> For the complete documentation index, see [llms.txt](https://zmnako.gitbook.io/datascience/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://zmnako.gitbook.io/datascience/disctionary.md). # Dictionary Dictionaries and lists share the following characteristics: * Both are mutable. * Both are dynamic. They can grow and shrink as needed. * Both can be nested. A list can contain another list. A dictionary can contain another dictionary. A dictionary can also contain a list, and vice versa. Dictionaries differ from lists primarily in how elements are accessed: * List elements are accessed by their position in the list, via indexing. * Dictionary elements are accessed via keys. ### What is dictionary in Python? Python dictionary is an unordered collection of elements. While other compound data types have only value as an element, a dictionary has a **key:value** pair. Dictionaries are optimised to retrieve values when the key is known. ### Defining a Dictionary Dictionaries are Python’s implementation of a data structure that is more generally known as an associative array. A dictionary consists of a collection of key-value pairs. Each key-value pair maps the key to its associated value. You can define a dictionary by enclosing a comma-separated list of key-value pairs in curly braces (`{}`). A colon (`:`) separates each key from its associated value: ```python dict = { : , : , . . . : } ``` The following defines a dictionary that maps a location to the name of its corresponding Major League Baseball team: ```python mlb_team = { 'Colorado' : 'Rockies', 'Boston' : 'Red Sox', 'Minnesota': 'Twins', 'Milwaukee': 'Brewers', 'Seattle' : 'Mariners' } ``` You can also construct a dictionary with the built-in `dict()` function. The argument to `dict()` should be a sequence of key-value pairs. A list of tuples works well for this: ```python d = dict([ (, ), (, , ) ]) ``` ```python mlb_team = dict([ ('Colorado', 'Rockies'), ('Boston', 'Red Sox'), ('Minnesota', 'Twins'), ('Milwaukee', 'Brewers'), ('Seattle', 'Mariners') ]) ``` If the key values are simple strings, they can be specified as keyword arguments. So here is yet another way to define `mlb_team`: ```python mlb_team = dict( Colorado='Rockies', Boston='Red Sox', Minnesota='Twins', Milwaukee='Brewers', Seattle='Mariners' ) ``` Once you’ve defined a dictionary, you can display its contents, the same as you can do for a list. All three of the definitions shown above appear as follows when displayed: ```python print(type(mlb_team)) # output print(mlb_team) # output {'Colorado': 'Rockies', 'Boston': 'Red Sox', 'Minnesota': 'Twins', 'Milwaukee': 'Brewers', 'Seattle': 'Mariners'} ``` Indexing do not work on dictionaries ```python print(mlb_team[1]) KeyError Traceback (most recent call last) in () ----> 1 print(mlb_team[0]) KeyError: 0 ``` ### How to access elements from a dictionary? While indexing is used with other container types to access values, dictionary uses keys. Key can be used either inside square brackets or with the `get()` method. The difference while using `get()` is that it returns `None` instead of `KeyError`, if the key is not found. {% tabs %} {% tab title="Code" %} ```python my_dict = {'name':'John Legend', 'age': 79} # method 1 print(my_dict['name']) # method 2 print(my_dict.get('age')) ``` {% endtab %} {% tab title="Output" %} ```python # method 1 output John Legend # method 2 output 79 ``` {% endtab %} {% endtabs %} ### How to change or add elements in a dictionary? Dictionary are mutable. We can add new elements or change the value of existing elements using assignment operator. If the key is already present, value gets updated, else a new key: value pair is added to the dictionary. #### Update a value {% tabs %} {% tab title="Code" %} ```python my_dict = {'name':'John', 'age': 72} # update a value my_dict['age'] = 27 print(my_dict) ``` {% endtab %} {% tab title="Output" %} ```python {'name': 'John', 'age': 27} ``` {% endtab %} {% endtabs %} #### Add an element (key and value pair) {% tabs %} {% tab title="Code" %} ```python my_dict = {'name':'John', 'age': 72} # add element my_dict['address'] = 'Krakow' print(my_dict) ``` {% endtab %} {% tab title="Output" %} ```python {'name': 'John', 'age': 72, 'address': 'Krakow'} ``` {% endtab %} {% endtabs %} #### Delete elements We can remove a particular element in a dictionary by using the method `pop()`. This method removes an element with the provided key and returns the value. The method, `popitem()` can be used to