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 use global.

I'm using Python 2.7 and I tried this little test

>>> sub = ['0', '0', '0', '0']
>>> def getJoin():
...     return '.'.join(sub)
...
>>> getJoin()
'0.0.0.0'

It seems things are working fine even without global. I was able to access global variable without any problem.

Am I missing anything? Also, following is from Python documentation:

Names listed in a global statement must not be defined as formal parameters or in a for loop control target, class definition, function definition, or import statement.

While formal parameters and class definition make sense to me, I'm not able to understand the restriction on for loop control target and function definition.

Answer 1

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 2

The other answers answer your question. Another important thing to know about names in Python is that they are either local or global on a per-scope basis.

Consider this, for example:

value = 42

def doit():
    print value
    value = 0

doit()
print value

You can probably guess that the value = 0 statement will be assigning to a local variable and not affect the value of the same variable declared outside the doit() function. You may be more surprised to discover that the code above won't run. The statement print value inside the function produces an UnboundLocalError.

The reason is that Python has noticed that, elsewhere in the function, you assign the name value, and also value is nowhere declared global. That makes it a local variable. But when you try to print it, the local name hasn't been defined yet. Python in this case does not fall back to looking for the name as a global variable, as some other languages do. Essentially, you cannot access a global variable if you have defined a local variable of the same name anywhere in the function.

Answer 3

It means that you should not do the following:

x = 1

def myfunc():
  global x

  # formal parameter
  def localfunction(x):
    return x+1

  # import statement
  import os.path as x

  # for loop control target
  for x in range(10):
    print x

  # class definition
  class x(object):
    def __init__(self):
      pass

  #function definition
  def x():
    print "I'm bad"

Answer 4

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 5

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 6

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.