traceback
— 打印或检索堆栈回溯
¶
源代码： Lib/traceback.py
This module provides a standard interface to extract, format and print stack traces of Python programs. It exactly mimics the behavior of the Python interpreter when it prints a stack trace. This is useful when you want to print stack traces under program control, such as in a “wrapper” around the interpreter.
The module uses traceback objects — this is the object type that is stored in the
sys.last_traceback
变量，且作为第 3 项返回，来自
sys.exc_info()
.
模块定义了下列函数：
traceback.
print_tb
(
tb
,
limit=None
,
file=None
)
¶
Print up to
limit
stack trace entries from traceback object
tb
(starting from the caller’s frame) if
limit
is positive. Otherwise, print the last
abs(limit)
entries. If
limit
被省略或
None
, all entries are printed. If
file
被省略或
None
, the output goes to
sys.stderr
; otherwise it should be an open file or file-like object to receive the output.
3.5 版改变： Added negative limit 支持。
traceback.
print_exception
(
etype
,
value
,
tb
,
limit=None
,
file=None
,
chain=True
)
¶
Print exception information and stack trace entries from traceback object
tb
to
file
. This differs from
print_tb()
in the following ways:
None
, it prints a header
Traceback
(most
recent
call
last):
SyntaxError
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.
可选
limit
argument has the same meaning as for
print_tb()
。若
chain
is true (the default), then chained exceptions (the
__cause__
or
__context__
attributes of the exception) will be printed as well, like the interpreter itself does when printing an unhandled exception.
3.5 版改变： etype argument is ignored and inferred from the type of value .
traceback.
print_exc
(
limit=None
,
file=None
,
chain=True
)
¶
这是简写的
print_exception(*sys.exc_info(),
limit,
file,
chain)
.
traceback.
print_last
(
limit=None
,
file=None
,
chain=True
)
¶
这是简写的
print_exception(sys.last_type,
sys.last_value,
sys.last_traceback,
limit,
file,
chain)
. In general it will work only after an exception has reached an interactive prompt (see
sys.last_type
).
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
被省略或
None
, all entries are printed. The optional
f
argument can be used to specify an alternate stack frame to start. The optional
file
argument has the same meaning as for
print_tb()
.
3.5 版改变： Added negative limit 支持。
traceback.
extract_tb
(
tb
,
limit=None
)
¶
返回
StackSummary
object representing a list of “pre-processed” stack trace entries extracted from the traceback object
tb
. It is useful for alternate formatting of stack traces. The optional
limit
argument has the same meaning as for
print_tb()
. A “pre-processed” stack trace entry is a
FrameSummary
object containing attributes
filename
,
lineno
,
name
，和
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
.
traceback.
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()
.
traceback.
format_list
(
extracted_list
)
¶
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
.
traceback.
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 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 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()
.
3.5 版改变： etype argument is ignored and inferred from the type of value .
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 traceback
tb
by calling the
clear()
method of each frame object.
3.4 版新增。
traceback.
walk_stack
(
f
)
¶
Walk a stack following
f.f_back
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
StackSummary.extract()
.
3.5 版新增。
traceback.
walk_tb
(
tb
)
¶
Walk a traceback following
tb_next
yielding the frame and line number for each frame. This helper is used with
StackSummary.extract()
.
3.5 版新增。
The module also defines the following classes:
TracebackException
对象
¶
3.5 版新增。
TracebackException
objects are created from actual exceptions to capture data for later printing in a lightweight fashion.
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
and
capture_locals
are as for the
StackSummary
类。
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__
¶
__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.
from_exception
(
exc
,
*
,
limit=None
,
lookup_lines=True
,
capture_locals=False
)
¶
Capture an exception for later rendering.
limit
,
lookup_lines
and
capture_locals
are as for the
StackSummary
类。
Note that when locals are captured, they are also shown in the traceback.
format
(
*
,
chain=True
)
¶
Format the exception.
若
chain
不是
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.
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, 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.
StackSummary
对象
¶
3.5 版新增。
StackSummary
objects represent a call stack ready for formatting.
traceback.
StackSummary
¶
extract
(
frame_gen
,
*
,
limit=None
,
lookup_lines=True
,
capture_locals=False
)
¶
构造
StackSummary
object from a frame generator (such as is returned by
walk_stack()
or
walk_tb()
).
若
limit
is supplied, only this many frames are taken from
frame_gen
。若
lookup_lines
is
False
, the returned
FrameSummary
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.
from_list
(
a_list
)
¶
构造
StackSummary
object from a supplied list of
FrameSummary
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 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.
3.6 版改变： Long sequences of repeated frames are now abbreviated.
FrameSummary
对象
¶
3.5 版新增。
FrameSummary
objects represent a single frame in a traceback.
traceback.
FrameSummary
(
filename
,
lineno
,
name
,
lookup_line=True
,
locals=None
,
line=None
)
¶
Represent a single frame in the traceback or stack that is being formatted or printed. It may optionally have a stringified version of the frames locals included in it. If
lookup_line
is
False
, the source code is not looked up until the
FrameSummary
拥有
line
attribute accessed (which also happens when casting it to a tuple).
line
may be directly provided, and will prevent line lookups happening at all.
locals
is an optional local variable dictionary, and if supplied the variable representations are stored in the summary for later display.
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
code
模块。
import sys, traceback
def run_user_code(envdir):
source = input(">>> ")
try:
exec(source, envdir)
except Exception:
print("Exception in user code:")
print("-"*60)
traceback.print_exc(file=sys.stdout)
print("-"*60)
envdir = {}
while True:
run_user_code(envdir)
The following example demonstrates the different ways to print and format the exception and traceback:
import sys, traceback
def lumberjack():
bright_side_of_death()
def bright_side_of_death():
return tuple()[0]
try:
lumberjack()
except IndexError:
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 later
traceback.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 later
print(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:
>>> import traceback
>>> def another_function():
... lumberstack()
...
>>> def lumberstack():
... 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_function
lumberstack()
File "<doctest>", line 6, in lumberstack
traceback.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:
>>> import traceback
>>> 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']