[Python] Why do I get a 'referenced before assignment' error, for a variable that has been...

This code:

my_num = 0

def my_func():
my_num = my_num + 1
my_func()


causes the following error:

Traceback (most recent call last): File "main.py", line 5, in my_func() File "main.py", line 4, in my_func my_num = my_num + 1 UnboundLocalError: local variable 'my_num' referenced before assignment
​

Why does the interpreter not read the expression as:

my_num = 0 + 1

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