Merge Sort returns the same array in Python - python-2.7

Question

for merge sort i wrote this code:
I have tested merge function that works correctly. but in mergeSort function i coudn't handle the arrays. it returns the same list as the input list.
def mergeSort(a):
l, h = 0, len(a)-1
mid = (l+h)/2
if (l<h-1): #the lowest length must be 2
mergeSort(a[l:mid+1])
mergeSort(a[mid+1:h+1])
return merge(a[l:mid+1],a[mid+1:h+1])
def merge(a,b):
n_a = len(a)
n_b = len(b)
c = [[] for i in range(n_a + n_b)]
i,j,k=0,0,0
while (i<n_a and j<n_b):
if a[i]<b[j]:
c[k] = a[i]
i += 1
else:
c[k]= b[j]
j += 1
k += 1
while(i<n_a):
c[k] = a[i]
k+=1
i+=1
while(j< n_b):
c[k] = b[j]
k+=1
j+=1
return c

Answer 1

I would rewrite merge as so:
def mergeSort(a):
h = len(a)
mid = h / 2
if h >= 2:
return merge(mergeSort(a[:mid]), mergeSort(a[mid:]))
else:
return a
A few notes:
l is always 0, might as well remove it
h is len(a) - 1, but then you use h + 1, might as well use h = len(a)
Actually writting h >= 2 makes it clearer that you need at least 2 items in your list
mid can be len(a) / 2.
when taking the complete beginning/end of an array in a slice, the first/last bound is not required