yield "{}: {}{}\n".format(stype, msg, filename_suffix) def format(self, *, 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. """ if chain: if self.__cause__ is not None: yield from self.__cause__.format(chain=chain) yield _cause_message elif (self.__context__ is not None and not self.__suppress_context__): yield from self.__context__.format(chain=chain) yield _context_message if self._truncated: yield ( 'Chained exceptions have been truncated to avoid ' 'stack overflow in traceback formatting:\n') if self.stack: yield 'Traceback (most recent call last):\n' yield from self.stack.format() yield from self.format_exception_only()
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