1. Define a dictionary in Python
- Dictionaries in python are unordered collections of objects that is made up of elements and each element consists of a key:value pair.
- In other programming languages, we talk about associative arrays or hashes.
- Like lists, dictionaries are mutable and dynamic objects. They can be modified and extended according to your needs.
- A dictionary can contain objects of any type and even include other dictionaries.
- It is thanks to these characteristics that dictionaries are often used to create complex data structures where several elements are nested inside each other!
- A dictionary can be defined by enclosing braces {} with a list of key-value pairs separated by commas.
Syntax
dic = {key1: value1, key2: value2, key3: value3, ...}
To access a value from the dictionary, simply use the name of the dictionary followed by the corresponding key in square brackets:
Example
dic = {key1: value1, key2: value2, key3: value3, ...}
print (dic [key1]) # output : value1
Example: phone book
phoneBook = {"Elysa": "333678765", "Robert": "4544333456", "Andres": "333245511", "Walid": "333445566"}
print(phoneBook ["Robert"]) # output: "4544333456"
2. Browse the values and keys of a Python dictionary
A Python dictionary has a method called values() that allows you to browse its values, and another named keys() to browse its keys.
Example: print values from a dictionary
phoneBook = { "Alberto": "3336633558", "Jean": "3337958414", "Bachlar": "3339584758"}
for value in phoneBook.values():
print(value)
"""
output:
3336633558
3337958414
3339584758
"""
Example: browse dictionary keys
phoneBook = { "Farid": "3336633558", "Najib": "3337958414", "Bernardo": "3339584758"}
for key in phoneBook.keys ():
print(key)
"""
output:
Farid
Najib
Bernardo
"""
Note
You can also browse keys and values at the same time by using the items() method
Example: browsing through keys and values
phoneBook = { "Majid": "0556633558", "Tomas": "0587958414", "Bernard": "0669584758"}
for key, value in phoneBook.items ():
print (key, value)
"""
output:
Majid 0556633558
Tomas 0587958414
Bernard 0669584758
"""
3. Update Python dictionary: add or delete elements
3.1 Update a dictionary element
You can update a dictionary item directly by assigning a value to a key:
Example: Stock Manager
stock = {"USB_Key": 29, "Printer": 45, "Routers": 13 , "Laptops": 31}
#modification of the "printer key value
stock [ "Printer"] = 22
print (stock)
# output: {"USB_Key": 29, "Printer": 22, "Routers": 13 , "Laptops": 31}
3.2 Add an element to the dictionary
In the case of a non-existent key, the same method mentioned above, allows to add elements to the dictionary:
Example: Add an item to the stock
stock = {"Laptop": 15, "Printer": 35, "Tablet": 27}
# Added the "Ipad" element: 21
stock [ "Ipad"] = 21
print (stock)
# output: {'Laptop': 15, 'Printer': 35, 'Tablet': 27, 'Ipad': 21}
3.3 Delete an element from the dictionary
You can remove an element from the dictionary by specifying its key in the pop() method
Example: deleting a dictionary element
stock = {'Laptop': 15, 'Printer': 35, 'Tablet': 27, 'Ipad': 22}
# Delete the "Printer" element: 35
stock.pop ( "Printer")
print (stock)
# output: {'Laptop': 15, 'Tablet': 27, 'Ipad': 22}
Note
A dictionary has another method: popitem() that removes the last element
Example: Deleting the last item
stock = {'Laptop': 15, 'Printer': 35, 'Tablet': 27, 'Ipad': 22}
# Deleting the last element
stock.popitem ()
print (stock)
# displays: {'Laptop': 15, 'Printer': 35, 'Tablet': 27}
3.4 Emptying a dictionary
A Python dictionary can be empty using the clear() method
Example: empty a dictionary
stock = {'Laptop': 15, 'Printer': 35, 'Tablet': 27, 'Ipad': 22}
# empty the dictionary
stock.clear ()
print (stock)
# displays an empty dictionary: {}
4. Summary of dictionary associated methods
Here is a summary of the main methods associated with a dictionary object:
- clear(): removes all items from the dictionary.
- copy(): returns a shallow copy of the dictionary.
- fromkeys (seq [, v]): Returns a new dictionary with the keys of seq and a value equal to v (the default is None).
- get(key [, d]): returns the value of key. If the key does not exit, returns d (the default is None).
- items(): returns a new view of dictionary items (key, value).
- keys(): Returns a new view of the dictionary keys.
- pop(key [, d]): removes the element with key and returns its value or d if key is not found. If d is not provided and the key can not be found, raises KeyError.
- popitem(): delete and return an arbitrary element (key, value). Lift KeyError if the dictionary is empty.
- setdefault(key [, d]): if key is in the dictionary, return its value. Otherwise, insert the key with the value d and return d (the default is None).
- update([other]): Update the dictionary with the key / value pairs of other existing keys.
- values(): returns a new view of dictionary values
Younes Derfoufi
my-courses.net
