Python: pick appropriate datatype size (int) automatically

Create a mapping of maximum value to type, and then look for the smallest value larger than N.

typemap = {
  256: uint8,
  65536: uint16,
   ...
}

return typemap.get(min((x for x in typemap.iterkeys() if x > N)))

What about writing a simple function to do the job?

import numpy as np

def type_chooser(N):
    for dtype in [np.uint8, np.uint16, np.uint32, np.uint64]:
        if N <= dtype(-1):
            return dtype
    raise Exception('{} is really big!'.format(N))

Example usage:

>>> type_chooser(255)
<type 'numpy.uint8'>
>>> type_chooser(256)
<type 'numpy.uint16'>
>>> type_chooser(18446744073709551615)
<type 'numpy.uint64'>
>>> type_chooser(18446744073709551616)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "spam.py", line 6, in type_chooser
    raise Exception('{} is really big!'.format(N))
Exception: 18446744073709551616 is really big!