105 CHAPTER NINE TOP-LEVEL COMPONENTS 9.1 Complete Python programs 9.2 File input 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. 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$_{__main__}$ . 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. 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 string passed to the$_{exec()}$ function; • when parsing a module; • when parsing a complete Python program (from a file or from a string);