35.10. "pipes" --- Interface to shell pipelines
***********************************************

**源代码：** Lib/pipes.py

======================================================================

The "pipes" module defines a class to abstract the concept of a
*pipeline* --- a sequence of converters from one file to  another.

Because the module uses **/bin/sh** command lines, a POSIX or
compatible shell for "os.system()" and "os.popen()" is required.

The "pipes" module defines the following class:

class pipes.Template

   An abstraction of a pipeline.

示例:

   >>> import pipes
   >>> t = pipes.Template()
   >>> t.append('tr a-z A-Z', '--')
   >>> f = t.open('pipefile', 'w')
   >>> f.write('hello world')
   >>> f.close()
   >>> open('pipefile').read()
   'HELLO WORLD'


35.10.1. Template Objects
=========================

Template objects following methods:

Template.reset()

   Restore a pipeline template to its initial state.

Template.clone()

   Return a new, equivalent, pipeline template.

Template.debug(flag)

   If *flag* is true, turn debugging on. Otherwise, turn debugging
   off. When debugging is on, commands to be executed are printed, and
   the shell is given "set -x" command to be more verbose.

Template.append(cmd, kind)

   Append a new action at the end. The *cmd* variable must be a valid
   bourne shell command. The *kind* variable consists of two letters.

   The first letter can be either of "'-'" (which means the command
   reads its standard input), "'f'" (which means the commands reads a
   given file on the command line) or "'.'" (which means the commands
   reads no input, and hence must be first.)

   Similarly, the second letter can be either of "'-'" (which means
   the command writes to standard output), "'f'" (which means the
   command writes a file on the command line) or "'.'" (which means
   the command does not write anything, and hence must be last.)

Template.prepend(cmd, kind)

   Add a new action at the beginning. See "append()" for explanations
   of the arguments.

Template.open(file, mode)

   Return a file-like object, open to *file*, but read from or written
   to by the pipeline.  Note that only one of "'r'", "'w'" may be
   given.

Template.copy(infile, outfile)

   Copy *infile* to *outfile* through the pipe.
