Timing the cost of a try/except block vs the cost of a key lookup...

eafbvslbyl.py

from timeit import Timer

def eafp(d):
    try:
        return d["key"]
    except KeyError:
        return None

def lbyl(d):
    if "key" in d:
        return d["key"]
    return None

dicts = [{"key":1}, {"nokey":1}]

t1 = Timer("[eafp(d) for d in dicts]", 'from __main__ import dicts, eafp')
t2 = Timer("[lbyl(d) for d in dicts]", 'from __main__ import dicts, lbyl')

print t1.timeit()
print t2.timeit()