A few of the most common Python questions revolve around global variables. How can I create or use a global variable in a function? If you create a global variable in one function, how can it be used as a global variable in another function?
Check out our example of using global variables in a function below.
testvar = 0
def set_testvar_to_one():
global testvar # Needed to modify global copy of testvar
testvar = 1
def print_testvar():
print(testvar) # No need for global declaration to read value of testvar
set_testvar_to_one()
print_testvar() # Prints 1
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 new value anywhere within the function’s body, it’s assumed to be a local. If a variable is ever assigned a new value inside the function, the variable is implicitly local, and you need to explicitly declare it 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.
