However...

    $ python -m timeit -s "a={k:k  for k in range(10000)}" 'if 10001 in a:
       b=a[10001]
    else:
       pass'
    100000000 loops, best of 3: 0.0187 usec per loop

I gotta say, I didn't expect that. I thought the try..except.. for keys in the dict would be faster than this form. TIL