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.. _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: 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 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 :func:`exec` function; .. _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 :func:`eval` is used for expression input. It ignores leading whitespace. The string argument to :func:`eval` must have the following form: .. productionlist:: eval_input: `expression_list` NEWLINE*