(Im)mutability of OrderedDict-CodePudding

Is Python's OrderedDict data structure a mutable collection?

The documentation does not say anything about it, and after a quick search on the web, I have not found any answer yet.

CodePudding user response：

The documentation does not say anything about it, and after a quick search on the web, I have not found any answer yet.

When in doubt regarding mutability of class you might run following test

from collections import OrderedDict
from collections.abc import MutableSequence, MutableSet, MutableMapping
print(issubclass(OrderedDict,(MutableSequence,MutableSet,MutableMapping)))

output

True

See Collections Abstract Base Classes in collections.abc docs for other things which can be checked this way.

CodePudding user response：

OrderedDict is mutable:

from collections import OrderedDict

d = OrderedDict.fromkeys('abcde')
d['a']= 'aretor'
print(d)

>> OrderedDict([('a', 'aretor'), ('b', None), ('c', None), ('d', None), ('e', None)])

print(d['a'])
>> aretor

CodePudding user response：

From realpython.com:

Since OrderedDict is a mutable data structure, you can perform mutating operations on its instances. You can insert new items, update and remove existing items, and so on. If you insert a new item into an existing ordered dictionary, then the item is added to the end of the dictionary:

They provide a couple of examples for mutable operations like insert:

>>> from collections import OrderedDict

>>> numbers = OrderedDict(one=1, two=2, three=3)
>>> numbers
OrderedDict([('one', 1), ('two', 2), ('three', 3)])

>>> numbers["four"] = 4
>>> numbers
OrderedDict([('one', 1), ('two', 2), ('three', 3), ('four', 4)])

Generally, if a datatype supports the following operations, then it is considered a mutable sequence type.