10.3 Dictionary operations

Python dictionaries are versatile data structures that store key-value pairs. They're mutable, allowing dynamic modifications like adding, updating, or removing items. This flexibility makes dictionaries ideal for organizing and accessing data efficiently.

Dictionaries offer various methods for manipulation and iteration. You can access values using keys, add new pairs, remove items, and iterate through components. Understanding these operations is crucial for effective dictionary usage in Python programming.

Dictionary Fundamentals

Mutability of Python dictionaries

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• Dictionaries are mutable data structures in Python allowing for modification after creation
  • Key-value pairs can be added, updated, or removed dynamically (adding "age" key, updating "name" value)
  • Dictionary size adjusts automatically as key-value pairs are added or removed
• Modifying a dictionary directly updates the existing object without creating a new one
• Assigning a dictionary to another variable creates a reference pointing to the same underlying dictionary object
• Keys in dictionaries must be immutable (immutability of keys)

Dictionary access via keys

• Dictionary values are retrieved using their associated keys enclosed in square brackets

  dictionary[key]

  • Accessing an existing key returns the corresponding value (

    student["name"]

    returns "John")
  • Attempting to access a non-existent key raises a

    [KeyError](https://www.fiveableKeyTerm:KeyError)

    exception
• Values can be updated by assigning a new value to an existing key

  dictionary[key] = new_value

• The

  [get()](https://www.fiveableKeyTerm:get())

  method provides an alternative way to access values

  dictionary.get(key, default_value)

  • Returns the value if the key exists, otherwise returns the

    default_value

    (

    None

    by default)

Dictionary Modification

Adding key-value pairs

• New key-value pairs can be added to a dictionary using assignment

  dictionary[new_key] = value

  • Creates a new key-value pair if the key doesn't exist (

    student["age"] = 25

    adds "age" key)
  • Updates the value if the key already exists (

    student["name"] = "Jane"

    updates "name" value)
• The

  [update()](https://www.fiveableKeyTerm:update())

  method merges another dictionary or an iterable of key-value pairs into the dictionary

  • dictionary.update(other_dict)

    merges

    other_dict

    into

    dictionary

  • dictionary.update(iterable)

    merges key-value pairs from

    iterable

    into

    dictionary

Removing dictionary items

• The

  [del](https://www.fiveableKeyTerm:del)

  statement removes a specific key-value pair from the dictionary

  del dictionary[key]

  • Removes the key-value pair if the key exists (

    del student["age"]

    removes "age" key)
  • Raises a

    KeyError

    if the key doesn't exist
• The

  [pop()](https://www.fiveableKeyTerm:pop())

  method removes a key-value pair and returns the corresponding value

  value = dictionary.pop(key, default_value)

  • Removes and returns the value if the key exists (

    age = student.pop("age")

    removes and returns the "age" value)
  • Returns the

    default_value

    if the key doesn't exist (raises

    KeyError

    if

    default_value

    not provided)
• The

  [popitem()](https://www.fiveableKeyTerm:popitem())

  method removes and returns an arbitrary key-value pair as a tuple

  (key, value) = dictionary.popitem()

• The

  [clear()](https://www.fiveableKeyTerm:clear())

  method removes all key-value pairs, emptying the dictionary

  dictionary.clear()

Iterating through dictionary components

• The

  [keys()](https://www.fiveableKeyTerm:keys())

  method returns a view object containing all the dictionary keys

  for key in dictionary.keys():

  • Iterating directly over the dictionary

    for key in dictionary:

    achieves the same result
• The

  [values()](https://www.fiveableKeyTerm:values())

  method returns a view object containing all the dictionary values

  for value in dictionary.values():

• The

  [items()](https://www.fiveableKeyTerm:items())

  method returns a view object containing all the key-value pairs as tuples

  for key, value in dictionary.items():

  • Allows simultaneous access to both keys and values during iteration (

    for name, grade in student.items():

    )
• Dictionary view objects are dynamic, reflecting any changes made to the dictionary in real-time

Dictionary Performance and Behavior

Efficiency and internal structure

• Dictionaries use a hash function to achieve constant-time average case complexity for key lookups
• The unordered nature of dictionaries allows for efficient insertion and deletion operations
• Lookup efficiency in dictionaries is typically O(1), making them suitable for large datasets

Specialized dictionary types

• The defaultdict is a subclass of dict that automatically initializes new keys with a default value