Python Language Lesson: local and global variables

Python Language Lesson: local and global variables

By default, variables that are introduced in functions have only a
limited lifetime and visibility. They are visible only in the function in
which they were created and they disappear when execution returns
from the function.


In  the variable y is local to the function first
function(). You cannot use it outside this function. In contrast,
keyword global declares that the variable x belongs to the global
scope, which covers all functions within a module. For this reason, you
can use x also in the function second function(). If a variable
is defined outside any function, it belongs automatically to the global
scope, such as the variable z.
A good rule of thumb is that if the same variable must be used in
more than one function, it must be declared as global or it must be
passed as a parameter to the functions.
Note that you have to assign a value to a variable before it can
be used. It is meaningless to refer to a variable which does not
have any content. Thus, in the example above you must call the
function first function() before calling second function()
or Python will complain NameError: global variable "x"is
not defined. This error means that no value has been assigned to
the variable yet or it was not declared as global.