Variables¶

Local, non-local and global variables¶

Here you return to the definition of fact from the beginning of this Functions chapter:

>>> def fact(n):
...     """Return the factorial of the given number."""
...     f = 1
...     while n > 0:
...         f = f * n
...         n = n - 1
...     return f

Both the variables f and n are local to a particular call to the function fact; changes made to them during the execution of the function have no effect on variables outside the function. All variables in the parameter list of a function and all variables created within a function by an assignment, such as f = 1, are local to the function.

You can explicitly make a variable a global variable by declaring it with the global statement before it is used. Global variables can be accessed and changed by the function. They exist outside the function and can also be accessed and changed by other functions that declare them as global, or by code that is not inside a function. Here is an example that illustrates the difference between local and global variables:

>>> def my_func():
...     global x
...     x = 1
...     y = 2

>>> x = 3
>>> y = 4
>>> my_func()
>>> x
1
>>> y
4

In this example, a function is defined that treats x as a global variable and y as a local variable, and attempts to change both x and y. The assignment to x within my_func is an assignment to the global variable x, which also exists outside my_func. Since x is designated as global in my_func, the assignment changes this global variable so that it retains the value 1 instead of the value 3. However, the same is not true for y; the local variable y inside my_func initially refers to the same value as the variable y outside my_func, but the assignment causes y to refer to a new value that is local to the my_func function.