9. 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.
9.1. 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.
9.2. 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;
9.3. 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.