Side Effects in Python

Question

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

Answer 1

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

When the docs say that global provides a bar against unintended side-effects, it means that it safeguards you against situations where a function could change its outer scope because of name aliasing.

Here is an example:

x = 5

def safedouble(x)
    x = 2 * x
    return x

def unsafedouble(x)
    global x
    x = 2 * x
    return x

print(x) # 5
print(safedouble(x)) # 10
print(x) # 5 The x inside of the function does not interfere with x outside =)
print(unsafedouble(x)) # 10
print(x) # 10 ... do we really want this to happen?!

Requiring global means that functions only mutate the outer scope if the programmer explicitly asks for it.

Answer 2

No, you misunderstand. Here is what the docs state in context:

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.

Though a bit surprising at first, a moment’s consideration explains this. On one hand, requiring global for assigned variables provides a bar against unintended side-effects. On the other hand, if global was required for all global references, you’d be using global all the time. You’d have to declare as global every reference to a built-in function or to a component of an imported module. This clutter would defeat the usefulness of the global declaration for identifying side-effects.

Assignment to a name does not require global. You can use assignment without global, but the docs are telling you that if you do use assignment in a local scope without the global directive, the name will be considered a local variable by default! The reasoning is as stated, if assigned-to variables inside a function could be global by default, then you might accidentally assign to a global variable and inadvertently cause a side-effect. But if you require a global directive, then you will know that your function produces a side-effect. It isn't stating that global prevents side-effects, it prevents inadvertent side-effects.