python_doc_md/Intermediate/reference/toplevel_components.md

Top-level components {#top-level}

::: 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.

Complete Python programs {#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 sys{.interpreted-text role="mod"} (various system services), builtins{.interpreted-text role="mod"} (built-in functions, exceptions and None) and __main__{.interpreted-text role="mod"}. 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 __main__{.interpreted-text role="mod"}.

::: 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 -c{.interpreted-text role="option"} 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:

::: {.grammar-snippet group="python-grammar"} file_input: (NEWLINE | [statement]{.title-ref})* ENDMARKER :::

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{.interpreted-text role="func"} function;

Interactive input {#interactive}

Input in interactive mode is parsed using the following grammar:

::: {.grammar-snippet group="python-grammar"} interactive_input: [[stmt_list]{.title-ref}] NEWLINE | [compound_stmt]{.title-ref} NEWLINE | ENDMARKER :::

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

::: index single: input :::

::: index pair: built-in function; eval :::

eval{.interpreted-text role="func"} is used for expression input. It ignores leading whitespace. The string argument to eval{.interpreted-text role="func"} must have the following form:

::: {.grammar-snippet group="python-grammar"} eval_input: [expression_list]{.title-ref} NEWLINE* ENDMARKER :::