Is if-checking or try-assert-except more appropriate and why

Question

Which method or checking conditions is more suggested to use:

import os

# method 1
if not os.path.exists("somedir"):
    os.makedirs("somedir")

# method 2
try:
    assert not os.path.exists("somedir")
except AssertionError:
    pass
else:
    os.makedirs("somedir")

# method 3
try:
    os.makedirs("somedir")
except OSError:
    pass

Would method 3 still break The Zen of Python ("Errors should never pass silently." if there was some logging?

It's not an answer to the question as this os.makedirs() was just an example but thank you I didn't know about this option ;) — Kuba Chrabański, Sep 27 '18 at 18:10
`assert` is typically associated with a debugging scenario: alluded to in [the docs](https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement) and [other resources](https://stackoverflow.com/a/41721518/2823755) using the search terms "python assert usage" — wwii, Sep 27 '18 at 18:42

score 1 · Accepted Answer · answered Sep 27 '18 at 18:08

Method 3 is, in general, superior. The other methods introduce a race condition, where another process could create the directory after you test for its existence but before you actually try to create it.

As pointed out in a comment, the exist_ok option does this for you, by simply not raising an exception if the exception is caused by a pre-existing directory (while still raising an exception for other errors).

Is if-checking or try-assert-except more appropriate and why

1 Answers1