Python defaultdict

In Python, defaultdict is a data type that belongs to the collections module. It is a dictionary subclass that automatically provides a default value for missing keys.

Syntax

collections.defaultdict(default_factory)

Parameters:

• default_factory: A callable (such as a function or type like int, list, set) that provides the default value for missing keys. If set to None, a KeyError is raised when accessing a missing key.

Return value:

Returns a defaultdict object, which behaves like a dictionary but creates default values for missing keys using the specified default_factory.

Example 1: Using a Custom Function

The following example demonstrates the defaultdict data type with a custom function as default_factory argument:

from collections import defaultdict

def default_value():
  return "Not Declared"

myDefaultDict = defaultdict(default_value)

myDefaultDict["first"] = 100
myDefaultDict["second"] = 90

print(myDefaultDict["first"])
print(myDefaultDict["second"])
print(myDefaultDict["third"])

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Here is the output for the above code:

100
90
Not Declared

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Example 2: Using Built-in Callables

This example demonstrates defaultdict with built-in types (int, list, str, set) as the default_factory:

from collections import defaultdict

intDefaultDict = defaultdict(int)
listDefaultDict = defaultdict(list)
strDefaultDict = defaultdict(str)
setDefaultDict = defaultdict(set)

print(intDefaultDict[0])
print(listDefaultDict['zero'])
print(strDefaultDict['0'])
print(setDefaultDict['a'])

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Here is the output of the above code:

0
[]

set()

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Example 3: Working with Lists

This example shows how list as default_factory allows appending to keys that don’t yet exist:

from collections import defaultdict

myDefaultDict = defaultdict(list)

myDefaultDict['apple'].append(1)
# myDefaultDict['apple'] does not exist so it defaults to empty list [],
# then 1 is appended to it.

myDefaultDict['orange'] = 2
#The empty list [] is replaced by integer 2 here.

print(myDefaultDict['apple'])
print(myDefaultDict['orange'])

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Here is the output of the above code:

[1]
2

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Codebyte Example

Run the following codeblock and explore more about the defaultdict data type:

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