Data loss when sorting points

After finding a large set of points I’m now trying to sort them such that:

arr = [(1,2)(1,3)(1,4)(2,1)(2,3)]

becomes:

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

In other words I’m trying to group the points by their x coordinate.

I wrote below function to do this:

def sortPoints(points):
    prev = NULL
    sorted = []
    groupe = []
    for p in points:
        if prev == NULL : prev = p[0]
        if p[0] == prev:
            groupe.append(p)
        else :
            sorted.append(groupe[:])
            del groupe[:]
            prev = p[0]
    return sorted

When I run the code I get an array with sorted points as expected, however, a large part of the points also vanish in the process, and I cannot figure out why.

If anyone could help me, that would be great and very appreciated. Thank you in advance.

>Solution :

As mark suggested you should use itertools.groupby for such cases

from itertools import groupby
arr = [(1,2),(1,3),(1,4),(2,1),(2,3)]
arr.sort(key=lambda x: x[0])
[list(v) for _,v in groupby(arr, lambda y: y[0])]
# output
[[(1, 2), (1, 3), (1, 4)], [(2, 1), (2, 3)]]