Doc/ref8.tex

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