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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?
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The reason global variables are bad is that they enable functions to have hidden (non-obvious, surprising, hard to detect, hard to diagnose) side effects, leading to an increase in complexity, potentially leading to Spaghetti code.
The basic rules for global keyword in Python are: When we create a variable inside a function, it is local by default. When we define a variable outside of a function, it is global by default. You don't have to use global keyword.
The global keyword is used to create global variables from a no-global scope, e.g. inside a function.
A global keyword is a keyword that allows a user to modify a variable outside the current scope. It is used to create global variables in Python from a non-global scope, i.e. inside a function. Global keyword is used inside a function only when we want to do assignments or when we want to change a variable.
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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