Python OrderedDict – Reordering, Delete and Reinsert New OrderedDict

Free Python course with 25 projects (coupon code: DATAFLAIR_PYTHON) Start Now

1. Python OrderedDict

We’ve been talking about classes from Python collections modulepython namedtuple, and python defaultdict. Today, we base our article on Python OrderedDict, another class in ‘collections’. Here we study what is Python OrderedDict with its syntax and examples. Moreover, we will study comparing to a regular dictionary, and reordering a OrderedDict in Python. At last, we will cover how to delete and reinsert a key for new ordereddict, and popitem method.

So, let’s start the Python OderedDict Tutorial.

Python OrderedDict - Reordering, Delete and Reinsert New OrderedDict

Python OrderedDict – Reordering, Delete and Reinsert New OrderedDict

2. Introduction to Python OrderedDict

Python OrderedDict is just like a regular dictionary in python, but it also saves the order in which pairs were added. Actually, it is a subclass of ‘dict’.

>>> issubclass(OrderedDict,dict)


3. Python OrderedDict Syntax

To define an OrderedDict in python, we import it from the module ‘collections’. Let us see this Python ordereddict example.

>>> from collections import OrderedDict

Then, we call this factory function and assign it to a variable in python.

>>> d=OrderedDict()

We now add some key-value pairs to it.

>>> d['a']=1
>>> d['e']=5
>>> d['d']=4
>>> d['b']=2
>>> d['c']=3

Now, if we access ‘d’, we see that it preserved the order in which we defined the key-value pairs.

>>> d

OrderedDict([(‘a’, 1), (‘e’, 5), (‘d’, 4), (‘b’, 2), (‘c’, 3)])

4. Comparing to a Regular Dictionary

Let’s try this with a regular dictionary. If we declared this as a regular dictionary, it would produce the pairs according to how the keys are stored in a hash table. This depends on a random value to reduce collisions.

a. Equality

To a regular dictionary, the order does not matter. Let u take another Python ordereddict example.

>>> {'a': 1, 'e': 5, 'd': 4, 'b': 2, 'c': 3}=={'a': 1, 'b': 2, 'c': 3, 'd': 4, 'e': 5}

To Python, these two are equal. Let’s create another OrderedDict, and check if it is the same as the OrderedDict ‘d’ we defined above.

>>> e=OrderedDict()
>>> e['a']=1
>>> e['b']=2
>>> e['c']=3
>>> e['d']=4
>>> e['e']=5
>>> e

OrderedDict([(‘a’, 1), (‘b’, 2), (‘c’, 3), (‘d’, 4), (‘e’, 5)])
Now, let’s check if they’re equal.

>>> d==e

As you can see, they’re not.

5. How to Reorder Python OrderedDict?

To reorder a key-value pair to the front or the end of an OrderedDict in python, we have the move_to_end() method. Look, this is ‘d’:

>>> d

OrderedDict([(‘a’, 1), (‘e’, 5), (‘d’, 4), (‘b’, 2), (‘c’, 3)])
Now, let’s move (‘e’,5) to the end.

>>> d.move_to_end('e')
>>> d

OrderedDict([(‘a’, 1), (‘d’, 4), (‘b’, 2), (‘c’, 3), (‘e’, 5)])

This method takes a key as an argument. But it may also take another argument- ‘last’. This may take one of two values: True or False. When true, the item is moved to the end. And when false, it is moved to the front.

>>> d.move_to_end('a',last=True)
>>> d

OrderedDict([(‘d’, 4), (‘b’, 2), (‘c’, 3), (‘e’, 5), (‘a’, 1)])
Here, we moved (‘a’,1) to the end. Now, let’s move it back to the front.

>>> d.move_to_end('a',last=False)
>>> d

OrderedDict([(‘a’, 1), (‘d’, 4), (‘b’, 2), (‘c’, 3), (‘e’, 5)])
Now, to sort this, we do the following:

>>> d.move_to_end('b')
>>> d

OrderedDict([(‘a’, 1), (‘d’, 4), (‘c’, 3), (‘e’, 5), (‘b’, 2)])

>>> d.move_to_end('c')
>>> d

OrderedDict([(‘a’, 1), (‘d’, 4), (‘e’, 5), (‘b’, 2), (‘c’, 3)])

>>> d.move_to_end('d')
>>> d

OrderedDict([(‘a’, 1), (‘e’, 5), (‘b’, 2), (‘c’, 3), (‘d’, 4)])

>>> d.move_to_end('e')
>>> d

OrderedDict([(‘a’, 1), (‘b’, 2), (‘c’, 3), (‘d’, 4), (‘e’, 5)])

6. Assigning a Key Again in Python OrderedDict

Let’s define a new OrderedDict in Python for this example.

>>> f=OrderedDict()
>>> f['c']=3
>>> f['e']=5
>>> f['a']=1
>>> f['b']=2
>>> f['d']=4
>>> f

OrderedDict([(‘c’, 3), (‘e’, 5), (‘a’, 1), (‘b’, 2), (‘d’, 4)])
Now, let’s try changing the value of key ‘e’.

>>> f['e']=7
>>> f

OrderedDict([(‘c’, 3), (‘e’, 7), (‘a’, 1), (‘b’, 2), (‘d’, 4)])
Okay. Since the key ‘e’ already exists, the old order is preserved; it isn’t ordered to the end.

7. Deleting and Reinserting a Key

When we delete a key and then reinsert it, it is ordered to the end of the new order. Let’s try deleting ‘e’.

>>> f.pop('e')


>>> f

OrderedDict([(‘c’, 3), (‘a’, 1), (‘b’, 2), (‘d’, 4)])
Now, let’s insert it again.

>>> f

OrderedDict([(‘c’, 3), (‘a’, 1), (‘b’, 2), (‘d’, 4), (‘e’, 7)])
As you can see, it is at the back of the list now.

8. Python Popitem ()

The popitem() method in Python may take 0 or 1 argument- ‘last’. When ‘last’ is true, LIFO is followed to delete, otherwise, FIFO is followed. When we don’t pass an argument, last is assumed to be true.
Let’s take a new Python OrderedDict for this.

>>> g=OrderedDict()
>>> g['d']=4
>>> g['b']=2
>>> g['a']=1
>>> g['c']=3
>>> g['e']=5
>>> g

OrderedDict([(‘d’, 4), (‘b’, 2), (‘a’, 1), (‘c’, 3), (‘e’, 5)])
Now, let’s start popping.

>>> g.popitem(last=False)

(‘d’, 4)

>>> g.popitem()

(‘e’, 5)

>>> g.popitem(last=True)

(‘c’, 3)

So, this was all about Python OrderedDict Tutorial. Hope you like our explanation.

9. Conclusion

Hopefully, we’ve covered every important detail about python orderedDict in this article. We saw how to define them, Syntax, deleting and reinserting, assigning A key to python ordereddict, and we saw methods move_to_end(), popitem(), and pop(). Next, we will discuss Counter python module, yet another class from collections.
See Also- Python Features & Python Installation
For reference

1 Response

  1. Will says:

    I think you are missing a line of code. In section 7, the page reads:

    Now, let’s insert it again.

    >>> f

    should there be a line above this that says something along the lines of “>>> f[‘e’] = 7” ?

Leave a Reply

Your email address will not be published. Required fields are marked *

This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.