Why is the global keyword not required in this case?

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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.

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