Doc/lib/libfileinput.tex

   1 \section{\module{fileinput} ---
   2          Iterate over lines from multiple input streams}
   3 \declaremodule{standard}{fileinput}
   4 \moduleauthor{Guido van Rossum}{guido@python.org}
   5 \sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
   6
   7 \modulesynopsis{Perl-like iteration over lines from multiple input
   8 streams, with ``save in place'' capability.}
   9
  10
  11 This module implements a helper class and functions to quickly write a
  12 loop over standard input or a list of files.
  13
  14 The typical use is:
  15
  16 \begin{verbatim}
  17 import fileinput
  18 for line in fileinput.input():
  19     process(line)
  20 \end{verbatim}
  21
  22 This iterates over the lines of all files listed in
  23 \code{sys.argv[1:]}, defaulting to \code{sys.stdin} if the list is
  24 empty.  If a filename is \code{'-'}, it is also replaced by
  25 \code{sys.stdin}.  To specify an alternative list of filenames, pass
  26 it as the first argument to \function{input()}.  A single file name is
  27 also allowed.
  28
  29 All files are opened in text mode.  If an I/O error occurs during
  30 opening or reading a file, \exception{IOError} is raised.
  31
  32 If \code{sys.stdin} is used more than once, the second and further use
  33 will return no lines, except perhaps for interactive use, or if it has
  34 been explicitly reset (e.g. using \code{sys.stdin.seek(0)}).
  35
  36 Empty files are opened and immediately closed; the only time their
  37 presence in the list of filenames is noticeable at all is when the
  38 last file opened is empty.
  39
  40 It is possible that the last line of a file does not end in a newline
  41 character; lines are returned including the trailing newline when it
  42 is present.
  43
  44 The following function is the primary interface of this module:
  45
  46 \begin{funcdesc}{input}{\optional{files\optional{,
  47                        inplace\optional{, backup}}}}
  48   Create an instance of the \class{FileInput} class.  The instance
  49   will be used as global state for the functions of this module, and
  50   is also returned to use during iteration.
  51 \end{funcdesc}
  52
  53
  54 The following functions use the global state created by
  55 \function{input()}; if there is no active state,
  56 \exception{RuntimeError} is raised.
  57
  58 \begin{funcdesc}{filename}{}
  59   Return the name of the file currently being read.  Before the first
  60   line has been read, returns \code{None}.
  61 \end{funcdesc}
  62
  63 \begin{funcdesc}{lineno}{}
  64   Return the cumulative line number of the line that has just been
  65   read.  Before the first line has been read, returns \code{0}.  After
  66   the last line of the last file has been read, returns the line
  67   number of that line.
  68 \end{funcdesc}
  69
  70 \begin{funcdesc}{filelineno}{}
  71   Return the line number in the current file.  Before the first line
  72   has been read, returns \code{0}.  After the last line of the last
  73   file has been read, returns the line number of that line within the
  74   file.
  75 \end{funcdesc}
  76
  77 \begin{funcdesc}{isfirstline}{}
  78   Returns true the line just read is the first line of its file,
  79   otherwise returns false.
  80 \end{funcdesc}
  81
  82 \begin{funcdesc}{isstdin}{}
  83   Returns true if the last line was read from \code{sys.stdin},
  84   otherwise returns false.
  85 \end{funcdesc}
  86
  87 \begin{funcdesc}{nextfile}{}
  88   Close the current file so that the next iteration will read the
  89   first line from the next file (if any); lines not read from the file
  90   will not count towards the cumulative line count.  The filename is
  91   not changed until after the first line of the next file has been
  92   read.  Before the first line has been read, this function has no
  93   effect; it cannot be used to skip the first file.  After the last
  94   line of the last file has been read, this function has no effect.
  95 \end{funcdesc}
  96
  97 \begin{funcdesc}{close}{}
  98   Close the sequence.
  99 \end{funcdesc}
 100
 101
 102 The class which implements the sequence behavior provided by the
 103 module is available for subclassing as well:
 104
 105 \begin{classdesc}{FileInput}{\optional{files\optional{,
 106                              inplace\optional{, backup}}}}
 107   Class \class{FileInput} is the implementation; its methods
 108   \method{filename()}, \method{lineno()}, \method{fileline()},
 109   \method{isfirstline()}, \method{isstdin()}, \method{nextfile()} and
 110   \method{close()} correspond to the functions of the same name in the
 111   module.  In addition it has a \method{readline()} method which
 112   returns the next input line, and a \method{__getitem__()} method
 113   which implements the sequence behavior.  The sequence must be
 114   accessed in strictly sequential order; random access and
 115   \method{readline()} cannot be mixed.
 116 \end{classdesc}
 117
 118 \strong{Optional in-place filtering:} if the keyword argument
 119 \code{\var{inplace}=1} is passed to \function{input()} or to the
 120 \class{FileInput} constructor, the file is moved to a backup file and
 121 standard output is directed to the input file.
 122 This makes it possible to write a filter that rewrites its input file
 123 in place.  If the keyword argument \code{\var{backup}='.<some
 124 extension>'} is also given, it specifies the extension for the backup
 125 file, and the backup file remains around; by default, the extension is
 126 \code{'.bak'} and it is deleted when the output file is closed.  In-place
 127 filtering is disabled when standard input is read.
 128
 129 \strong{Caveat:} The current implementation does not work for MS-DOS
 130 8+3 filesystems.