Dictionaries

• Built-in data type.
• It is not a sequence.

• A set of key: value pairs.

  

• Indexed by keys.

  • The key is case-sensitive.
  • We use a key to get values from a dictionary.

  • They can be strings, numbers, tuples:

    info = {
        "name": "Claudius Ptolemy",
        100: ["Mathematician", "Astrologist", "Geologist"],
        ("Alexandria", "Egypt", "Roman Empire"): "Alexandria is the second largest city in Egypt and the largest city on the Mediterranean coast."
    }
    

    [!CAUTION]

    If a tuple contains any mutable object either directly or indirectly, it cannot be used as a key.

  • They have to be unique within one dictionary.

Get a Value

• get method:

  car = {
    "name": "BYD",
    "model_number": 123,
    "plaque_number": "px123"
  }
  print(car.get("name"))
  

• Key as index:

  print(car['name'])
  

[!TIP]

• get will return None when it cannot find a value for the provided key whereas the key as index approach will raise an exception in case it did not find it in the dictionary.
• Key as index is faster than get method.
• Sometimes we want our app to crash if the key was not present.

profile = {
    "username": "Genghis_khan"
}
print(profile["age"])  # raises an exception
print(profile.get("age"))  # returns None

• Get all keys like this:

  info = {
      "name": "Claudius Ptolemy",
      "fields": ["Mathematician", "Astrologist", "Geologist"],
      "home_town": "Alexandria is the second largest city in Egypt and the largest city on the Mediterranean coast.",
      "died": 100
  }
  keys = list(info)
  sorted_keys = sorted(info)
  print(keys, sorted_keys)
  

Delete a key-value Pair

• You can use del:

  me = {"test": 123}
  del me["test"]
  print(me)  # {}
  

• You can also use pop, note that this method returns the value of the deleted key:

  car.pop("name")
  

  And you can also specify a default value in case it did not exists in the dictionary.

  car.pop("mileage", 0)  # 0 will be returned if "mileage" does not exist in the car dictionary
  

  Note that pop works in LIFO (Last In First Out). BTW we have another method which returns both key and value for that key called popitem.

[!TIP]

del performs better than pop.

in Operator – Membership Test Operator

• In dictionaries the in operator looks up for the keys:

  if "production_date" in car:
      print(car['production_date'])
  

[!NOTE]

The in operator in a sequence (e.g. an array) looks for that value’s presence in the sequence.

names = ["Mohammad", "Jawad"]
if "Alex" in names:
    print("Alex exists in the list")

Loop Over a Dictionary

• You by default iterate over the keys:

  for key in car:
      print(key)
      print(car[key])  # This is how you can get the values
  

• items method:

  for key, value in car.items():  # Unpacking
      print(key)
      print(value)
  

[!TIP]

• The items method is more readable.
• The items method is more efficient.

Convert a List to Dictionary

• enumerate function:

  arr = ["asd"]
  
  for key, val in enumerate(arr):
      print(key, val)
  

• Build dictionary directly from sequences of key-value pairs with the dict() constructor:

  ai_figures = [("John McCarthy", 2011), ("Kate Crawford", None)]
  print(dict(ai_figures))
  # Yet another way:
  print(dict(key1="value1", key2="value2"))
  

Dictionary Comprehensions

numbers = {number: "even" if number % 2 == 0 else "odd"
           for number in range(1, 100)}
print(numbers)

setdefault Method

Instead of

account = {
    "credit": 123
}


def top_up(account):
    # TODO copy the object first
    if "credit" not in account:
        account["credit"] = 0

    account["credit"] += 10

    return account

You can do this:

# ...
account["credit"] = account.setdefault("credit", 0) + 10
return account

update Method

You can add key-value pairs of one dictionary to another by using this method:

user = { "name": "jawad" }
account = { "IBAN": "DE123456789012" }
user.update(account)

print(user)

Shallow Copy VS Deep Copy

• copy method does a shallow copy.

  user1 = {"schools": ["zoom"]}
  user2 = user1  # user2 = user1.copy()
  user2["schools"].append("elder")
  print(user1)
  print(user2)
  

  So basically these two objects are pointing to the same address of memory. It copies references and not the things it is referring to.

• deepcopy helper function in copy module:

  from copy import deepcopy
  user2 = deepcopy(user1)
  

[!TIP]

A simple deep copy helper function:

def simple_deepcopy(obj):
    res = {}

    for key, value in object.items():
        if value is Array:
            res[key] = value.copy()
            continue

        res[key] = value
    return res

fromkeys Method

• Creates a new dictionary from the given list.

YouTube/Aparat

• https://youtu.be/-QperLIB4b8.
• https://aparat.com/v/jhdj52n.

Ref

• 5. Data Structures.