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OrderedDict vs. defaultdict in Python

Dario Vincenzo Tarantini

OrderedDict and defaultdict are both subclasses of dict. They have different uses.

OrderedDict

OrderedDict keeps the elements in the order that they were in when they were first inserted.

If we remove an element and re-add it, the element will be pushed to the back. If an element’s key changes, the position does not change.

Code

from collections import OrderedDict

mydict = OrderedDict([('a', '1'), ('b', '2'), ('c', '3')])

for k in mydict.items():
  print(k)
print('\n')
mydict.pop('b')
mydict['b'] = '2'
for k in mydict.items():
  print(k)

In the code above, we created an OrderedDict, added elements to it, and then printed them. Then, we removed one item and reentered it in the dict. It is now at the last index because the order of entry has changed.

defaultdict

A defaultdict is a useful subclass that doesn’t raise a KeyError exception when trying to access an undefined key. However, you must define a function that defines what you want to happen when a key is searched for that doesn’t exist. Let’s look at an example:

Code

from collections import defaultdict

def defval():
  return 'default value'

mydict = defaultdict(defval)
mydict['a'] = 1
mydict['b'] = 2
mydict['c'] = 3

for k in mydict.items():
  print(k)

print('\n') 

# if we try to get 'd' 
print(mydict['d']) 
# with a 'generic' dict this will raise a KeyError exception

print('\n') 

# it also add it to the dict
for k in mydict.items():
  print(k)

In the code above, we defined a function (defval()) that returns “default value” when a KeyError exception would normally be raised. Then, we defined a defaultdict, added items to it, and printed them. Then, we tried to get d, which is a value that’s not present. This also adds that value to the dict.

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Dario Vincenzo Tarantini
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