Is it possible to define global variables in a function in Python

Question

How do I declare a global variable in a function in Python?

That is, so that it doesn't have to be declared before but can be used outside of the function.

Solution 1

Yes, but why?

def a():
    globals()['something'] = 'bob'

Solution 2

def function(arguments):
    global var_name
    var_name = value #must declare global prior to assigning value

This will work in any function, regardless of it is in the same program or not.

Here's another way to use it:

def function():
    num = #code assigning some value to num
    return num

NOTE: Using the return built-in will automatically stop the program (or the function), regardless of whether it is finished or not.

You can use this in a function like this:

if function()==5 #if num==5:
    #other code

This would allow you to use the variable outside of the function. Doesn't necessarily have to be declared global.

In addition, to use a variable from one function to another, you can do something like this:

import primes as p #my own example of a module I made
p.prevPrimes(10) #generates primes up to n
for i in p.primes_dict:
    if p.primes_dict[i]: #dictionary contains only boolean values
        print p.primes_dict[i]

This will allow you to use the variable in another function or program without having use a global variable or the return built-in.

Comments

[Ashwini Chaudhary]

just do, global something, it will create a new global variable if it doesn't exist.

[Jon Clements]

explicit rather than implicit... - this makes it more "obvious" as to it's a bad idea...

[Terrence Brannon]

it's not global is it? I think it's module-wide.

[Rushy Panchal]

Ah yes, it is only module-wide. However, using the return function should fix that.

[WoooHaaaa]

@F3AR3DLEGEND globals my_var does usable outside the function, i've tried that.

[ideasman42]

While correct, this isn't standard practice. in most cases its best to use global keyword.

[TheTechRobo Stands for Ukraine]

it may not be standard practice but it saved me a codefactor headache

[Azmisov]

I believe this is the only way to do it if "something" variable name is programmatically generated

[TheTechRobo Stands for Ukraine]

@Azmisov either that or exec

[Admin]

I can think of at least one use-case: I want to mass-produce a bunch of variables in a module (variables whose name would be determined by a set of arguments) and I want to be able to import these variables whenever the module is imported. Is there a cleaner way to do such a thing?