Python Cookbook

Dispatching Based on Pattern Matches

Credit: Michael Robin

Problem

You need to use regular expressions to match strings and then automatically call functions with arguments based on the matched strings.

Solution

Once again, a class offers a good way to package together some state and some behavior:

import re

class Dispatcher:

    def _dispatch(self, cmdList, str):
        """ Find a match for str in the cmdList and call the associated
           method with arguments that are the matching grouped subexpressions
           from the regex.
        """
        for comment, pattern, command in cmdList:
             found = pattern.match(str)   # or, use .search(  )
             if found: return command(self, *found.groups(  ))

    def runCommand(self, cmd):
        self._dispatch(Commands, cmd)

    # example methods

    def cmd1(self, num, name):
        print "The number for %s is %d" % (name, int(num))
        return 42

    def cmd2(self, partnum):
        print "Widget serial #: %d" % int(partnum)

Commands = [
      [ 'Number-to-name correspondence',
              r'X (?P<num>\d+),(?P<name>.*)$',
              Dispatcher.cmd1],
      [ 'Extract Widget part-number',
              r'Widget (?P<partnum>.*)$',
              Dispatcher.cmd2],
]

# Prepare the Commands list for execution by compiling each re for cmd in Commands:
    try:
     cmd[1] = re.compile( cmd[1] )
    except:
     print "Bad pattern for %s: %s" % ( cmd[0], cmd[1] )

Discussion

In Python, it’s generally best to compile regular expressions into re objects. The re module does some caching of string-form regular expressions that you use directly, but it’s still better to make sure that regular expressions are not needlessly recompiled. ...