Python weird thing with list local and global scope

Question

Why can we modify list with the append method but can't do the same with the list concatenation? I know about the local and global scope, I'm confused about why we can do it with append method, thanks in advance

some_list=[]

def foo():
    some_list.append('apple')


foo()
print(some_list)
#it works

with list concatenation

some_list=[]

def foo():
    some_list+=['apple']


foo()
print(some_list)
#UnboundLocalError: local variable 'some_list' referenced before assignment

Answer 1

Augmented operations like += reassign the original variable, even if its not strictly necessary.

Python's operators turn into calls to an object's magic methods: __iadd__ for +=. Immutable objects like int can't change themselves so you can't do an in-place += like you can in C. Instead, python's augmented methods return an object to be reassigned to the variable being manipulated. Mutable objects like lists just return themselves while immutable objects return a different object.

Since the variable is being reassigned, it has to follow the same scoping rules as any other variable. The reassignment causes python to assume that the variable is in the local namespace and you need the global keyword to override that assumption.