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February 24, 2021 04:22 pm GMT

TIGEW: defaultdict

Photo by Kelly Sikkema on Unsplash

In a recent PR review, using defaultdict instead of a regular dictionary was suggested. Having never worked with defaultdict I needed to take a moment to understand the suggested refactor.

Keeping with the theme, here's another installment of Things I Google Each Week focusing on defaultdict.

What is defaultdict?

The Python defaultdict behaves almost exactly like a regular Python dictionary, however, if you try to access or modify a missing key defaultdict will automatically create the key and generate a default value.

Remember: A Python dictionary holds key:value pairs, the key must be unique and immutable.

Defaultdict is a sub-class of the dict class that returns a dictionary-like object and never raises a KeyError due to the default value.

This makes defaultdict a valuable option for handling missing keys in dictionaries.

When/How to Use defaultdict?

Lets compare a dictionary and a defaultdict:

# Python dictionary st_dict = {1: 'Picard', 2: 'Spock', 3: 'Tasha Yar', 4: 'Data'}  print("Star Trek Characters:")  print(st_dict) print(st_dict[1]) print(st_dict[5])# print(st_dict[5]) will raise a KeyError as# the 5 key is not present in the dictionary

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Output:

Star Trek Characters:{1: 'Picard', 2: 'Spock', 3: 'Tasha Yar', 4: 'Data'}PicardTraceback (most recent call last): File "/home/dictionary_example.py",line 7, in print(st_dict[5])KeyError: 5

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Now here's a defualtdict:

# Python defaultdict from collections import defaultdict # Function to return a default value for keys that are missing def def_value():     return "Captains Log..."# Defining the defaultdict st_dict = defaultdict(def_value) st_dict["Picard"] = "Captain"st_dict["Spock"] = "Commander"st_dict["Tasha Yar"] = "Lieutenant"st_dict["Data"] = "Lieutenant Commander"print(st_dict["Picard"]) print(st_dict["Spock"]) print(st_dict["Geordi"])

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Output:

CaptainCommanderCaptains Log...

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See the difference? Which one might be easier to handle in your project?

What are some other interesting features of defaultdict?

Default_factory

The function provided in the defaultdict definition is known as the default_factory.

As a general FYI, if this argument is absent from the defaultdict it raises a KeyError when a missing key is encountered. Just like a dictionary.

# Python defaultdict - default_factoryfrom collections import defaultdict# Defining the defaultdict# using lambda this timest_dict = defaultdict(lambda: "Captain's Log...")st_dict["Picard"] = "Captain"st_dict["Spock"] = "Commander"st_dict["Tasha Yar"] = "Lieutenant"st_dict["Data"] = "Lieutenant Commander"print(st_dict["Picard"]) print(st_dict["Spock"]) print(st_dict["Geordi"])

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Output:

CaptainCommanderCaptains Log...

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missing()

A method to return the default value of a given defaultdict.

# Python defaultdict - __missing__()from collections import defaultdict# Defining the defaultdictst_dict = defaultdict(lambda: "Captain's Log...")st_dict["Picard"] = "Captain"st_dict["Spock"] = "Commander"st_dict["Tasha Yar"] = "Lieutenant"st_dict["Data"] = "Lieutenant Commander"print(st_dict.__missing__("Geordi"))print(st_dict.__missing__("Worf"))print(st_dict.__missing__("Picard"))

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Output:

Captain's Log...Captain's Log...Captain's Log...

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Using list as default_factory

When the list method is passed as the default_factory argument, then a defaultdict is created that will default new values to an empty list.

# Python defaultdict - default_factory with listfrom collections import defaultdict# Defining the defaultdictnumber_dict = defaultdict(list)for i in range(5):   number_dict[i].append(i)print("Dictionary with list value:")print(number_dict)

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Output:

Dictionary with list value:defaultdict(<class 'list'>, {0: [0], 1: [1], 2: [2], 3: [3], 4: [4]})

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Using int as default_factory

When the int method is passed as the default_factory argument, then a defaultdict is created with default value as zero.

# Python defaultdict - default_factory with intfrom collections import defaultdict# Defining the defaultdictnumbers_dict = defaultdict(int)numbers = [10, 20, 30 ]# Iterate through the listfor i in numbers:    numbers_dict[i] = i*2print(numbers_dict)print(numbers_dict[40])

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Output:

defaultdict(<class 'int'>, {10: 20, 20: 40, 30: 60})0

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Where Can I Learn More?

Original Link: https://dev.to/saramccombs/tigew-defaultdict-in-python-2e42

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