traceback —- 打印或检索堆栈回溯

源代码：Lib/traceback.py

该模块提供了一个标准接口来提取、格式化和打印 Python 程序的堆栈跟踪结果。它完全模仿Python 解释器在打印堆栈跟踪结果时的行为。当您想要在程序控制下打印堆栈跟踪结果时，例如在“封装”解释器时，这是非常有用的。

这个模块使用 traceback 对象 —— 这是存储在 sys.last_traceback 中的对象类型变量，并作为 sys.exc_info() 的第三项被返回。

这个模块定义了以下函数：

  • traceback.printtb(_tb, limit=None, file=None)
  • Print up to limit stack trace entries from traceback object tb (startingfrom the caller's frame) if limit is positive. Otherwise, print the lastabs(limit) entries. If limit is omitted or None, all entries areprinted. If file is omitted or None, the output goes tosys.stderr; otherwise it should be an open file or file-like object toreceive the output.

在 3.5 版更改: Added negative limit support.

  • traceback.printexception(_etype, value, tb, limit=None, file=None, chain=True)

  • 打印回溯对象 tb 到 file 的异常信息和整个堆栈回溯。这和 print_tb() 比有以下方面不同：

    • 如果 tb 不为 None，它将打印头部 Traceback (most recent call last):

    • it prints the exception etype and value after the stack trace

    • if type(value) is SyntaxError and value has the appropriateformat, it prints the line where the syntax error occurred with a caretindicating the approximate position of the error.

The optional limit argument has the same meaning as for print_tb().If chain is true (the default), then chained exceptions (thecause or context attributes of the exception) will beprinted as well, like the interpreter itself does when printing an unhandledexception.

在 3.5 版更改: The etype argument is ignored and inferred from the type of value.

  • traceback.printexc(_limit=None, file=None, chain=True)

  • This is a shorthand for print_exception(*sys.exc_info(), limit, file,chain).

  • traceback.printlast(_limit=None, file=None, chain=True)

  • This is a shorthand for print_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.printstack(_f=None, limit=None, file=None)

  • Print up to limit stack trace entries (starting from the invocationpoint) 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 stack frameto start. The optional file argument has the same meaning as forprint_tb().

在 3.5 版更改: Added negative limit support.

  • traceback.extracttb(_tb, limit=None)

  • Return a StackSummary object representing a list of "pre-processed"stack trace entries extracted from the traceback object tb. It is usefulfor alternate formatting of stack traces. The optional limit argument hasthe same meaning as for print_tb(). A "pre-processed" stack traceentry is a FrameSummary object containing attributesfilename, lineno,name, and line 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 is None.

  • traceback.extractstack(_f=None, limit=None)

  • Extract the raw traceback from the current stack frame. The return value hasthe same format as for extract_tb(). The optional f and _limit_arguments have the same meaning as for print_stack().

  • traceback.formatlist(_extracted_list)

  • Given a list of tuples or FrameSummary objects as returned byextract_tb() or extract_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 not None.

  • traceback.formatexception_only(_etype, value)

  • Format the exception part of a traceback. The arguments are the exceptiontype 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 SyntaxErrorexceptions, 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.formatexception(_etype, value, tb, limit=None, chain=True)

  • Format a stack trace and the exception information. The arguments have thesame meaning as the corresponding arguments to print_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 does print_exception().

在 3.5 版更改: The etype argument is ignored and inferred from the type of value.

  • traceback.formatexc(_limit=None, chain=True)

  • This is like print_exc(limit) but returns a string instead of printing toa file.

  • traceback.formattb(_tb, limit=None)

  • A shorthand for format_list(extract_tb(tb, limit)).

  • traceback.formatstack(_f=None, limit=None)

  • A shorthand for format_list(extract_stack(f, limit)).

  • traceback.clearframes(_tb)

  • Clears the local variables of all the stack frames in a traceback _tb_by calling the clear() method of each frame object.

3.4 新版功能.

  • traceback.walkstack(_f)
  • Walk a stack following f.fback from the given frame, yielding the frameand line number for each frame. If _f is None, the current stack isused. This helper is used with StackSummary.extract().

3.5 新版功能.

  • traceback.walktb(_tb)
  • Walk a traceback following tb_next yielding the frame and line numberfor each frame. This helper is used with StackSummary.extract().

3.5 新版功能.

The module also defines the following classes:

TracebackException Objects

3.5 新版功能.

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

  • class traceback.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 the StackSummary class.

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

  • cause

  • A TracebackException of the original cause.

  • context

  • A TracebackException of the original context.

  • suppress_context

  • The suppress_context value from the original exception.

  • stack

  • A StackSummary 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.

  • classmethod fromexception(_exc, *, limit=None, lookup_lines=True, capture_locals=False)

  • Capture an exception for later rendering. limit, lookup_lines andcapture_locals are as for the StackSummary class.

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

  • format(*, chain=True)
  • Format the exception.

If chain is not True, 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

3.5 新版功能.

StackSummary objects represent a call stack ready for formatting.

  • class traceback.StackSummary
    • classmethod extract(frame_gen, *, limit=None, lookup_lines=True, capture_locals=False)
    • Construct a StackSummary object from a frame generator (such asis returned by walk_stack() orwalk_tb()).

If limit is supplied, only this many frames are taken from frame_gen.If lookup_lines is False, the returned FrameSummaryobjects will not have read their lines in yet, making the cost ofcreating the StackSummary cheaper (which may be valuable if itmay not actually get formatted). If capture_locals is True thelocal variables in each FrameSummary are captured as objectrepresentations.

  • classmethod fromlist(_a_list)

  • Construct a StackSummary 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.

在 3.6 版更改: Long sequences of repeated frames are now abbreviated.

FrameSummary Objects

3.5 新版功能.

