Doc/lib/libxreadlines.tex

   1 \section{\module{xreadlines} ---
   2          Efficient iteration over a file}
   3
   4 \declaremodule{extension}{xreadlines}
   5 \modulesynopsis{Efficient iteration over the lines of a file.}
   6
   7 \versionadded{2.1}
   8
   9
  10 This module defines a new object type which can efficiently iterate
  11 over the lines of a file.  An xreadlines object is a sequence type
  12 which implements simple in-order indexing beginning at \code{0}, as
  13 required by \keyword{for} statement or the
  14 \function{filter()} function.
  15
  16 Thus, the code
  17
  18 \begin{verbatim}
  19 import xreadlines, sys
  20
  21 for line in xreadlines.xreadlines(sys.stdin):
  22     pass
  23 \end{verbatim}
  24
  25 has approximately the same speed and memory consumption as
  26
  27 \begin{verbatim}
  28 while 1:
  29     lines = sys.stdin.readlines(8*1024)
  30     if not lines: break
  31     for line in lines:
  32         pass
  33 \end{verbatim}
  34
  35 except the clarity of the \keyword{for} statement is retained in the
  36 former case.
  37
  38 \begin{funcdesc}{xreadlines}{fileobj}
  39   Return a new xreadlines object which will iterate over the contents
  40   of \var{fileobj}.  \var{fileobj} must have a \method{readlines()}
  41   method that supports the \var{sizehint} parameter.
  42 \end{funcdesc}
  43
  44 An xreadlines object \var{s} supports the following sequence
  45 operation:
  46
  47 \begin{tableii}{c|l}{code}{Operation}{Result}
  48  \lineii{\var{s}[\var{i}]}{\var{i}'th line of \var{s}}
  49 \end{tableii}
  50
  51 If successive values of \var{i} are not sequential starting from
  52 \code{0}, this code will raise \exception{RuntimeError}.
  53
  54 After the last line of the file is read, this code will raise an
  55 \exception{IndexError}.