You can consider a Python source file as a sequence of simple and compound statements. Unlike other languages, Python has no declarations or other top-level syntax elements, just statements.

A simple statement is one that contains no other statements. A simple statement lies entirely within a logical line. As in other languages, you may place more than one simple statement on a single logical line, with a semicolon (;) as the separator. However, one statement per line is the usual Python style, and makes programs more readable.

Lexical Structure

Any expression can stand on its own as a simple statement (I'll discuss expressions in detail in "Expressions and Operators" on page 50). The interactive interpreter shows the result of an expression statement you enter at the prompt (>>>) and binds the result to a variable named _ (a single underscore). Apart from interactive sessions, expression statements are useful only to call functions (and other callables) that have side effects (e.g., ones that perform output, change global variables, or raise exceptions).

An assignment is a simple statement that assigns values to variables, as I"ll discuss in "Assignment Statements" on page 47. Unlike in some other languages, an assignment in Python is a statement and can never be part of an expression.

A compound statement contains one or more other statements and controls their execution. A compound statement has one or more clauses, aligned at the same indentation. Each clause has a header starting with a keyword and ending with a colon (:), followed by a body, which is a sequence of one or more statements. When the body contains multiple statements, also known as a block, these statements should be placed on separate logical lines after the header line, indented four spaces rightward. The block lexically ends when the indentation returns to that of the clause header (or further left from there, to the indentation of some enclosing compound statement). Alternatively, the body can be a single simple statement, following the : on the same logical line as the header. The body may also consist of several simple statements on the same line with semicolons between them, but, as I've already indicated, this is not good style.

Please check back next week for the continuation of this series.