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), __builtin__ (built-in functions,
exceptions and None) and __main__. The latter is used
to provide the local and global name space 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 name space 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.