Python in a Nutshell, 2nd Edition

Variables and Other References

A Python program accesses data values through references. A reference is a name that refers to the location in memory of a value (object). References take the form of variables, attributes, and items. In Python, a variable or other reference has no intrinsic type. The object to which a reference is bound at a given time always has a type, but a given reference may be bound to objects of various types during the program’s execution.

Variables

In Python there are no declarations. The existence of a variable begins with a statement that binds the variable, or, in other words, sets a name to hold a reference to some object. You can also unbind a variable, resetting the name so it no longer holds a reference. Assignment statements are the most common way to bind variables and other references. The del statement unbinds references.

Binding a reference that was already bound is also known as rebinding it. Whenever I mention binding in this book, I implicitly include rebinding except where I explicitly exclude it. Rebinding or unbinding a reference has no effect on the object to which the reference was bound, except that an object disappears when nothing refers to it. The automatic cleanup of objects bereft of references is known as garbage collection.

You can name a variable with any identifier except the 30 that are reserved as Python’s keywords (see Keywords). A variable can be global or local. A global variable is an attribute of a module object (Chapter 7 ...