CODEWITHSUNDEEP
pythondictionaryset
jclancy
jclancy

Reputation: 52318

Checking for membership in dictionary: exceptions or set?

I want to process a list of elements ls, each of which may or may not have a corresponding element in a dictionary d. I can think of two ways to do it. Here, s is the set of all elements that are keys of d. When constructing d it is trivial to construct s as well, so take that as given.

for e in ls:
    if e in s:
        process(d[e])

and

for e in ls:
    try:
        process(d[e])
    except KeyError:
        pass

Of the two, which is faster? Furthermore I've heard that Python uses the principle of "Ask forgiveness, not permission". Does this mean that in general testing using if statements will be slower than using exceptions?

Upvotes: 1

Views: 104

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1122082

If order doesn't matter, use intersections:

for e in s.intersection(ls):
    # only elements that are in `s` *and* `ls` are iterated over
    process(d[e])

Most of your time is probably going to be spent in process() in any case, so don't micro-optimize. Instead optimize for readability (within reason).

As for the choice of in test vs. exception handling: exception handling will be faster if the number of misses is low (relatively few exceptions), but if you have a lot of misses the in test will be faster than handling loads of exceptions. If you really care, use the timeit module to find a balance. See my answer on the subject on Programmers.SE.

Upvotes: 4

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