Finding the distance between the two closest points in a 2-D plane
Coding style

Your code is difficult to read because it is written very condensed.

There is a well-established coding style for Python, the 
PEP8 coding style,
and conformance to that style can be checked online at
PEP8 online.

In your case it reports “missing space around operator” in almost every
code line, also “line too long” and “multiple statements on one line”
violations. As an example,

elif len(points)==3: return min(dist(points[0],points[1]),dist(points[0],points[2]),dist(points[1],points[2]))


is better written as

elif len(points) == 3:
    return min(dist(points[0], points[1]),
               dist(points[0], points[2]),
               dist(points[1], points[2]))


Variable naming

Another aspect which makes the code difficult to understand is the
naming of variables. Here are some examples:

points_y = sorted(points, key= lambda z:z[1])
points.sort(key=lambda g:g[0])


The parameter names z and g seem to be arbitrary. Why are they different
at all if they both refer to a point? Similarly at

yleft = [t for t in points_y if t[0]&lt;=points[ave-1][0]]
yright = [q for q in points_y if q[0]&gt;=points[ave][0]]


Why t and q? I'd suggest point or p in all those places.

def min_dist(points,points_y):


Both parameters contain all points, just sorted differently (by x and y,
respectively). Why the asymmetry in the parameter names? I'd suggest

def min_dist(points_x, points_y):


here, plus a doc comment explaining the meaning of the parameters.

ave = (len(points)+1)//2


makes one think of “average” but is just half of the list length.

Some simplifications

import sys


is not needed. The inner parentheses in

return math.hypot((b[0]-a[0]),(b[1]-a[1]))


are not needed. The elif in 

elif len(points)==3:


can be replaced by an if. The slicing in

d1 = min_dist(points[0:ave],yleft)
d2 = min_dist(points[ave:len(points)],yright)


can be simplified to

d1 = min_dist(points[:ave], yleft)
d2 = min_dist(points[ave:], yright)


Validate the input

The program aborts with RecursionError or IndexError if zero or
one point are passed to the function. You should validate the input
and throw an appropriate exception in those cases.

A possible problem

If points[ave-1][0] == points[ave][0] (which means that more than one point
lies on the dividing vertical line) then elements from the point_y
list are assigned to both yleft and yright:

yleft = [t for t in points_y if t[0]&lt;=points[ave-1][0]]
yright = [q for q in points_y if q[0]&gt;=points[ave][0]]


which means that in the recursive calls 

d1 = min_dist(points[0:ave],yleft)
d2 = min_dist(points[ave:len(points)],yright)


the second list is not necessarily a rearrangement of the first list anymore.

I haven't found an example where this leads to a wrong result, but it could
be an performance problem because “too large” lists are passed down the 
recursion. As an extremal case, if all points have the same x-coordinate
then yleft and yright will always be the complete initial points_y
list.

Performance improvements

points[ave][0]] is accessed multiple times, it might be advantageous to
assign that value to a variable once.

The initial value

d_=2*(10**18)


is a bit arbitrary, but actually not needed: In the following loop we are
only interested in points having a distance less than the previously 
computed minimum distance in the left and right half:

d = min(d1, d2)
# ...
for i in range(len(arr_split) - 1):
    for j in range(i+1, min(len(arr_split), i + 7)):
        temp = dist(arr_split[i], arr_split[j])
        if temp &lt; d:
            d = temp
return d


And while it is true that pairs of points having a distance less than d
are at most seven (or six?) indices apart, it seems to be more efficient
to leave the inner loop if the y-coordinates differ by d or more:

for i in range(len(arr_split) - 1):
    for j in range(i + 1, len(arr_split)):
        if arr_split[j][1] - arr_split[i][1] &gt;= d:
            break
        temp = dist(arr_split[i], arr_split[j])
        if temp &lt; d:
            d = temp
return d


Further suggestions

Instead of storing the point coordinates a a tuple you could define a 
custom class

class Point:
    __slots__ = ('x', 'y')

    def __init__(self, x, y):
        self.x = x
        self.y = y


so that you can access the x- and y-coordinates of a point with p.x
and p.y instead of subscripting.