Summary of the use of defaultdict in basic Python syntax

Python's defaultdict is a special dictionary type provided in the collections module. It has similar functions to ordinary dictionaries, but has a key difference: when accessing a non-existent key, the defaultdict will not throw a KeyError exception, but will automatically create a default value for the key. This feature makes it easier to handle situations where new keys need to be initialized.

createdefaultdictTo usedefaultdictFirst of all, we need tocollectionsModule imports it:

from collections import defaultdict

Then you can create adefaultdictInstance, pass in a callable object as the default_factory parameter.
This callable object determines the default value type that should be returned when accessing non-existent keys.
For example:

• useintAsdefault_factoryWill return integer0。
• uselistAsdefault_factoryWill return to an empty list[]。
• usesetAsdefault_factoryWill return an empty collectionset()。
• Use custom functions asdefault_factoryThe default value will be determined based on the return value of the function.

Example 1

from collections import defaultdict

bag = ['apple', 'orange', 'cherry', 'apple','apple', 'cherry', 'blueberry']
count = defaultdict(int)  # Use int as default_factoryprint(f"\n > > > > > > count:\n {count}")
print(f"\n > > > > > > type(count):\n {type(count)}")

for fruit in bag:
    count[fruit] += 1
print(f"\n > > > > > > count:\n {count}")

Output:

 > > > > > > count:
 defaultdict(<class 'int'>, {})

 > > > > > > type(count):
 <class ''>

 > > > > > > count:
 defaultdict(<class 'int'>, {'apple': 3, 'orange': 1, 'cherry': 2, 'blueberry': 1})

In this example, when operating on a non-existent key, defaultdict will automatically assign a default value of 0 to the key and allow direct incremental operation.

Example 2

from collections import defaultdict

names = [('group1', 'Alice'), ('group2', 'Bob'), ('group1', 'Charlie')]
grouped_names = defaultdict(list)
print(f"\n > > > > > > grouped_names:\n {grouped_names}")
print(f"\n > > > > > > type(grouped_names):\n {type(grouped_names)}")

for group, name in names:
    grouped_names[group].append(name)

print(f"\n > > > > > > grouped_names:\n {grouped_names}")

Output:

 > > > > > > grouped_names:
 defaultdict(<class 'list'>, {})

 > > > > > > type(grouped_names):
 <class ''>

 > > > > > > grouped_names:
 defaultdict(<class 'list'>, {'group1': ['Alice', 'Charlie'], 'group2': ['Bob']})

In this example, the names are grouped according to the first element. If using a normal dictionary, additional logic is needed to check and initialize each new group.

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