📓 Dictionaries¶

1. Definition and Initialization¶

In Python, a dictionary is an unordered collection of key-value pairs. Each key-value pair is separated by a colon ":" and the pairs are separated by commas. Dictionaries are written with curly brackets.

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# dictionary of integer keys and string values
d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}
print(d1)  # prints: {1: 'apple', 2: 'banana', 3: 'cherry'}

# dictionary of string keys and integer values
d2 = {'one': 1, 'two': 2, 'three': 3}
print(d2)  # prints: {'one': 1, 'two': 2, 'three': 3}

# mixed dictionary
d3 = {1: 'apple', 'two': 2, True: 3.14}
print(d3)  # prints: {1: 'apple', 'two': 2, True: 3.14}

# dictionary of integer keys and string values d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} print(d1) # prints: {1: 'apple', 2: 'banana', 3: 'cherry'} # dictionary of string keys and integer values d2 = {'one': 1, 'two': 2, 'three': 3} print(d2) # prints: {'one': 1, 'two': 2, 'three': 3} # mixed dictionary d3 = {1: 'apple', 'two': 2, True: 3.14} print(d3) # prints: {1: 'apple', 'two': 2, True: 3.14}

2. Accessing Dictionary Items¶

You can access dictionary items by referring to their key name.

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}
print(d1[1])  # prints: 'apple'

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} print(d1[1]) # prints: 'apple'

3. Modifying Dictionary Items¶

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

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}
d1[1] = 'avocado'
print(d1)  # prints: {1: 'avocado', 2: 'banana', 3: 'cherry'}

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} d1[1] = 'avocado' print(d1) # prints: {1: 'avocado', 2: 'banana', 3: 'cherry'}

4. Adding Items¶

Adding an item to the dictionary is done by using a new index key and assigning a value to it.

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}
d1[4] = 'dragonfruit'
print(d1)  # prints: {1: 'apple', 2: 'banana', 3: 'cherry', 4: 'dragonfruit'}

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} d1[4] = 'dragonfruit' print(d1) # prints: {1: 'apple', 2: 'banana', 3: 'cherry', 4: 'dragonfruit'}

5. Removing Items¶

There are several methods to remove items from a dictionary, such as del keyword, pop(), popitem(), and clear().

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry', 4: 'dragonfruit'}

del d1[1]
print(d1)  # prints: {2: 'banana', 3: 'cherry', 4: 'dragonfruit'}

d1.pop(2)
print(d1)  # prints: {3: 'cherry', 4: 'dragonfruit'}

d1.popitem()  # removes the last inserted item
print(d1)  # prints: {3: 'cherry'}

d1.clear()
print(d1)  # prints: {}

d1 = {1: 'apple', 2: 'banana', 3: 'cherry', 4: 'dragonfruit'} del d1[1] print(d1) # prints: {2: 'banana', 3: 'cherry', 4: 'dragonfruit'} d1.pop(2) print(d1) # prints: {3: 'cherry', 4: 'dragonfruit'} d1.popitem() # removes the last inserted item print(d1) # prints: {3: 'cherry'} d1.clear() print(d1) # prints: {}

6. Dictionary Methods¶

Python provides several methods to manipulate dictionaries, like keys(), values(), items(), get(), update(), and many more.

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}

print(d1.keys())     # prints: dict_keys([1, 2, 3])
print(d1.values())   # prints: dict_values(['apple', 'banana', 'cherry'])
print(d1.items())    # prints: dict_items([(1, 'apple'), (2, 'banana'), (3, 'cherry')])

# get() returns the value for the specified key if key is in dictionary.
print(d1.get(1))     # prints: 'apple'
print(d1.get(4))     # prints: None

# update() method updates the dictionary with the elements from another dictionary object or from an iterable of key/value pairs.
d1.update({4: 'dragonfruit'})
print(d1)  # prints: {1: 'apple', 2: 'banana', 3: 'cherry', 4: 'dragonfruit'}

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} print(d1.keys()) # prints: dict_keys([1, 2, 3]) print(d1.values()) # prints: dict_values(['apple', 'banana', 'cherry']) print(d1.items()) # prints: dict_items([(1, 'apple'), (2, 'banana'), (3, 'cherry')]) # get() returns the value for the specified key if key is in dictionary. print(d1.get(1)) # prints: 'apple' print(d1.get(4)) # prints: None # update() method updates the dictionary with the elements from another dictionary object or from an iterable of key/value pairs. d1.update({4: 'dragonfruit'}) print(d1) # prints: {1: 'apple', 2: 'banana', 3: 'cherry', 4: 'dragonfruit'}

7. Check if Key Exists¶

To determine if a specified key is present in a dictionary, you can use the in keyword.

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}
print(1 in d1)  # prints: True
print(4 in d1)  # prints: False

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} print(1 in d1) # prints: True print(4 in d1) # prints: False

8. Length of a Dictionary¶

You can get the number of items (key-value pairs) in a dictionary with the len() function.

In [ ]:

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'}
print(len(d1))  # prints: 3

d1 = {1: 'apple', 2: 'banana', 3: 'cherry'} print(len(d1)) # prints: 3

9. Nested Dictionaries¶

A dictionary can contain dictionaries, this is called nested dictionaries.

In [ ]:

nested_dict = {1: 'apple', 2: 'banana', 3: {'name': 'cherry', 'color': 'red'}}
print(nested_dict)  # prints: {1: 'apple', 2: 'banana', 3: {'name': 'cherry', 'color': 'red'}}

nested_dict = {1: 'apple', 2: 'banana', 3: {'name': 'cherry', 'color': 'red'}} print(nested_dict) # prints: {1: 'apple', 2: 'banana', 3: {'name': 'cherry', 'color': 'red'}}

10. Dictionary Comprehension¶

Dictionary comprehension is an elegant and concise way to create a new dictionary from an iterable in Python.

In [ ]:

# create a dictionary with integer keys and their corresponding square values
squares = {x: x**2 for x in range(6)}
print(squares)  # prints: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# create a dictionary with integer keys and their corresponding square values squares = {x: x**2 for x in range(6)} print(squares) # prints: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25}