Python quicksort

def qsort( ary , left, right ):
    l=left
    r=right
    ipivot = int ( ( l + r ) / 2 )
    pivot = ary [ ipivot ]

    while True:
        while ary [ l ] < pivot and l < right:
            l += 1
        while pivot < ary [ r ] and r > 0:
            r -= 1
        if l <= r:
            temp = ary [ l ]
            ary [ l ] = ary [ r ]
            ary [ r ] = temp
            l += 1
            r -= 1
        if l > r:
            break
    if left < r:
        qsort ( ary, left , r  )
    if l < right:
        qsort ( ary , l, right )

ary= [ 12, 5, 1, 7, 5, 3, 2, 2, 13, 7, 3, 3, 3, 2, 8, 1 , 2, 3, 4 ]
print ( ary )
qsort( ary, 0, len( ary ) - 1 )
print ( ary )