Python Library Reference

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3.9 UserString -- Class wrapper for string objects

Note: This UserString class from this module is available for backward compatibility only. If you are writing code that does not need to work with versions of Python earlier than Python 2.2, please consider subclassing directly from the built-in str type instead of using UserString (there is no built-in equivalent to MutableString).

This module defines a class that acts as a wrapper around string objects. It is a useful base class for your own string-like classes, which can inherit from them and override existing methods or add new ones. In this way one can add new behaviors to strings.

It should be noted that these classes are highly inefficient compared to real string or Unicode objects; this is especially the case for MutableString.

The UserString module defines the following classes:

class UserString(

[sequence])

Class that simulates a string or a Unicode string object. The instance's content is kept in a regular string or Unicode string object, which is accessible via the data attribute of UserString instances. The instance's contents are initially set to a copy of sequence. sequence can be either a regular Python string or Unicode string, an instance of UserString (or a subclass) or an arbitrary sequence which can be converted into a string using the built-in str() function.

class MutableString(

[sequence])

This class is derived from the UserString above and redefines strings to be mutable. Mutable strings can't be used as dictionary keys, because dictionaries require immutable objects as keys. The main intention of this class is to serve as an educational example for inheritance and necessity to remove (override) the __hash__() method in order to trap attempts to use a mutable object as dictionary key, which would be otherwise very error prone and hard to track down.

In addition to supporting the methods and operations of string and Unicode objects (see section 2.3.6, ``String Methods''), UserString instances provide the following attribute:

data

A real Python string or Unicode object used to store the content of the UserString class.

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Python Library Reference

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Release 2.4.1, documentation updated on 30 March 2005.