Python 3.6.5 Documentation >  Top-level components

Top-level components
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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
================

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

eval_input ::= expression_list NEWLINE*