Scope of variables

The scope of a variable is the part of the program where it can be used. A variable is either global or local.

• A global variable is defined outside all functions. It can be used (read) inside any function defined after it.
• A local variable is defined inside a function (or block). It can be used only there, and it exists only while the function is running. Trying to use it outside the function gives a NameError (Program 7-14).

If a function assigns a value to a variable with the same name as a global one, Python treats it as a new local variable inside that function. The local one hides the global one, and the global variable is not changed.

To change a global variable from inside a function, declare it first with the keyword global:

• count = 0
• def addOne(): global count; count = count + 1

After addOne() is called, count is 1 everywhere in the program (the book's Program 7-15 does the same with num).