Global Variables

Inside and outside

A name assigned at the top level of a file is global: every function in that file can read it.

site = "MEPX"

def show():
    print("reading:", site)

show()
print("still here:", site)

reading: MEPX
still here: MEPX

Assigning inside a function makes a new, local name

This is the part that surprises people. Assigning to a name inside a function creates a separate local name. The global is untouched:

count = 0

def bump():
    count = 99
    print("inside:", count)

bump()
print("outside:", count)

inside: 99
outside: 0

Python decided count was local the moment it saw an assignment to it anywhere in the function body.

The global keyword

To rebind the global from inside a function, say so:

count = 0

def bump():
    global count
    count += 1

bump()
bump()
print(count)

2

Changing versus rebinding

global is only needed to point the name somewhere new. Changing an object in place needs nothing, because the name still points at the same object:

scores = []

def record(value):
    scores.append(value)

record(10)
record(20)
print(scores)

[10, 20]

Prefer arguments and return values

Globals are convenient and they scale badly: any function might change one, so tracking down a wrong value means reading everything. Passing values in and handing results back keeps each function readable on its own:

def bump(value):
    return value + 1

count = 0
count = bump(count)
count = bump(count)
print(count)

2

Constants are the honourable exception. A configuration value that is set once and only read is a fine global, and the convention is to shout its name:

MAX_RETRIES = 3

def describe():
    return f"giving up after {MAX_RETRIES} tries"

print(describe())

giving up after 3 tries