python - Nunmpy sum() function + comprehensions: int32 or in64? -


why numpy.sum , numpy.prod function return int32 when input list of int, , int64 if it's generator same list? what's best way coerce them use int64 when operating on list?

e.g.

sum([x x in range(800000)]) == -2122947200 sum((x x in range(800000))) == 319999600000l

python 2.7

you using numpy.sum instead of built-in sum, side effect of from numy import *. advised not so, cause no end of confusion. instead, use import numpy np, , refer numpy namespace short np prefix.

to answer question, numpy.sum makes accumulator type same type of array. on 32-bit system, numpy coerces list int32 array, causes numpy.sum use 32-bit accumulator. when given generator expression, numpy.sum falls calling sum, promotes integers longs. force use of 64-bit accumulator array/list input, use dtype parameter:

>>> np.sum([x x in range(800000)], dtype=np.int64) 319999600000

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