remove and return an arbitrary or last element (key, value) from the dictionary. All the elements can be removed at once using the `clear()` method. We can also use the `del` keyword to remove individual elements or the entire dictionary itself. **`pop()`** `pop()` method expects a location of the key and value pair. Index in dictionary starts from **ONE**. {% tabs %} {% tab title="Code" %} ```python # create a dictionary tens = {1:10, 2:20, 3:30, 4:40, 5:50} # remove a particular element print(tens.pop(2)) # The remaining dictionary pairs print(tens) ``` {% endtab %} {% tab title="Output" %} ```python # print(tens.pop(2)) 20 # print(tens) {1: 10, 3: 30, 4: 40, 5: 50} ``` {% endtab %} {% endtabs %} **`popitem()`** `popitem()` remove the last key and value pair from the dictionary. {% tabs %} {% tab title="Code" %} ```python # create a dictionary tens = {1:10, 2:20, 3:30, 4:40, 5:50} # remove a particular element print(tens.popitem()) # The remaining dictionary pairs print(tens) ``` {% endtab %} {% tab title="Output" %} ```python # print(tens.popitem()) (5, 50) # print(tens) {1: 10, 2: 20, 3: 30, 4: 40} ``` {% endtab %} {% endtabs %} **`del`** You can use `del` method to delete a key and value pair based on the index location, remember index starts from **ONE** in the dictionaries. You can use `del` to delete the whole dictionary as well. {% tabs %} {% tab title="Code" %} ```python # create a dictionary tens = {1:10, 2:20, 3:30, 4:40, 5:50} # delete a particular element del tens[2] print(tens) ``` {% endtab %} {% tab title="Output" %} ```python # print(tens) {1: 10, 3: 30, 4: 40, 5: 50} ``` {% endtab %} {% endtabs %} **`clear()`** `clear()` method can be use to delete all key and pair values in the dictionary, it returns an empty dictionary. {% tabs %} {% tab title="Code" %} ```python # create a dictionary tens = {1:10, 2:20, 3:30, 4:40, 5:50} # remove all elements tens.clear() print(tens) ``` {% endtab %} {% tab title="Output" %} ```python # print(tens) {} ``` {% endtab %} {% endtabs %} ### Python Dictionary Methods Methods that are available with dictionary are tabulated below. Some of them have already been used in the above examples. | Method | Description | | -------------------- | ---------------------------------------------------------------------------------------------------------------------------------------- | | clear() | Remove all elements form the dictionary. | | copy() | Return a shallow copy of the dictionary. | | fromkeys(seq\[, v]) | Return a new dictionary with keys from seq and value equal to v (defaults to `None`). | | get(key\[,d]) | Return the value of key. If key doesnot exit, return d (defaults to `None`). | | items() | Return a new view of the dictionary's elements (key, value). | | keys() | Return a new view of the dictionary's keys. | | pop(key\[,d]) | Remove the element with key and return its value or d if key is not found. If d is not provided and key is not found, raises `KeyError`. | | popitem() | Remove and return a random element (key, value). Raises `KeyError` if the dictionary is empty. | | setdefault(key\[,d]) | If key is in the dictionary, return its value. If not, insert key with a value of d and return d (defaults to `None`). | | update(\[other]) | Update the dictionary with the key/value pairs from other, overwriting existing keys. | | values() | Return a new view of the dictionary's values | ### Python Dictionary Comprehension Dictionary comprehension is an elegant and concise way to create new dictionary from an iterable in Python. Dictionary comprehension consists of an expression pair (key: value) followed by `for` statement inside `curly brackets {}`. Here is an example to make a dictionary with each item being a pair of a number and its square. {% tabs %} {% tab title="Code" %} ```python tens = {x: x*10 for x in range(1, 6)} print(tens) ``` {% endtab %} {% tab title="Output" %} ```python {1: 10, 2: 20, 3: 30, 4: 40, 5: 50} ``` {% endtab %} {% endtabs %} A dictionary comprehension can optionally contain more **for** or **if statements**. An optional `if` statement can filter out items to form the new dictionary. {% tabs %} {% tab title="Code" %} ```python tens = {x: x*10 for x in range(1, 20) if x%3==1} print(tens) ``` {% endtab %} {% tab title="Output" %} ```python # print(tens) {1: 10, 4: 40, 7: 70, 10: 100, 13: 130, 16: 160, 19: 190} ``` {% endtab %} {% endtabs %} --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. 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