traceback shows up until decorator

Question

This nice little Python decorator can configurably disabled decorated functions:

enabled = get_bool_from_config()

def run_if_enabled(fn):
    def wrapped(*args, **kwargs):
        try:
            return fn(*args, **kwargs) if enabled else None
        except Exception:
            log.exception('')
            return None
    return wrapped

alas, if an exception is raised within fn() the traceback shows only up to the wrapper:

Traceback (most recent call last):
  File "C:\my_proj\run.py", line 46, in wrapped
    return fn(*args, **kwargs) if enabled else None
  File "C:\my_proj\run.py", line 490, in a_decorated_function
    some_dict['some_value']
KeyError: 'some_value'

1. Why?
2. Can I workaround to see the full traceback?

Answer 1

Ah! Ok, now this is an interesting question!

Here is is the same approximate function, but grabbing the exception directly from sys.exc_info():

import sys
import traceback

def save_if_allowed(fn):
    def wrapped(*args, **kwargs):
        try:
            return fn(*args, **kwargs) if enabled else None
        except Exception:
            print "The exception:"
            print "".join(traceback.format_exception(*sys.exc_info()))
            return None
    return wrapped

@save_if_allowed
def stuff():
    raise Exception("stuff")


def foo():
    stuff()

foo()

And it's true: no higher stack frames are included in the traceback that's printed:

$ python test.py
The exception:
Traceback (most recent call last):
  File "x.py", line 21, in wrapped
    return fn(*args, **kwargs) if enabled else None
  File "x.py", line 29, in stuff
    raise Exception("stuff")
Exception: stuff

Now, to narrow this down a bit, I suspect it's happening because the stack frame only includes stack information up until the most recent try/except block… So we should be able to recreate this without the decorator:

$ cat test.py
def inner():
    raise Exception("inner")

def outer():
    try:
        inner()
    except Exception:
        print "".join(traceback.format_exception(*sys.exc_info()))

def caller():
    outer()

caller()

$ python test.py
Traceback (most recent call last):
  File "x.py", line 42, in outer
    inner()
  File "x.py", line 38, in inner
    raise Exception("inner")
Exception: inner

Ah ha! Now, on reflection, this does make sense in a certain kind of way: at this point, the exception has only encountered two stack frames: that of inner() and that of outer() — the exception doesn't yet know from whence outer() was called.

So, to get the complete stack, you'll need to combine the current stack with the exception's stack:

$ cat test.py
def inner():
    raise Exception("inner")

def outer():
    try:
        inner()
    except Exception:
        exc_info = sys.exc_info()
        stack = traceback.extract_stack()
        tb = traceback.extract_tb(exc_info[2])
        full_tb = stack[:-1] + tb
        exc_line = traceback.format_exception_only(*exc_info[:2])
        print "Traceback (most recent call last):"
        print "".join(traceback.format_list(full_tb)),
        print "".join(exc_line)

def caller():
    outer()

caller()

$ python test.py
Traceback (most recent call last):
  File "test.py", line 56, in <module>
    caller()
  File "test.py", line 54, in caller
    outer()
  File "test.py", line 42, in outer
    inner()
  File "test.py", line 38, in inner
    raise Exception("inner")
Exception: inner

See also:

• http://docs.python.org/2/library/sys.html
• http://docs.python.org/2/library/traceback.html