Python in a Nutshell

Variables and Other References

A Python program accesses data values through references. A reference is a name that refers to the specific 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 does have a type, however. Any given reference may be bound to objects of different types during the execution of a program.

Variables

In Python, there are no declarations. The existence of a variable depends on a statement that binds the variable, or, in other words, that sets a name to hold a reference to some object. You can also unbind a variable by 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 binding is mentioned in this book, rebinding is implicitly included except where it is explicitly excluded. 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 to which there are no references is known as garbage collection.

You can name a variable with any identifier except the 29 that are reserved as Python’s keywords (see Section 4.1.2.2 earlier in this chapter). A variable ...