Package: immutable-views
OverviewTheimmutable-viewspackage provides collection classes that are immutable
views on other (mutable) collection objects:DictView-
immutable view on another mapping (dictionary) object.ListView-
immutable view on another sequence (list) object.SetView-
immutable view on another set object.An important behavior of views is that they are “live”: Since the view classes
delegate to the underlying collection, any modification of the underlying
collection object will be visible in the view object.Creating an immutable view on a collection does not copy the collection and
is therefore much faster than creating an immutable copy of the collection.The memory overhead of using immutable views is very small: An object
of any of the view classes in theimmutable-viewspackage occupies 40 Bytes
(measured in CPython 3.9 on macOS), and because the view object only has a
reference to its underlying collection object, that size is independent of the
number of items in the collection.The compute overhead is also very small, it is basically an additional function
call to the corresponding function of the underlying collection.Immutable views are useful if a method or function maintains data in form of a
mutable collection and is intended to return that data but users should not be
able to modify the data. The underlying collection can be updated by the method
or function as needed, but the caller only gets an immutable view on it.The view classes in theimmutable-viewspackage implement the complete
behavior of the corresponding Python collection types except for any
operations that would modify the underlying collection object.The view classes delegate all operations to the underlying collection object
they are a view of. Therefore, the underlying collection can be any kind of
collection implementation (i.e. not just the standard Python collection
classes).Note that the immutability of the view objects only applies to the view object
itself and to its underlying collection, but not to the items in the underlying
collection. So if the underlying collection contains mutable objects, they will
still be mutable when accessed through the view objects.The standard Python classtypes.MappingProxyTypeserves the same purpose as theDictViewclass but it does not support pickling or hashing and was added only in
Python 3.3.
Thedictproxyclass from thedictproxyhackpackage on Pypi supports Python 2 and Python 3 and uses Python classes where
available (e.g.MappingProxyTypeon Python 3.3 and later, and the internalmappingproxyclass used for__dict__on CPython) but also does not
support pickling or hashing.
The lack of support for standard dictionary behaviors prevents their use in
cases where the view class is used as a read-only replacement for the standard
dictionary.Note that there are several packages on Pypi that provide immutable
collections, but they all are collections on their own, and not views on
other collections. Here is a notable subset of such packages:immutablespyimmutablefrozenordereddictimmutabledictfrozendictitypesHashableDictshoobx.immutableimmutable-collectionDict-Path-ImmutableExamplesExample with dictionaries:$python>>>fromimmutable_viewsimportDictView>>>dict1={'a':1,'b':2}>>>dictview1=DictView(dict1)# Read-only access to the underlying collection through the view is supported:>>>dictview1['a']1# Modifying the underlying collection through the view is rejected:>>>dictview1['a']=2Traceback(mostrecentcalllast):File"<stdin>",line1,in<module>TypeError:'DictView'objectdoesnotsupportitemassignment# Modifications of the underlying collection are visible in the view:>>>dict1['a']=2>>>dictview1['a']2Example with lists:$python>>>fromimmutable_viewsimportListView>>>list1=['a','b']>>>listview1=ListView(list1)# Read-only access to the underlying collection through the view is supported:>>>listview1[0]'a'# Modifying the underlying collection through the view is rejected:>>>listview1[0]='c'Traceback(mostrecentcalllast):File"<stdin>",line1,in<module>TypeError:'ListView'objectdoesnotsupportitemassignment# Modifications of the underlying collection are visible in the view:>>>list1[0]='c'>>>listview1[0]'c'Example with sets:$python>>>fromimmutable_viewsimportSetView>>>set1={'a','b'}>>>setview1=SetView(set1)# Read-only access to the underlying collection through the view is supported:>>>'a'insetview1True# Modifying the underlying collection through the view is rejected:>>>setview1.add('c')Traceback(mostrecentcalllast):File"<stdin>",line1,in<module>AttributeError:'SetView'objecthasnoattribute'add'# Modifications of the underlying collection are visible in the view:>>>set1.add('c')>>>'c'insetview1TrueDocumentation and change logDocumentationChange logLicenseTheimmutable-viewsproject is provided under theApache Software License 2.0.