Need explanation for simple program | assertion

Question

the program is

def a(x):
x+=1
assert (x>=0),'negative'

print(a(10))

output is:
>>>
None
>>>

but I expect out is < 11 >

some one please explain how the out is came 'None'
what is the meaning of 'None'

Answer 1

None is same as null,Nil in other programming language.
In python function that doesn't return anything always returns None .
assert is used here to raise assertion error if x is less than 0 .

Answer 2

This function doesn't return any value. None is returned in such cases.
None represents absence of value.