How to set initial size for a dictionary in Python?

With performance issues it’s always best to measure. Here are some timings:

 d = {}
 for i in xrange(4000000):
     d[i] = None
 # 722ms

 d = dict(itertools.izip(xrange(4000000), itertools.repeat(None)))
 # 634ms

 dict.fromkeys(xrange(4000000))
 # 558ms

 s = set(xrange(4000000))
 dict.fromkeys(s)
 # Not including set construction 353ms

The last option doesn’t do any resizing, it just copies the hashes from the set and increments references. As you can see, the resizing isn’t taking a lot of time. It’s probably your object creation that is slow.