FrameSummary objects represent a single frame in a traceback.

  • class traceback.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. If lookup_line is False, the source code is notlooked up until the FrameSummary has the lineattribute 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 the codemodule.

  1. import sys, traceback
  2.  
  3. def run_user_code(envdir):
  4.     source = input(">>> ")
  5.     try:
  6.         exec(source, envdir)
  7.     except Exception:
  8.         print("Exception in user code:")
  9.         print("-"*60)
  10.         traceback.print_exc(file=sys.stdout)
  11.         print("-"*60)
  12.  
  13. envdir = {}
  14. while True:
  15.     run_user_code(envdir)

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

  1. import sys, traceback
  2.  
  3. def lumberjack():
  4.     bright_side_of_death()
  5.  
  6. def bright_side_of_death():
  7.     return tuple()[0]
  8.  
  9. try:
  10.     lumberjack()
  11. except IndexError:
  12.     exc_type, exc_value, exc_traceback = sys.exc_info()
  13.     print("*** print_tb:")
  14.     traceback.print_tb(exc_traceback, limit=1, file=sys.stdout)
  15.     print("*** print_exception:")
  16.     # exc_type below is ignored on 3.5 and later
  17.     traceback.print_exception(exc_type, exc_value, exc_traceback,
  18.                               limit=2, file=sys.stdout)
  19.     print("*** print_exc:")
  20.     traceback.print_exc(limit=2, file=sys.stdout)
  21.     print("*** format_exc, first and last line:")
  22.     formatted_lines = traceback.format_exc().splitlines()
  23.     print(formatted_lines[0])
  24.     print(formatted_lines[-1])
  25.     print("*** format_exception:")
  26.     # exc_type below is ignored on 3.5 and later
  27.     print(repr(traceback.format_exception(exc_type, exc_value,
  28.                                           exc_traceback)))
  29.     print("*** extract_tb:")
  30.     print(repr(traceback.extract_tb(exc_traceback)))
  31.     print("*** format_tb:")
  32.     print(repr(traceback.format_tb(exc_traceback)))
  33.     print("*** tb_lineno:", exc_traceback.tb_lineno)

The output for the example would look similar to this:

  1. *** print_tb:
  2.   File "<doctest...>", line 10, in <module>
  3.     lumberjack()
  4. *** print_exception:
  5. Traceback (most recent call last):
  6.   File "<doctest...>", line 10, in <module>
  7.     lumberjack()
  8.   File "<doctest...>", line 4, in lumberjack
  9.     bright_side_of_death()
  10. IndexError: tuple index out of range
  11. *** print_exc:
  12. Traceback (most recent call last):
  13.   File "<doctest...>", line 10, in <module>
  14.     lumberjack()
  15.   File "<doctest...>", line 4, in lumberjack
  16.     bright_side_of_death()
  17. IndexError: tuple index out of range
  18. *** format_exc, first and last line:
  19. Traceback (most recent call last):
  20. IndexError: tuple index out of range
  21. *** format_exception:
  22. ['Traceback (most recent call last):\n',
  23.  '  File "<doctest...>", line 10, in <module>\n    lumberjack()\n',
  24.  '  File "<doctest...>", line 4, in lumberjack\n    bright_side_of_death()\n',
  25.  '  File "<doctest...>", line 7, in bright_side_of_death\n    return tuple()[0]\n',
  26.  'IndexError: tuple index out of range\n']
  27. *** extract_tb:
  28. [<FrameSummary file <doctest...>, line 10 in <module>>,
  29.  <FrameSummary file <doctest...>, line 4 in lumberjack>,
  30.  <FrameSummary file <doctest...>, line 7 in bright_side_of_death>]
  31. *** format_tb:
  32. ['  File "<doctest...>", line 10, in <module>\n    lumberjack()\n',
  33.  '  File "<doctest...>", line 4, in lumberjack\n    bright_side_of_death()\n',
  34.  '  File "<doctest...>", line 7, in bright_side_of_death\n    return tuple()[0]\n']
  35. *** tb_lineno: 10

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

  1. >>> import traceback
  2. >>> def another_function():
  3. ...     lumberstack()
  4. ...
  5. >>> def lumberstack():
  6. ...     traceback.print_stack()
  7. ...     print(repr(traceback.extract_stack()))
  8. ...     print(repr(traceback.format_stack()))
  9. ...
  10. >>> another_function()
  11.   File "<doctest>", line 10, in <module>
  12.     another_function()
  13.   File "<doctest>", line 3, in another_function
  14.     lumberstack()
  15.   File "<doctest>", line 6, in lumberstack
  16.     traceback.print_stack()
  17. [('<doctest>', 10, '<module>', 'another_function()'),
  18.  ('<doctest>', 3, 'another_function', 'lumberstack()'),
  19.  ('<doctest>', 7, 'lumberstack', 'print(repr(traceback.extract_stack()))')]
  20. ['  File "<doctest>", line 10, in <module>\n    another_function()\n',
  21.  '  File "<doctest>", line 3, in another_function\n    lumberstack()\n',
  22.  '  File "<doctest>", line 8, in lumberstack\n    print(repr(traceback.format_stack()))\n']

This last example demonstrates the final few formatting functions:

  1. >>> import traceback
  2. >>> traceback.format_list([('spam.py', 3, '<module>', 'spam.eggs()'),
  3. ...                        ('eggs.py', 42, 'eggs', 'return "bacon"')])
  4. ['  File "spam.py", line 3, in <module>\n    spam.eggs()\n',
  5.  '  File "eggs.py", line 42, in eggs\n    return "bacon"\n']
  6. >>> an_error = IndexError('tuple index out of range')
  7. >>> traceback.format_exception_only(type(an_error), an_error)
  8. ['IndexError: tuple index out of range\n']