How and When Should You Use Defaultdict in Python?

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On this tutorial you’ll learn to use it default from Python’s assortment module (to higher deal with KeyErrors) when working with Python dictionaries.

In Python, a dictionary is a strong built-in information construction that shops information in key-value pairs. You utilize the keys to faucet into the dictionary and entry the values.

Nevertheless, when you’ve got a number of dictionaries in your Python script which might be modified throughout code execution, you’ll usually encounter KeyErrors. And there are just a few other ways you possibly can take care of it.

On this tutorial you’ll be taught:

  • What KeyErrors are and why they happen
  • Easy methods to take care of KeyErrors
  • Easy methods to use Python’s defaultdict, a subclass that inherits from the built-in dict class, to higher deal with lacking keys

Let’s begin!

What are KeyErrors in Python?

1

When defining a Python dictionary, be sure to maintain the next:

  • The keys have to be distinctive – with none repetition.
  • When utilizing an current iterable because the keys of a dictionary, you must favor to make use of an immutable assortment akin to a tuple.

So a key’s solely legitimate whether it is current within the dictionary; in any other case it results in KeyErrors.

Contemplate the next dictionary, books_authorsthe place the keys are the names of the books and the values ​​are the names of the authors.

You may code in a Python REPL together with this tutorial.

books_authors = {
    'Deep Work':'Cal Newport',
    'Hyperfocus':'Chris Bailey',
    'Pivot':'Jenny Blake',
    'The Happiness Equation':'Neil Pasricha'
}

You should use the important thing (identify of the guide) to entry the creator’s identify.

books_authors['Hyperfocus']
'Chris Bailey'

To entry all key-value pairs within the dictionary, you should use the gadgets() technique on the dictionary object, as proven under:

for guide,creator in books_authors.gadgets():
  print(f"'{guide}' by {creator}")
'Deep Work' by Cal Newport
'Hyperfocus' by Chris Bailey
'Pivot' by Jenny Blake
'The Happiness Equation' by Neil Pasricha

In case you attempt to entry the worth of a key that’s not current within the dictionary, the Python interpreter throws a KeyError. We encounter KeyError once we attempt to entry the worth of keys that do not exist, particularly ‘Grit’ and ‘non-existent key’.

books_authors['Grit']
---------------------------------------------------------------------------
KeyError                                  Traceback (most up-to-date name final)
<ipython-input-6-e1a4486f5ced> in <module>
----> 1 books_authors['Grit']

KeyError: 'Grit'
books_authors['non-existent-key']
---------------------------------------------------------------------------
KeyError                                  Traceback (most up-to-date name final)
<ipython-input-7-a3efd56f69e5> in <module>
----> 1 books_authors['non-existent-key']

KeyError: 'non-existent-key'

So how do you take care of KeyErrors in Python?

There are some methods to do that, and we’ll be taught them within the subsequent part.

Easy methods to take care of key errors in Python

How to deal with key errors in Python

Let’s learn to deal with KeyErrors utilizing:

  • If-else conditional statements
  • Attempt-except blocks
  • The dictionary technique .get()

#1. Utilizing If-Else Conditional Statements

One of many easiest methods to deal with KeyErrors in Python is to make use of the if-else conditional statements.

In Python, if-else statements have the next normal syntax:

 if situation:
 	# do that 
 else:
    # do one thing else
  • If the situation is Truethe statements within the if physique is executed, and
  • If the situation is Falsethe statements within the else physique is executed.

On this instance, the situation is to verify if the bottom line is current within the dictionary.

If the bottom line is current within the dictionary, the in operator will return Trueand if the physique is executed, the corresponding worth will probably be printed.

key = 'The Happiness Equation'
if key in books_authors:
  print(books_authors[key])
else:
  print('Sorry, this key doesn't exist!')

# Output
# Neil Pasricha

If the bottom line is not current within the dictionary, the in operator returns False and the else physique will probably be executed. A message is printed that the bottom line is not current.

key = 'non-existent-key'
if key in books_authors:
  print(books_authors[key])
else:
  print('Sorry, this key doesn't exist!')

# Output
# Sorry, this key doesn't exist!

#2. Use Attempt-Besides statements

2

One other frequent technique to deal with KeyError is to make use of the try-except statements in Python.

Learn by the next code block:

key = 'non-existent-key'
attempt:
  print(books_authors[key])
besides KeyError:
  print('Sorry, this key doesn't exist!')
  • The try block makes an attempt to retrieve the worth equivalent to the required key.
  • If the bottom line is not current, the interpreter throws a KeyError that throws an exception inside the besides block.

#3. Utilizing the .get() technique

In Python, you should use the built-in dictionary technique .get() to deal with lacking keys.

The final syntax for utilizing the get() technique is dict.get(key,default_value) The place dict is a legitimate dictionary object in Python.

– If the bottom line is current within the dictionary, then the get() technique returns the worth.
– In any other case it returns the default worth.

