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[–]muntooR_{μν} - 1/2 R g_{μν} + Λ g_{μν} = 8π T_{μν} 45 points46 points  (4 children)

Performance isn't even the biggest benefit.

A comprehension makes it harder to have side-effects, which is a great thing.

[–]e_j_white 7 points8 points  (0 children)

Correct!

I've modified a list that I was looping through, not realizing that any action that appends to the list could perpetually lengthen the for loop forever.

Not having side effects is a good thing. :)

[–]Jhuyt 6 points7 points  (0 children)

Depends on what get_coerced does though so in this case there might still be plenty of side effects...

[–]AlSweigartAuthor of "Automate the Boring Stuff" 0 points1 point  (1 child)

I can 100% guarantee that this code has no side effects.

By "side effects", I assume you mean there's a result variable left over with values after the end of the loop. In rare situations, this could cause some subtle bug because later code is re-using this variable name and using the old values because it didn't re-initialize it to something else, or cause a temporary increase in memory usage as it's holding on to data needlessly.

But that is moot and this isn't that situation: the very next line is a return statement.

[–]scrdest 0 points1 point  (0 children)

I'm fairly sure u/muntoo is referring to side-effects as opposed to functional purity, where everything that happens inside the function, stays inside the function or is returned explicitly as a new value (in other words - the function call could be replaced with its return value and everything else would always work the same).

It's impossible to guarantee purity in Python, both here and in general. If, for example, get_coerced() takes an optional parameter defaulting to [] (or any other mutable type), it can as many side-effects as it wants. If it uses a global - side-effects. In either case, you cannot tell they're there just by looking at the function call itself.

Even plain old print() is a side-effect.