Top-level components

.. index:: single: interpreter

The Python interpreter can get its input from a number of sources: from a script passed to it as standard input or as program argument, typed in interactively, from a module source file, etc. This chapter gives the syntax used in these cases.

Complete Python programs

.. index:: single: program

.. index::
   pair: module; sys
   pair: module; __main__
   pair: module; builtins

While a language specification need not prescribe how the language interpreter is invoked, it is useful to have a notion of a complete Python program. A complete Python program is executed in a minimally initialized environment: all built-in and standard modules are available, but none have been initialized, except for :mod:`sys` (various system services), :mod:`builtins` (built-in functions, exceptions and None) and :mod:`__main__`. The latter is used to provide the local and global namespace for execution of the complete program.

The syntax for a complete Python program is that for file input, described in the next section.

.. index::
   single: interactive mode
   pair: module; __main__

The interpreter may also be invoked in interactive mode; in this case, it does not read and execute a complete program but reads and executes one statement (possibly compound) at a time. The initial environment is identical to that of a complete program; each statement is executed in the namespace of :mod:`__main__`.

.. index::
   single: UNIX
   single: Windows
   single: command line
   single: standard input

A complete program can be passed to the interpreter in three forms: with the :option:`-c` string command line option, as a file passed as the first command line argument, or as standard input. If the file or standard input is a tty device, the interpreter enters interactive mode; otherwise, it executes the file as a complete program.

File input

All input read from non-interactive files has the same form:

.. grammar-snippet::
   :group: python-grammar

   file_input: (NEWLINE | `statement`)* ENDMARKER

This syntax is used in the following situations:

when parsing a complete Python program (from a file or from a string);
when parsing a module;
when parsing a string passed to the :func:`exec` function;

Interactive input

Input in interactive mode is parsed using the following grammar:

.. grammar-snippet::
   :group: python-grammar

   interactive_input: [`stmt_list`] NEWLINE | `compound_stmt` NEWLINE | ENDMARKER

Note that a (top-level) compound statement must be followed by a blank line in interactive mode; this is needed to help the parser detect the end of the input.

Expression input

.. index:: single: input

.. index:: pair: built-in function; eval

:func:`eval` is used for expression input. It ignores leading whitespace. The string argument to :func:`eval` must have the following form:

.. grammar-snippet::
   :group: python-grammar

   eval_input: `expression_list` NEWLINE* ENDMARKER

Uh oh!