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formerly_proven • yesterday at 10:54 AM • 4 replies • view on HN

All iterators in Python use exceptions for flow control, as do all context managers for the abort/rollback case, and it is generally considered Pythonic to use single-indexing (EAFP) instead of check-then-get (LBYL) - generally with indexing and KeyError though and less commonly with attribute access and AttributeError.

[heavy green check mark]

    try:
        data = collection['key']
    except KeyError:
        data = ..try something else..
[red x]

    if 'key' in collection:
         data = collection['key']
    else:
         data = ..try something else..
The latter form also has the genuine disadvantage that nothing ensures the two keys are the same. I've seen typos there somewhat often in code reviews.

Replies

sevensor • yesterday at 2:01 PM

Last time I measured it, handling KeyError was also significantly faster than checking with “key in collection.” Also, as I was surprised to discover, Python threads are preemptively scheduled, GIL notwithstanding, so it’s possible for the key to be gone from the dictionary by the time you use it, even if it was there when you checked it. Although if you’re creating a situation where this is a problem, you probably have bigger issues.

agumonkey • yesterday at 11:23 AM

I thought I knew enough about python culture but TIL

https://realpython.com/python-lbyl-vs-eafp/#errors-and-excep...

andy99 • yesterday at 2:33 PM

to me something like

  for key in possible_keys:
    if key in collection:
      ...
is fine and isn’t subject to your disadvantage.
IshKebab • yesterday at 11:35 AM

You should do normally do

    data = collection.get("key")
    if data is not None:
         ...
    else:
         ....