Python Variable

A Python variable is a name bound to an object. The assignment max_age = 34 evaluates the right-hand side to the integer object 34 and binds the name max_age to it. The single equals sign means “bind this name,” not “these are equal”; equality is tested with ==.

The name has no fixed type; the object does. After max_age = "34", the same name refers to a string. This is dynamic typing: Python checks types when an operation runs rather than requiring a type declaration for the variable. Reassignment changes what the name refers to. After resetting max_age = 34, max_age + 1 produces 35 without rebinding the name. max_age = max_age + 1 rebinds it to that result. If the name still referred to "34", adding 1 would instead raise TypeError.

The “labeled box” picture helps at first but misleads for shared objects. After a = [1, 2] and b = a, both names refer to one list, so b.append(3) makes a show [1, 2, 3] too. Integers and strings cannot be modified in place; operations do not alter the existing integer or string object. A result need not always be a newly allocated object.

A valid name starts with a letter or underscore and continues with letters, digits, or underscores; it cannot contain spaces or symbols such as & or -. Names are case-sensitive, so age and Age are different. Keywords such as if, for, and class cannot be names. Built-in names such as print, list, and str are allowed, but assigning to one hides the built-in in that scope and causes confusing errors later. Python 3 accepts many non-ASCII letters, yet ASCII names in snake_case are the usual convention for shared code.

References: Python Reference: Naming and binding, Identifiers and keywords, PEP 8 naming conventions. See it in use in Python Foundations: Variables, Types, and the Notebook.