Source code: Lib/UserDict.py
The module defines a mixin,
DictMixin, defining all dictionary methods for classes that already have a minimum mapping interface. This greatly simplifies writing classes that need to be substitutable for dictionaries (such as the shelve module).
This module also defines a class,
UserDict, that acts as a wrapper around dictionary objects. The need for this class has been largely supplanted by the ability to subclass directly from
dict (a feature that became available starting with Python version 2.2). Prior to the introduction of
UserDict class was used to create dictionary-like sub-classes that obtained new behaviors by overriding existing methods or adding new ones.
Class that simulates a dictionary. The instance’s contents are kept in a regular dictionary, which is accessible via the
data attribute of
UserDict instances. If initialdata is provided,
data is initialized with its contents; note that a reference to initialdata will not be kept, allowing it be used for other purposes.
For backward compatibility, instances of
UserDict are not iterable.
UserDict that supports direct iteration (e.g.
for key in
A real dictionary used to store the contents of the
This mixin should be used as a superclass. Adding each of the above methods adds progressively more functionality. For instance, defining all but
__delitem__() will preclude only
popitem() from the full interface.
When Python 2.2 was released, many of the use cases for this class were subsumed by the ability to subclass
list directly. However, a handful of use cases remain.
This module provides a list-interface around an underlying data store. By default, that data store is a
list; however, it can be used to wrap a list-like interface around other objects (such as persistent storage).
In addition, this class can be mixed-in with built-in classes using multiple inheritance. This can sometimes be useful. For example, you can inherit from
str at the same time. That would not be possible with both a real
list and a real
This module defines a class that acts as a wrapper around list objects. It is a useful base class for your own list-like classes, which can inherit from them and override existing methods or add new ones. In this way one can add new behaviors to lists.
Class that simulates a list. The instance’s contents are kept in a regular list, which is accessible via the
data attribute of
UserList instances. The instance’s contents are initially set to a copy of list, defaulting to the empty list
. list can be any iterable, e.g. a real Python list or a
In addition to supporting the methods and operations of mutable sequences (see section Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange),
UserList instances provide the following attribute:
A real Python list object used to store the contents of the
Subclassing requirements: Subclasses of
UserList are expected to offer a constructor which can be called with either no arguments or one argument. List operations which return a new sequence attempt to create an instance of the actual implementation class. To do so, it assumes that the constructor can be called with a single parameter, which is a sequence object used as a data source.
If a derived class does not wish to comply with this requirement, all of the special methods supported by this class will need to be overridden; please consult the sources for information about the methods which need to be provided in that case.
Changed in version 2.0: Python versions 1.5.2 and 1.6 also required that the constructor be callable with no parameters, and offer a mutable
data attribute. Earlier versions of Python did not attempt to create instances of the derived class.
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
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
UserString module defines the following classes:
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
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.
Deprecated since version 2.6: The
MutableString class has been removed in Python 3.
A real Python string or Unicode object used to store the content of the
© 2001–2017 Python Software Foundation
Licensed under the PSF License.