[Python] What's wrong with this recursive code for permutations? [duplicate]

I am trying to write code to output permutations. I do not understand why I am getting the TypeError? How to fix it? Thanks!

A=[1,2,3]
c=[]
def perm(a):
global c
if len(a)==1:
p=a
else:
for i in a:
p=+perm(a.remove(i))
c+=[p]
return p
perm(A)
print(c)

TypeError: object of type 'NoneType' has no len()

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