Why is the global keyword not required in this case?

Question

cache = {}
def func():
    cache['foo'] = 'bar'
print cache['foo'] 

output

bar

Why does this work and why doesn't it require use of the global keyword?

Answer 1

Because you are not assigning to cache, you are changing the dictionary itself instead. cache is still pointing to the dictionary, thus is itself unchanged. The line cache['foo'] = 'bar' translates to cache.__setitem__('foo', 'bar'). In other words, the value of cache is a python dict, and that value is itself mutable.

If you tried to change what cache refers to by using cache = 'bar' instead, you would be changing what cache points to and then you need the global keyword.

Perhaps this older answer of mine to a similar question helps you understand the difference: Python list doesn't reflect variable change.