A cheat sheet for Python dictionaries

python_dictionaries.py

# Create an empty dictionary using {}.
employees = {}
print(employees) # Output: {}
print(type(employees)) # Output: <class 'dict'>

# Create a dictionary with items using {}.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees) # Output: {1: 'Tom', 2: 'Macy', 3: 'Sam'}

# Create an empty dictionary using dict() constructor.
employees = dict()
print(employees) # Output: {}
print(type(employees)) # Output: <class 'dict'>

# Create a dictionary using dict() with a mapping(in this case, another dict) as input.
employees = dict({1: 'Tom', 2: 'Macy', 3: 'Sam'})
print(employees) # Output: {1: 'Tom', 2: 'Macy', 3: 'Sam'}

## Create a dictionary using dict() with an iterable as input.
# Input is a list of tuples, each of which becomes a key-value pair.
employees = dict([(1, 'Tom'), (2, 'Macy'), (3, 'Sam')])
print(employees) # Output: {1: 'Tom', 2: 'Macy', 3: 'Sam'}
# Input is an iterable created using zip() method that is taking lists of keys and values.
employees = dict(zip([1, 2, 3], ['Tom', 'Macy', 'Sam']))
print(employees) # Output: {1: 'Tom', 2: 'Macy', 3: 'Sam'}

# Create dictionary using dict() with keyword arguments as input(when keys are strings).
numbers = dict(one=1, two=2, three=3)
print(numbers) # Output: {'one': 1, 'two': 2, 'three': 3}

# Dictionary keys have to be unique.
employees = {1: 'Tom', 1: 'Macy'}
# Note that the second assignment overwrites the value associated with the key.
print(employees) # Output: {1: 'Macy'}

# Dictionary keys and values can be of different data types.
employees = {1: 'Tom', 'Macy': 2, (1, '1'): 'Sam'}
print(employees) # Output: {1: 'Tom', 'Macy': 2, (1, '1'): 'Sam'}

# Length of a dictionary, or number of key-value pairs in a dictionary.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(len(employees)) # Output: 3

# Check if a key is present in a dictionary using 'in'.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(1 in employees) # Output: True
print(5 in employees) # Output: False
print(1 not in employees) # Output: False
print(5 not in employees) # Output: True

# Access elements in a dictionary by indexing using the key.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees[1]) # Output: 'Tom'
# If key is not present, it raises a KeyError.
print(employees[5]) # Error -> KeyError: 5

# Get elements in a dictionary using get().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees.get(1)) # Output: 'Tom'
# If key is not present, the retrn value defaults to None, so it doesn't raise a KeyError.
print(employees.get(5)) # Output: None
# We can specify default values if key is not present.
print(employees.get(5, 'Unknown')) # Output: 'Unknown'

# Add items(key-value pairs) to a dictionary by indexing using key.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
employees[4] = 'Lucy'
print(employees) # Output: {1: 'Tom', 2: 'Macy', 3: 'Sam', 4: 'Lucy'}

# Remove items from a dictionary.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
del employees[3]
print(employees) # Output: {1: 'Tom', 2: 'Macy'}

# Remove items and get its value using pop().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees.pop(3)) # Output: 'Sam'
print(employees) # Output: {1: 'Tom', 2: 'Macy'}

# Remove items and get the item using popitem().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees.popitem()) # Output: (3, 'Sam') <- Note: popitem() follows LIFO
print(employees) # Output: {1: 'Tom', 2: 'Macy'}

# Update items in a dictionary.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
employees[1] = 'Max'
print(employees) # Output: {1: 'Max', 2: 'Macy', 3: 'Sam'}

# Get or add an item using setdefault().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
# If key is present, its value is returned without modifying.
print(employees.setdefault(1, 'Max')) # Output: 'Tom'
print(employees) # Output: {1: 'Tom', 2: 'Macy', 3: 'Sam'}
# If key is not present, it is added to the dictionary.
print(employees.setdefault(4, 'Lucy')) # Output: 'Lucy'
print(employees) # Output: {1: 'Max', 2: 'Macy', 3: 'Sam', 4: 'Lucy'}

# Get all items in a dictionary using items().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees.items()) # Output: dict_items([(1, 'Tom'), (2, 'Macy'), (3, 'Sam')])

# Get all keys in a dictionary using keys().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees.keys()) # Output: dict_keys([1, 2, 3])

# Get all values in a dictionary using values().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
print(employees.values()) # Output: dict_values(['Tom', 'Macy', 'Sam'])

# Get a key iterator for a dictionary using iter().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
employees_key_iterator = iter(employees)
print(employees_key_iterator) # Output: <dict_keyiterator object at 0x10d8fdea8>
for i in employees_key_iterator:
  print(i)
# Output: 1 2 3

# Remove all items in a dictionary using clear().
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
employees.clear()
print(employees) # Output: {}

# Delete dictionary using 'del' keyword.
employees = {1: 'Tom', 2: 'Macy', 3: 'Sam'}
del employees