.. _top-level:

********************

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.

.. _programs:

Complete Python programs

========================

.. index:: single: program

.. index::

   module: sys

   module: __main__

   module: __builtin__

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:`__builtin__` (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

   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: command line

   single: standard input

Under Unix, 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:

File input

==========

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

.. productionlist::

   file_input: (NEWLINE | `statement`)*

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 :keyword:`exec` statement;

.. _interactive:

Interactive input

=================

Input in interactive mode is parsed using the following grammar:

.. productionlist::

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

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:

Expression input

================

.. index:: single: input

.. index:: builtin: eval

There are two forms of expression input.  Both ignore leading whitespace. The

string argument to :func:`eval` must have the following form:

.. productionlist::

   eval_input: `expression_list` NEWLINE*

.. index:: builtin: input

The input line read by :func:`input` must have the following form:

.. productionlist::

   input_input: `expression_list` NEWLINE

.. index::

   object: file

   single: input; raw

   single: raw input

   builtin: raw_input

   single: readline() (file method)

Note: to read 'raw' input line without interpretation, you can use the built-in

function :func:`raw_input` or the :meth:`readline` method of file objects.