How do I check that multiple keys are in a dict in a single pass?

Question

Well, you could do this:

>>> if all (k in foo for k in ("foo","bar")):
...     print "They're there!"
...
They're there!

if {"foo", "bar"} <= myDict.keys(): ...

If you're still on Python 2, you can do

if {"foo", "bar"} <= myDict.viewkeys(): ...

If you're still on a really old Python <= 2.6, you can call set on the dict, but it'll iterate over the whole dict to build the set, and that's slow:

if set(("foo", "bar")) <= set(myDict): ...

Simple benchmarking rig for 3 of the alternatives.

Put in your own values for D and Q

>>> from timeit import Timer
>>> setup='''from random import randint as R;d=dict((str(R(0,1000000)),R(0,1000000)) for i in range(D));q=dict((str(R(0,1000000)),R(0,1000000)) for i in range(Q));print("looking for %s items in %s"%(len(q),len(d)))'''

>>> Timer('set(q) <= set(d)','D=1000000;Q=100;'+setup).timeit(1)
looking for 100 items in 632499
0.28672504425048828

#This one only works for Python3
>>> Timer('set(q) <= d.keys()','D=1000000;Q=100;'+setup).timeit(1)
looking for 100 items in 632084
2.5987625122070312e-05

>>> Timer('all(k in d for k in q)','D=1000000;Q=100;'+setup).timeit(1)
looking for 100 items in 632219
1.1920928955078125e-05

You don't have to wrap the left side in a set. You can just do this:

if {'foo', 'bar'} <= set(some_dict):
    pass

This also performs better than the all(k in d...) solution.