
Top-level components
********************

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

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 ``sys`` (various
system services), ``builtins`` (built-in functions, exceptions and
``None``) and ``__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.

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 ``__main__``.

Under Unix, a complete program can be passed to the interpreter in
three forms: with the *-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:

   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 ``exec()`` function;


Interactive input
=================

Input in interactive mode is parsed using the following 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
================

There are two forms of expression input.  Both ignore leading
whitespace. The string argument to ``eval()`` must have the following
form:

   eval_input ::= expression_list NEWLINE*

Note: to read 'raw' input line without interpretation, you can use the
the ``readline()`` method of file objects, including ``sys.stdin``.
