python numpy range() and sum(-1)

Question

Python question:

print(sum(range(5),-1)) 
from numpy import * 
print(sum(range(5),-1))

9
10

What is logical behind it? Thank you

Numpy must be redefining range, and it has different boundry rules. — Carcigenicate
– Carcigenicate, Commented Mar 22, 2017 at 14:59
The first line is equivalent to 0+1+2+3+4+(-1). The third line is equivalent to 0+1+2+3+4 on axis=-1. Use import numpy as np and then rewrite the third line as np.sum(range(5),-1). And read the documentation for np.sum() — jkr
– jkr, Commented Mar 22, 2017 at 14:59
Numpy does not redefine range. The second argument in np.sum() is axis. It will only sum the items in the first argument. — jkr
– jkr, Commented Mar 22, 2017 at 15:00
Ahh, my bad. Shot in the dark. I didn't think a summing function could have different behavior. — Carcigenicate
– Carcigenicate, Commented Mar 22, 2017 at 15:01

leovp · Accepted Answer · 2017-03-22 15:01:21Z

4

numpy.sum() signature is as follows (with some arguments omitted):

numpy.sum(a, axis=None, dtype=None, out=None, ...)

Python's sum signature:

sum(iterable, start=0)

sum iterates over supplied iterable, sums the values, and then adds -1 (i.e. substracts 1). numpy.sum just sums all the values from supplied iterable, and receives an axis parameter as 1, which in your case doesn't change the behaviour.

answered Mar 22, 2017 at 15:01

leovp

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1 Comment

miradulo Over a year ago

It's not so important, but sum doesn't add the start value at the end of the operation - it rather starts with the start value which defaults to 0, and adds the sequence to it. Like in CPython here.

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