Doc/ref/ref8.tex

   1 \chapter{Top-level components\label{top-level}}
   2
   3 The Python interpreter can get its input from a number of sources:
   4 from a script passed to it as standard input or as program argument,
   5 typed in interactively, from a module source file, etc.  This chapter
   6 gives the syntax used in these cases.
   7 \index{interpreter}
   8
   9
  10 \section{Complete Python programs\label{programs}}
  11 \index{program}
  12
  13 While a language specification need not prescribe how the language
  14 interpreter is invoked, it is useful to have a notion of a complete
  15 Python program.  A complete Python program is executed in a minimally
  16 initialized environment: all built-in and standard modules are
  17 available, but none have been initialized, except for \module{sys}
  18 (various system services), \module{__builtin__} (built-in functions,
  19 exceptions and \code{None}) and \module{__main__}.  The latter is used
  20 to provide the local and global namespace for execution of the
  21 complete program.
  22 \refbimodindex{sys}
  23 \refbimodindex{__main__}
  24 \refbimodindex{__builtin__}
  25
  26 The syntax for a complete Python program is that for file input,
  27 described in the next section.
  28
  29 The interpreter may also be invoked in interactive mode; in this case,
  30 it does not read and execute a complete program but reads and executes
  31 one statement (possibly compound) at a time.  The initial environment
  32 is identical to that of a complete program; each statement is executed
  33 in the namespace of \module{__main__}.
  34 \index{interactive mode}
  35 \refbimodindex{__main__}
  36
  37 Under {\UNIX}, a complete program can be passed to the interpreter in
  38 three forms: with the \programopt{-c} \var{string} command line option, as a
  39 file passed as the first command line argument, or as standard input.
  40 If the file or standard input is a tty device, the interpreter enters
  41 interactive mode; otherwise, it executes the file as a complete
  42 program.
  43 \index{UNIX}
  44 \index{command line}
  45 \index{standard input}
  46
  47
  48 \section{File input\label{file-input}}
  49
  50 All input read from non-interactive files has the same form:
  51
  52 \begin{productionlist}
  53   \production{file_input}
  54              {(NEWLINE | \token{statement})*}
  55 \end{productionlist}
  56
  57 This syntax is used in the following situations:
  58
  59 \begin{itemize}
  60
  61 \item when parsing a complete Python program (from a file or from a string);
  62
  63 \item when parsing a module;
  64
  65 \item when parsing a string passed to the \keyword{exec} statement;
  66
  67 \end{itemize}
  68
  69
  70 \section{Interactive input\label{interactive}}
  71
  72 Input in interactive mode is parsed using the following grammar:
  73
  74 \begin{productionlist}
  75   \production{interactive_input}
  76              {[\token{stmt_list}] NEWLINE | \token{compound_stmt} NEWLINE}
  77 \end{productionlist}
  78
  79 Note that a (top-level) compound statement must be followed by a blank
  80 line in interactive mode; this is needed to help the parser detect the
  81 end of the input.
  82
  83
  84 \section{Expression input\label{expression-input}}
  85 \index{input}
  86
  87 There are two forms of expression input.  Both ignore leading
  88 whitespace.
  89 The string argument to \function{eval()} must have the following form:
  90 \bifuncindex{eval}
  91
  92 \begin{productionlist}
  93   \production{eval_input}
  94              {\token{expression_list} NEWLINE*}
  95 \end{productionlist}
  96
  97 The input line read by \function{input()} must have the following form:
  98 \bifuncindex{input}
  99
 100 \begin{productionlist}
 101   \production{input_input}
 102              {\token{expression_list} NEWLINE}
 103 \end{productionlist}
 104
 105 Note: to read `raw' input line without interpretation, you can use the
 106 built-in function \function{raw_input()} or the \method{readline()} method
 107 of file objects.
 108 \obindex{file}
 109 \index{input!raw}
 110 \index{raw input}
 111 \bifuncindex{raw_input}
 112 \withsubitem{(file method)}{\ttindex{readline()}}