[Python] Mergesort and index out of range in Python

I wrote a merge sort algorithm and trying to figure out index out of range error. I checked the code in depth but could not see it so I do not have many things to write but I tried to follow the basic process of merge sort algorithm.

def merge(a,lower,middle,upper,b):
i=lower
j=middle+1
k=lower
while(i<=middle and j<=upper):
if(a<=a[j]):
b[k]=a
i=i+1
else:
b[k]=a[j]
j=j+1
k=k+1
if(i>middle):
while(j<=upper):
b[k]=a[j]
j=j+1
k=k+1
elif(j>upper):
while(i<=middle):
b[k]=a
i=i+1
k=k+1
return b

def mergesort(a,lower,upper):
if(lower<upper):
middle=(lower+upper)//2
mergesort(a,lower,middle)
mergesort(a,middle+1,upper)
s=merge(a,lower,middle,upper,[])

mergesort([1,3,2,7,11,10],0,5)

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