Python Gotcha's?

[Tim Chase]

On 04/04/12 17:34, Miki Tebeka wrote:
> Greetings,
>
> I'm going to give a "Python Gotcha's" talk at work.
> If you have an interesting/common "Gotcha" (warts/dark corners ...) please share.
>
> (Note that I want over http://wiki.python.org/moin/PythonWarts already).

1) While I believe it was fixed in more recent releases (perhaps 
Py3 or later, most of my code is still in 2.x), leaking of 
list-comprehension variables into the surrounding scope has stung 
me on occasion:

   val = something_important
   whatever = [val for val in iterable if condition(val)]
   assert val == something_important, "ug!"


2) While totally understandable, the significance of leading 
whitespace in docstrings/triple-quoted items occasionally catches 
me in places I didn't intentionally want it:

   class Foo:
     def frobniculate(self, x, y):
       """Frobniculate the x & y
       x is the macrowobble variance
       y is the miniwibble grobulation
       """
       pass

(there's now leading whitespace on lines 2 & 3, and an extra 
trailing line of pure whitespace).

3) the peculiarities of old-style classes and new-style classes 
in 2.x (mooted by 3.x) take careful reading of the docs if you're 
overriding __getattr__ or __getattribute__, as well as possible 
other old-vs-new gotchas.

4) the __del__ method may have things in the containing scopes 
(such as modules imported at the top of the containing module) 
that get garbage-collected before the __del__ is called.  In 
general, __del__ stinks, and is better replaced by "with" 
statements using context managers.

5) the masking of system modules.  On multiple occasions I've 
created a local module named "email.py", and then upon importing 
something else (I think it may have been smtplib...not sure), it 
failed with a confusing error because the imported module found 
mine rather than the system module.

Those are my off-the-top-of-the-head gotchas.

-tkc