Dictionaries in Python

In Python, you can safely use a trailing comma in dictionaries (and other collections like lists, tuples, and sets).

my_dict = {

    "x": 10,

    "y": 20,

}

In Python, dictionary keys have some important rules and behaviors.

animals = {

    'a': 'ape',

    'b': 'bear',

    'c': 'cat',

}

animals.keys()

Output: dict_keys(['a', 'b', 'c'])

animals.values()

Output: dict_values(['ape', 'bear', 'cat'])

list(animals.keys())

Output: ['a', 'b', 'c']

animals.get('a')

Output: 'ape'

animals = {

    'a': ['ape', 'abalone'],

    'b': ['bear'],

}

Each key ('a', 'b') maps to a list of values.

This means you can store multiple animals under the same starting letter.

animals['b'].append('bat')

You’re accessing the list stored under key 'b' → currently ['bear'].




.append('bat') adds 'bat' to that list.

.append() modifies the list in place (it doesn’t return a new list).

if 'c' not in animals:

    animals['c'] = []

    

animals['c'].append('cat')

Check if key 'c' exists




if 'c' not in animals: → looks for 'c' in the dictionary keys.

If 'c' is missing, it creates a new entry with an empty list

Append 'cat' to the list

Now animals['c'] is an empty list [].




.append('cat') adds 'cat' to that list.

defaultdict is a special type of dictionary from Python’s collections module. It works just like a normal dictionary, but with one big advantage: if you try to access a key that doesn’t exist, it automatically creates it with a default value instead of raising a KeyError.

from collections import defaultdict

>>> animals = defaultdict(list)

>>> animals

Output:

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

This is the string representation (__repr__) of a defaultdict object.

Inside the angular brackets < >, Python is showing you the default factory function used to create missing values.




<class 'list'> means: whenever you access a missing key, Python will automatically create a new empty list for it.

The {} part after the comma is the current contents of the dictionary (empty at the moment).

})

So the whole thing reads as, “This is a defaultdict whose default factory is the list class, and right now it contains an empty dictionary.”

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