On this instance keys is an inventory of keys whose values ​​we wish to entry. We undergo the important thing record to get the corresponding values ​​from the books_authors dictionary.

Right here we used the .get() technique with ‘Doesn’t exist’ because the default worth.

keys = ['Grit','Hyperfocus','Make Time','Deep Work']
for key in keys:
  print(books_authors.get(key,'Doesn't exist'))

Within the code above:

  • For keys current within the books_authors dictionary, the .get() technique returns the corresponding values.
  • When the keys don’t exist, on this case ‘Grit’ and ‘Make Time’, the .get() technique returns the default worth ‘Doesn’t exist’.
# Output

Doesn't exist
Chris Bailey
Doesn't exist
Cal Newport

All of the above strategies will assist us take care of main errors. Nevertheless, they’re in depth and require us to deal explicitly with the lacking keys. You may simplify this course of through the use of a default as an alternative of a daily dictionary.

Commonplace dict in Python

python-defaultdict

The defaultdict is a subclass of the dictionary (dict) class. So it inherits the habits of a Python dictionary. Furthermore, it additionally handles lacking keys by default.

The default is a container information sort constructed into Python’s customary library – contained in the collections module.

So it’s good to import it into your working surroundings:

from collections import defaultdict

Right here is the overall syntax to make use of defaultdict:

defaultdict(default_factory)

You may specify a callable worth, akin to int, float, or record, because the default_factory attribute. If you don’t specify a worth for the default_factoryit’s on by default None.

When the important thing you’re on the lookout for isn’t current, the __missing__() technique is triggered and derives the default worth from the default_factory. Then this default worth is returned.

In abstract:

  • In Python, a defaultdict returns the default worth if the bottom line is not current.
  • It additionally provides this key-default-value pair to the dictionary, which you’ll then modify.

Python Defaultdict Examples

Defaultdict-Examples-1

Subsequent, we’ll code just a few examples to grasp how Python defaultdict works.

Defaultdict in Python with default integer worth

Import first defaultdict of the collections module.

from collections import defaultdict
import random

Let’s create a typical dictation costs.

costs = defaultdict(int)

We now fill the costs dictionary utilizing the gadgets of the fruits record as keys. And we randomly take values ​​from the price_list to get the values.

price_list = [10,23,12,19,5]
fruits = ['apple','strawberry','pomegranate','blueberry']

for fruit in fruits:
  costs[fruit] = random.selection(price_list)

Let’s take a look at the key-value pairs within the costs default.

print(costs.gadgets())
dict_items([('apple', 12), ('blueberry', 19), ('pomegranate', 5), ('strawberry', 10)])

Identical to with a daily Python dictionary, you possibly can entry the values ​​of the costs defaultdict utilizing the keys:

costs['apple']
# 23

Now let’s attempt to entry the value of a fruit that’s not current, for instance “orange”. We see that the customary worth of zero.

costs['orange']
# 0

Once we print the dictionary, we see {that a} new key ‘orange’ has been added with the default integer worth of zero.

print(costs.gadgets())
dict_items([('apple', 12), ('blueberry', 19), ('pomegranate', 5), ('strawberry', 10), ('orange', 0)])

Defaultdict in Python with Listing as default worth

Let’s outline students_majors like a defaultdict of lists. The names of the majors are the keys. And the values ​​are the lists of scholars taking every of the majors, akin to math, economics, pc science, and extra.

from collections import defaultdict
students_majors = defaultdict(record)

If we attempt to entry the coed record equivalent to ‘Economics’, defaultdict returns an empty record; no key errors!

students_majors['Economics']
# []

We have now now assigned an empty record to the “Economics” main. So we are able to now add components to this record utilizing the record technique .append().

students_majors['Economics'].append('Alex')

An entry has been created for ‘Economic system’ within the students_majors default dictionary.

print(students_majors)
defaultdict(<class 'record'>, {'Economics': ['Alex']})

You may add extra college students to the Economics main record, add a brand new main, and far more!

students_majors['Economics'].append('Bob')
students_majors['Math'].append('Laura')
print(students_majors)
defaultdict(<class 'record'>, {'Economics': ['Alex', 'Bob'], 'Math': ['Laura']})

Conclusion

I hope this tutorial helped you perceive how and when to make use of it default in Python. After operating the code samples on this tutorial, you possibly can attempt utilizing defaultdict as the popular information construction in your tasks if wanted.

Here is a abstract of what you have realized on this tutorial.

  • When working with a Python dictionary, you usually encounter KeyErrors.
  • To deal with such KeyErrors, you should use some elaborate strategies. You should use conditional statements, try-except blocks, or the .get() technique. However the defaultdict information sort within the assortment module can simplify this KeyError dealing with.
  • You should use default_dict(default_factory) The place default_factory is a legitimate callable worth.
  • When the bottom line is not current within the default dict, the default worth (derived from default_factory) and the bottom line is added to the usual dictation.

Subsequent, take a look at the Python map operate tutorial.

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