Use of “global” keyword in Python

Question

What I understand from reading the documentation is that Python has a separate namespace for functions, and if I want to use a global variable in that function, I need to us

Answer 1

While you can access global variables without the global keyword, if you want to modify them you have to use the global keyword. For example:

foo = 1
def test():
    foo = 2 # new local foo

def blub():
    global foo
    foo = 3 # changes the value of the global foo

In your case, you're just accessing the list sub.

Answer 2

This is explained well in the Python FAQ

What are the rules for local and global variables in Python?

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.

https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python

Answer 3

This is the difference between accessing the name and binding it within a scope.

If you're just looking up a variable to read its value, you've got access to global as well as local scope.

However if you assign to a variable who's name isn't in local scope, you are binding that name into this scope (and if that name also exists as a global, you'll hide that).

If you want to be able to assign to the global name, you need to tell the parser to use the global name rather than bind a new local name - which is what the 'global' keyword does.

Binding anywhere within a block causes the name everywhere in that block to become bound, which can cause some rather odd looking consequences (e.g. UnboundLocalError suddenly appearing in previously working code).

>>> a = 1
>>> def p():
    print(a) # accessing global scope, no binding going on
>>> def q():
    a = 3 # binding a name in local scope - hiding global
    print(a)
>>> def r():
    print(a) # fail - a is bound to local scope, but not assigned yet
    a = 4
>>> p()
1
>>> q()
3
>>> r()
Traceback (most recent call last):
  File "<pyshell#35>", line 1, in <module>
    r()
  File "<pyshell#32>", line 2, in r
    print(a) # fail - a is bound to local scope, but not assigned yet
UnboundLocalError: local variable 'a' referenced before assignment
>>> 

Answer 4

Any variable declared outside of a function is assumed to be global, it's only when declaring them from inside of functions (except constructors) that you must specify that the variable be global.

Answer 5

Global makes the variable "Global"

def out():
    global x
    x = 1
    print(x)
    return


out()

print (x)

This makes 'x' act like a normal variable outside the function. If you took the global out then it would give an error since it cannot print a variable inside a function.

def out():
     # Taking out the global will give you an error since the variable x is no longer 'global' or in other words: accessible for other commands
    x = 1
    print(x)
    return


out()

print (x)

Answer 6

Accessing a name and assigning a name are different. In your case, you are just accessing a name.

If you assign to a variable within a function, that variable is assumed to be local unless you declare it global. In the absence of that, it is assumed to be global.

>>> x = 1         # global 
>>> def foo():
        print x       # accessing it, it is global

>>> foo()
1
>>> def foo():   
        x = 2        # local x
        print x 

>>> x            # global x
1
>>> foo()        # prints local x
2