Python: map() with partial arguments

Question

I got a multi-argument function. I simplified the function to map, and here's my function to test with:

def _dist(I,J,X,Y):
    return abs(I-X)+abs(J-Y)

And in some case, I just want to hold X,Y as constant and iterate I,J over u_x and u_y, respectively. I write down something below (while SyntaxError raised)

new=map(_dist(,,cur.x(),cur.y()),u_x,u_y)

where the cur.x() and cur.y() are constant. Anyway to do it with map()?

Answer 1

You can partially apply _dist:

from functools import partial
_mapfunc = partial(_dist, X=cur.x(), Y=cur.y())
new = map(_mapfunc, u_x, u_y)

although Chris Taylor's list comprehension is probably more efficient.

Answer 2

Can you not use a list comprehension?

new = [_dist(x,y,i,j) for x in ux for y in uy]

where i and j are your constants.

Note that this is equivalent to

new = map(lambda (x,y): _dist(x,y,i,j), [(x,y) for x in ux for y in uy])

but somewhat shorter (and probably more efficient).

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Edit (in response to @chepner's comment) - if you want to iterate through the zipped product of ux and uy then you can do

new = [_dist(x,y,i,j) for (x,y) in zip(ux,uy)]

instead. Note the difference -

>> ux = [1, 2, 3]
>> uy = [4, 5, 6]
>> [(x,y) for x in ux for y in uy]

[(1, 4), (1, 5), (1, 6), (2, 4), (2, 5), (2, 6), (3, 4), (3, 5), (3, 6)]

>> zip(ux, uy)

[(1, 4), (2, 5), (3, 6)]