Python in a Nutshell, 2nd Edition

Lexical Structure

The lexical structure of a programming language is the set of basic rules that govern how you write programs in that language. It is the lowest-level syntax of the language and specifies such things as what variable names look like and which characters denote comments. Each Python source file, like any other text file, is a sequence of characters. You can also usefully consider it as a sequence of lines, tokens, or statements. These different lexical views complement and reinforce each other. Python is very particular about program layout, especially with regard to lines and indentation, so you’ll want to pay attention to this information if you are coming to Python from another language.

Lines and Indentation

A Python program is composed of a sequence of logical lines, each made up of one or more physical lines. Each physical line may end with a comment. A hash sign (#) that is not inside a string literal begins a comment. All characters after the # and up to the physical line end are part of the comment, and the Python interpreter ignores them. A line containing only whitespace, possibly with a comment, is known as a blank line, and Python totally ignores it. In an interactive interpreter session, you must enter an empty physical line (without any whitespace or comment) to terminate a multiline statement.

In Python, the end of a physical line marks the end of most statements. Unlike in other languages, you don’t normally terminate Python statements with a delimiter, ...