Fix Python – How does functools partial do what it does?

I am not able to get my head on how the partial works in functools.
I have the following code from here:
>>> sum = lambda x, y : x + y
>>> sum(1, 2)
3
>>> incr = lambda y : sum(1, y)
>>> incr(2)
3
>>> def sum2(x, y):
return x + y

>>> incr2 = functools.partial(sum2, 1)
>>> incr2(4)
5

Now in the line
incr = lambda y : sum(1, y)

I get that wha….