At this point, you might be wondering: how does Python know whether a variable used inside a function is a global variable or a local variable? Here are the rules that Python uses:

• If a variable appears on the left side of an assignment (to the left of the = symbol) and a global statement has been specified for the variable, it is a global variable.
• If a variable appears on the left side of an assignment and global has not been specified, a local variable is created. If a global variable of that name already exists, the global variable is ignored.
• If a variable appears on the right side of an assignment and has been defined as a local variable, the local variable is used.
• If a variable appears on the right side of an assignment and has not been defined as a local variable, the global variable is used.

See Sample Code for an illustration of these rules.

	If a local variable is created inside a function, it must be assigned to before it is used. For example, the following code is not legal: def FtoC(degf): mytemp = degc degc = (degf - 32) / 1.8 Here, because degc is assigned to, and global has not been specified, degc is defined as a local variable. The assignment to mytemp is an error because the local variable degc has not yet been defined.