The following snippet is an example where we face an UnboundLocalError when accessing a nonlocal variable.
def foo():
a = 1
def bar():
a += 1 # Raises UnboundLocalError
print(a)
bar()
foo()
a += 1 is evaluated as a = a+1. Here, the a on the RHS leads to an UnboundLocalError.
It can be fixed with the nonlocal keyword like so:
def foo():
a = 1
def bar():
nonlocal a # Fixes the UnboundLocalError
a += 1
print(a)
bar()
foo() # Prints 2
In the following snippet, we try to access a nonlocal list, but it does not raise an UnboundLocalError.
def egg():
l = [[1 for _ in range(3)] for _ in range(3)]
def chicken():
l[0][0] -= 1 # Does not raise an UnboundLocalError
l[1][1] -= 1
l[2][2] -= 1
print(l)
chicken()
egg() # Prints [[0, 1, 1], [1, 0, 1], [1, 1, 0]]
The way I understand this, here as well, l[0][0] -= 1 should have been evaluated as l[0][0] = l[0][0]-1 which should have resulted in an UnboundLocalError. Why is that not the case?
