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
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_life()
def bright_side_of_life():
return tuple()[0]
try:
lumberjack()
except IndexError as exc:
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:
>>> 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(an_error)
['IndexError: tuple index out of range\n']
With the helper class, we have more options:
>>> import sys
>>> from traceback import TracebackException
>>>
>>> def lumberjack():
... bright_side_of_life()
...
>>> def bright_side_of_life():
... t = "bright", "side", "of", "life"
... return t[5]
...
>>> try:
... lumberjack()
... except IndexError as e:
... exc = e
...
>>> try:
... try:
... lumberjack()
... except:
... 1/0
... except Exception as e:
... 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 lumberjack
bright_side_of_life()
~~~~~~~~~~~~~~~~~~~^^
File "<python-input-1>", line 10, in bright_side_of_life
return t[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 lumberjack
bright_side_of_life()
~~~~~~~~~~~~~~~~~~~^^
File "<python-input-1>", line 10, in bright_side_of_life
return t[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 lumberjack
bright_side_of_life()
~~~~~~~~~~~~~~~~~~~^^
File "<python-input-8>", line 11, in bright_side_of_life
return t[5]
~^^^
IndexError: tuple index out of range
During 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
