Side Effects in Python

I read this in the python docs: Docs

requiring global for assigned variables provides a bar against unintended side-effects.

Let's put this in code:

def double(n):
  global y 
  y = 2 * n # this turns global as we have state y explicitly global

y = 5 
double(y)
print y # outputs 10

I would like to double check my understanding if the above code has a side effect at global y if so I feel this is contradictory to the statement in the docs, basically I think requiring global for assigned variables does not guard against side effects.

Please correct me if I'm wrong.

Thanks