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Python Dictionaries

Introduction to Dictionaries

Dictionaries in Python are used to store data values in key-value pairs.

Basic Usage

Dictionaries are defined within braces {} with each item being a pair in the form key: value. Keys must be unique and immutable types (such as strings, numbers, or tuples), while values can be of any data type.

Example

my_dict = {
    "brand": "Ford",
    "model": "Mustang",
    "year": 1964
}

Accessing Items

You can access the items of a dictionary by referring to its key name.

Example

x = my_dict["model"]

Changing Values

You can change the value of a specific item by referring to its key name.

Example

my_dict["year"] = 2018

Dictionary Methods

keys()

Returns a list of all the keys in the dictionary. The order of the keys is arbitrary.

Example

keys = my_dict.keys()
print(keys) # ["brand", "model", "year"]

values()

Returns a list of all the values in the dictionary. The order of the values is arbitrary.

Example

values = my_dict.values()
print(values) # ["Ford", "Mustang", 2018]

items()

Returns a list of tuples, each tuple containing a key-value pair.

Example

items = my_dict.items()
print(items) # [("brand", "Ford"), ("model", "Mustang"), ("year", 2018)]

Dictionaries are mutable

pop()

Removes the element with the specified key.

Example

my_dict.pop("model")

clear()

Removes all the elements from the dictionary.

Example

my_dict.clear()

Dictionaries are unordered

Recap

Operation Description Example Result
d[key] Accesses the value associated with the specified key value = d['key'] value is the value associated with 'key'
d[key] = value Sets the value associated with the specified key d['new_key'] = 42 Adds a new key-value pair to the dictionary
del d[key] Removes the key-value pair with the specified key del d['key'] Removes the key 'key' and its value
key in d True if the key is in the dictionary, False otherwise exists = 'key' in d exists is True if 'key' is in the dictionary, False otherwise
key not in d True if the key is not in the dictionary, False otherwise not_exists = 'key' not in d not_exists is True if 'key' is not in the dictionary, False otherwise
d.keys() List of all keys in the dictionary all_keys = d.keys() all_keys is a view of all keys in d
d.values() List of all values in the dictionary all_values = d.values() all_values is a view of all values in d
d.items() List of all key-value pairs in the dictionary all_items = d.items() all_items is a view of all key-value pairs in d
d.get(key, fallback) Returns the value for the specified key or a default value if the key is not present value = d.get('key', default_value) value is the value associated with 'key', fallback if 'key' is not present
d.pop(key, fallback) Removes the key and returns its value; returns default if key is not present value = d.pop('key', default_value) value is the value associated with 'key' or default_value if 'key' is not present; removes the key 'key'
d.clear() Removes all items from the dictionary d.clear() Removes all key-value pairs from the dictionary
len(d) Returns the number of key-value pairs in the dictionary size = len(d) size is the number of key-value pairs in the dictionary
dict.copy() Returns a shallow copy of the dictionary d_copy = d.copy() d_copy is a shallow copy of the dictionary

Pass by Reference