Question

I use list comprehensions probably about as much as any intermediate or advanced Python programmer. I try not to use over do it.

I’m curious if this would seem obscure or merely terse:

some_count = len([x for x in some_list if x in some_dict])

… in lieu of:

some_count = 0
for x in some_list:
    if x in some_dict:
        some_count += 1

In the actual case I’m looking at I could even use:

some_count = len(set(some_list) & set(some_dict))

(Given that the items in some_list are guaranteed to be distinct).

In particular I have a function with returns a (possibly empty) list of strings (from an external, proprietary data store). It should be the case that only one of these is a valid key into a dictionary in my code. If it’s zero I should post a warning, if it’s one I should just the the value from my code, and if it’s more than one I should emit an error.

I’m just soliciting stylistic opinions here.

Answer 1

The condition x in some_dict(x) is very obscure; looks like you need to lose the (x).

You don’t need to build a list; try this:

some_count = sum(1 for x in some_list if x in whatever)

Your set approach seems to be the most understandable. It’s not likely to be the fastest, however.