How to use a global variable in a function?

How do I create or use a global variable inside a function?

How do I use a global variable that was defined in one function inside other functions?

_{Failing to use the global keyword where appropriate often causes UnboundLocalError. The precise rules for this are explained at UnboundLocalError on local variable when reassigned after first use. Generally, please close other questions as a duplicate of that question when an explanation is sought, and this question when someone simply needs to know the global keyword.}

You can use a global variable within other functions by declaring it as global within each function that assigns a value to it:

globvar = 0

def set_globvar_to_one():
    global globvar    # Needed to modify global copy of globvar
    globvar = 1

def print_globvar():
    print(globvar)     # No need for global declaration to read value of globvar

set_globvar_to_one()
print_globvar()       # Prints 1

Since it's unclear whether globvar = 1 is creating a local variable or changing a global variable, Python defaults to creating a local variable, and makes you explicitly choose the other behavior with the global keyword.

See other answers if you want to share a global variable across modules.