Python Cookbook

Replacing Multiple Patterns in a Single Pass

Credit: Xavier Defrang

Problem

You need to perform several string substitutions on a string.

Solution

Sometimes regular expressions afford the fastest solution even in cases where their applicability is anything but obvious. In particular, the sub method of re objects makes regular expressions a good way to perform string substitutions. Here is how you can produce a result string from an input string where each occurrence of any key in a given dictionary is replaced by the corresponding value in the dictionary:

# requires Python 2.1 or later
from _ _future_ _ import nested_scopes

import re

# the simplest, lambda-based implementation
def multiple_replace(adict, text):
  # Create a regular expression from all of the dictionary keys
  regex = re.compile("|".join(map(re.escape, adict.keys(  ))))

  # For each match, look up the corresponding value in the dictionary
  return regex.sub(lambda match: adict[match.group(0)], text)

A more powerful and flexible approach is to wrap the dictionary into a callable object that directly supports the lookup and replacement idea, which you can use directly as the callback in the sub method. This object-oriented approach is more flexible because the callable object can keep its own state and therefore is easily extensible to other tasks. In Python 2.2 and later, you can create a class for this object by extending the dict built-in type, while in older Python versions you must fall back on UserDict.UserDict (built-in ...