13.2 The global Statement

The global statement is the only thing that's remotely like a declaration statement in Python. It's not a type or size declaration, though, it's a namespace declaration. It tells Python that a function plans to change global names—names that live in the enclosing module's scope (namespace). We've talked about global in passing already; as a summary:

• global means "a name at the top-level of the enclosing module file."

• Global names must be declared only if they are assigned in a function.

• Global names may be referenced in a function without being declared.

The global statement is just the keyword global, followed by one or more names separated by commas. All the listed names will be mapped to the enclosing module's scope when assigned or referenced within the function body. For instance:

X = 88          # Global X

def func(  ):
    global X
    X = 99      # Global X: outside def

func(  )
print X         # Prints 99

We've added a global declaration to the example here, such that the X inside the def now refers to the X outside the def; they are the same variable this time. Here is a slightly more involved example of global at work:

y, z = 1, 2         # Global variables in module

def all_global(  ):
    global x        # Declare globals assigned.
    x = y + z       # No need to declare y,z: LEGB rule

Here, x, y, and z are all globals inside the function all_global. y and z are global because they aren't assigned in the function; x is global because it was listed in a global statement to map it to the module's scope explicitly. Without the global here, x would be considered local by virtue of the assignment.

Notice that y and z are not declared global; Python's LEGB lookup rule finds them in the module automatically. Also notice that x might not exist in the enclosing module before the function runs; if not, the assignment in the function creates x in the module.

If you want to change names outside functions, you have to write extra code (global statements); by default, names assigned in functions are locals. This is by design—as is common in Python, you have to say more to do the "wrong" thing. Although there are exceptions, changing globals can lead to well-known software engineering problems: because the values of variables are dependent on the order of calls to arbitrarily distant functions, programs can be difficult to debug. Try to minimize use of globals in your code.