Python Dictionaries

Dictionaries are used to store data values in key:value pairs.

A dictionary is a collection which is ordered*, changeable and do not allow duplicates.

Dictionaries are written with curly brackets, and have keys and values:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict)

Dictionary Items

Dictionary items are ordered, changeable, and does not allow duplicates.

Dictionary items are presented in key:value pairs, and can be referred to by using the key name.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(thisdict["brand"])

Ordered or Unordered?

When we say that dictionaries are ordered, it means that the items have a defined order, and that order will not change.

Unordered means that the items does not have a defined order, you cannot refer to an item by using an index.

Changeable

Dictionaries are changeable, meaning that we can change, add or remove items after the dictionary has been created.

Duplicates Not Allowed

Dictionaries cannot have two items with the same key:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964,
  "year": 2020
}
print(thisdict)

Dictionary Length

To determine how many items a dictionary has, use the len() function:

print(len(thisdict))

Dictionary Items - Data Types

The values in dictionary items can be of any data type:

thisdict = {
  "brand": "Ford",
  "electric": False,
  "year": 1964,
  "colors": ["red", "white", "blue"]
}

type()

From Python's perspective, dictionaries are defined as objects with the data type 'dict':

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
print(type(thisdict))

The dict() Constructor

It is also possible to use the dict() constructor to make a dictionary.

thisdict = dict(name = "John", age = 36, country = "Norway")
print(thisdict)

Accessing Items

You can access the items of a dictionary by referring to its key name, inside square brackets:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
x = thisdict["model"]

There is also a method called get() that will give you the same result:

x = thisdict.get("model")

Get Keys

The keys() method will return a list of all the keys in the dictionary.

x = thisdict.keys()

The list of the keys is a view of the dictionary, meaning that any changes done to the dictionary will be reflected in the keys list.

car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.keys()

print(x) #before the change

car["color"] = "white"

print(x) #after the change

Get Values

The values() method will return a list of all the values in the dictionary.

x = thisdict.values()

The list of the values is a view of the dictionary, meaning that any changes done to the dictionary will be reflected in the values list.

car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.values()

print(x) #before the change

car["year"] = 2020

print(x) #after the change

Add a new item to the original dictionary, and see that the values list gets updated as well:

car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.values()

print(x) #before the change

car["color"] = "red"

print(x) #after the change

Get Items

The items() method will return each item in a dictionary, as tuples in a list.

x = thisdict.items()

The returned list is a view of the items of the dictionary, meaning that any changes done to the dictionary will be reflected in the items list.

car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.items()

print(x) #before the change

car["year"] = 2020

print(x) #after the change

Add a new item to the original dictionary, and see that the items list gets updated as well:

car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}

x = car.items()

print(x) #before the change

car["color"] = "red"

print(x) #after the change

Check if Key Exists

To determine if a specified key is present in a dictionary use the in keyword:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
if "model" in thisdict:
  print("Yes, 'model' is one of the keys in the thisdict dictionary")

Change Values

You can change the value of a specific item by referring to its key name:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict["year"] = 2018

Update Dictionary

The update() method will update the dictionary with the items from the given argument.

The argument must be a dictionary, or an iterable object with key:value pairs.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"year": 2020})

Adding Items

Adding an item to the dictionary is done by using a new index key and assigning a value to it:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict["color"] = "red"
print(thisdict)

Update Dictionary

The update() method will update the dictionary with the items from a given argument. If the item does not exist, the item will be added.

The argument must be a dictionary, or an iterable object with key:value pairs.

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.update({"color": "red"})

Removing Items

There are several methods to remove items from a dictionary:

The pop() method removes the item with the specified key name:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.pop("model")
print(thisdict)

The popitem() method removes the last inserted item (in versions before 3.7, a random item is removed instead):

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.popitem()
print(thisdict)

The del keyword removes the item with the specified key name:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
del thisdict["model"]
print(thisdict)

The clear() method empties the dictionary:

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
thisdict.clear()
print(thisdict)

Loop Through a Dictionary

You can loop through a dictionary by using a for loop.

When looping through a dictionary, the return value are the keys of the dictionary, but there are methods to return the values as well.

for x in thisdict:
  print(x)

for x in thisdict:
  print(thisdict[x])

for x in thisdict.values():
  print(x)

for x in thisdict.keys():
  print(x)

for x, y in thisdict.items():
  print(x, y)

Copy a Dictionary

You cannot copy a dictionary simply by typing dict2 = dict1, because: dict2 will only be a reference to dict1, and changes made in dict1 will automatically also be made in dict2.

There are ways to make a copy, one way is to use the built-in Dictionary method copy().

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
mydict = thisdict.copy()
print(mydict)

thisdict = {
  "brand": "Ford",
  "model": "Mustang",
  "year": 1964
}
mydict = dict(thisdict)
print(mydict)

Nested Dictionaries

A dictionary can contain dictionaries, this is called nested dictionaries.

myfamily = {
  "child1" : {
    "name" : "Emil",
    "year" : 2004
  },
  "child2" : {
    "name" : "Tobias",
    "year" : 2007
  },
  "child3" : {
    "name" : "Linus",
    "year" : 2011
  }
}

Or, if you want to add three dictionaries into a new dictionary:

child1 = {
  "name" : "Emil",
  "year" : 2004
}
child2 = {
  "name" : "Tobias",
  "year" : 2007
}
child3 = {
  "name" : "Linus",
  "year" : 2011
}

myfamily = {
  "child1" : child1,
  "child2" : child2,
  "child3" : child3
}

Access Items in Nested Dictionaries

To access items from a nested dictionary, you use the name of the dictionaries, starting with the outer dictionary:

print(myfamily["child2"]["name"])

Dictionary Methods

Python has a set of built-in methods that you can use on dictionaries.

Below YouTube content is helpful for better understanding: