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https://github.com/andy-maier/immutable-views

Immutable views on other collection objects
https://github.com/andy-maier/immutable-views

collections immutable python view

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Immutable views on other collection objects

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immutable-views - Immutable views on other collection objects

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Overview

--------



The **immutable-views** package 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 the **immutable-views** package 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 the **immutable-views** package 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 class

`types.MappingProxyType `_

serves the same purpose as the

`DictView `_

class but it does not support pickling or hashing and was added only in

Python 3.3.

The ``dictproxy`` class from the

`dictproxyhack `_

package on Pypi supports Python 2 and Python 3 and uses Python classes where

available (e.g. ``MappingProxyType`` on Python 3.3 and later, and the internal

``mappingproxy`` class 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:



* `immutables `_

* `pyimmutable `_

* `frozenordereddict `_

* `immutabledict `_

* `frozendict `_

* `itypes `_

* `HashableDict `_

* `shoobx.immutable `_

* `immutable-collection `_

* `Dict-Path-Immutable `_



.. _`Examples`:



Examples

--------



Example with dictionaries:



.. code-block:: bash



    $ python

    >>> from immutable_views import DictView

    >>> 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'] = 2

    Traceback (most recent call last):

      File "", line 1, in 

    TypeError: 'DictView' object does not support item assignment



    # Modifications of the underlying collection are visible in the view:

    >>> dict1['a'] = 2

    >>> dictview1['a']

    2



Example with lists:



.. code-block:: bash



    $ python

    >>> from immutable_views import ListView

    >>> 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 (most recent call last):

      File "", line 1, in 

    TypeError: 'ListView' object does not support item assignment



    # Modifications of the underlying collection are visible in the view:

    >>> list1[0] = 'c'

    >>> listview1[0]

    'c'



Example with sets:



.. code-block:: bash



    $ python

    >>> from immutable_views import SetView

    >>> set1 = {'a', 'b'}

    >>> setview1 = SetView(set1)



    # Read-only access to the underlying collection through the view is supported:

    >>> 'a' in setview1

    True



    # Modifying the underlying collection through the view is rejected:

    >>> setview1.add('c')

    Traceback (most recent call last):

      File "", line 1, in 

    AttributeError: 'SetView' object has no attribute 'add'



    # Modifications of the underlying collection are visible in the view:

    >>> set1.add('c')

    >>> 'c' in setview1

    True



Documentation and change log

----------------------------



* `Documentation `_

* `Change log `_



License

-------



The **immutable-views** project is provided under the

`Apache Software License 2.0 `_.