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__
.
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.
9.4. 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*