Common Pitfalls in Python

Posted by Anurag Uniyal on Stack Overflow See other posts from Stack Overflow or by Anurag Uniyal
Published on 2009-06-18T08:19:11Z Indexed on 2010/05/02 15:18 UTC
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Today I was bitten again by "Mutable default arguments" after many years. I usually don't use mutable default arguments unless needed but I think with time I forgot about that, and today in the application I added tocElements=[] in a pdf generation function's argument list and now 'Table of Content' gets longer and longer after each invocation of "generate pdf" :)

My question is what other things should I add to my list of things to MUST avoid?

Mutable default arguments

  • Import modules always same way e.g. from y import x and import x are different things, they are treated as different modules.

  • Do not use range in place of lists because range() will become an iterator anyway, the following will fail:

    myIndexList = [0,1,3]
    isListSorted = myIndexList == range(3) # will fail in 3.0
    isListSorted = myIndexList == list(range(3)) # will not

same thing can be mistakenly done with xrange:

`myIndexList == xrange(3)`.

Catching multiple exceptions

try:
    raise KeyError("hmm bug")
except KeyError,TypeError:
    print TypeError

It prints "hmm bug", though it is not a bug, it looks like we are catching exceptions of type KeyError,TypeError but instead we are catching KeyError only as variable TypeError, use this instead:

try:
    raise KeyError("hmm bug")
except (KeyError,TypeError):
    print TypeError

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