Should I check if an item is already in a set before adding it?

Question

If foo is a builtin set that I know contains "bar", which of these is faster? Which is more Pythonic?

foo.add("bar")

or

if "bar" not in foo:
    foo.add("bar")

Answer 1

Actually, the second may be faster (output from IPython):

In [2]: %timeit s.add("a")
The slowest run took 68.27 times longer than the fastest. This could mean that an intermediate result is being cached 
10000000 loops, best of 3: 73.3 ns per loop

In [3]: %timeit if not "a" in s: s.add("a")
10000000 loops, best of 3: 37.1 ns per loop

But anyway, the first one is more Pythonic, I agree.

Answer 2

The pythonic way is to do first, ask later. Just add it to the set.

Asking first is more common in languages such as C.

Performance is usually not key in python code. Readability is usually much more important, so writing ideomatic code is good practice.