.. _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::
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:
File input
==========
All input read from non-interactive files has the same form:
.. productionlist:: python-grammar
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 :func:`exec` function;
.. _interactive:
Interactive input
=================
Input in interactive mode is parsed using the following grammar:
.. productionlist:: python-grammar
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:: 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:
.. productionlist:: python-grammar
eval_input: `expression_list` NEWLINE*
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