Normally, a Python source file must be entirely made up of characters from the ASCII set (character codes between 0 and 127). However, you may choose to tell Python that in a certain source file you are using a character set that is a superset of ASCII. In this case, Python allows that specific source file to contain characters outside the ASCII set, but only in comments and string literals. To accomplish this, start your source file with a comment whose form must be as rigid as the following:

  # -*- coding: utf-8 -*-

Between the coding: and the -*-, write the name of a codec known to Python, such as utf-8 or
iso-8859-1. Note that this coding directive comment is taken as such only if it is at the start of a source file (possibly after the "shebang line" covered in "Running Python Programs" on page 28), and that the only effect of a coding directive is to let you use non-ASCII characters in string literals and comments.

Python breaks each logical line into a sequence of elementary lexical components known as tokens. Each token corresponds to a substring of the logical line. The normal token types are identifiers, keywords, operators, delimiters, and literals, as covered in the following sections. You may freely use whitespace between tokens to separate them. Some whitespace separation is necessary between logically adjacent identifiers or keywords; otherwise, Python would parse them as a single, longer identifier. For example, printx is a single identifier; to write the keyword print followed by the identifier x, you need to insert some whitespace (e.g., print x).

Identifiers

An identifier is a name used to identify a variable, function, class, module, or other object. An identifier starts with a letter (A to Z or a to z) or an underscore (_) followed by zero or more letters, underscores, and digits (0 to 9). Case is significant in Python: lowercase and uppercase letters are distinct. Python does not allow punctuation characters such as @, $, and % within identifiers.

Normal Python style is to start class names with an uppercase letter and all other identifiers with a lowercase letter. Starting an identifier with a single leading underscore indicates by convention that the identifier is meant to be private. Starting an identifier with two leading underscores indicates a strongly private identifier; if the identifier also ends with two trailing underscores, the identifier is a language-defined special name. The identifier _ (a single underscore) is special in interactive interpreter sessions: the interpreter binds _ to the result of the last expression statement it has evaluated interactively, if any.