`if key in dict` vs. `try/except` - which is more readable idiom?

python idioms readability defaultdict code-readability
LeeMobile picture LeeMobile · Dec 22, 2010 · Viewed 70.8k times · Source

I have a question about idioms and readability, and there seems to be a clash of Python philosophies for this particular case:

I want to build dictionary A from dictionary B. If a specific key does not exist in B, then do nothing and continue on.

Which way is better?

try:
    A["blah"] = B["blah"]
except KeyError:
    pass

or 

if "blah" in B:
    A["blah"] = B["blah"]

"Do and ask for forgiveness" vs. "simplicity and explicitness".

Which is better and why?

Answer

Glenn Maynard picture Glenn Maynard · Dec 22, 2010

Exceptions are not conditionals.

The conditional version is clearer. That's natural: this is straightforward flow control, which is what conditionals are designed for, not exceptions.

The exception version is primarily used as an optimization when doing these lookups in a loop: for some algorithms it allows eliminating tests from inner loops. It doesn't have that benefit here. It has the small advantage that it avoids having to say "blah" twice, but if you're doing a lot of these you should probably have a helper move_key function anyway.

In general, I'd strongly recommend sticking with the conditional version by default unless you have a specific reason not to. Conditionals are the obvious way to do this, which is usually a strong recommendation to prefer one solution over another.

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