Dictionary in Python is a collection of keys values, used to store data values like a map, which, unlike other data types which hold only a single value as an element.
Example of Dictionary in Python Dictionary holds key:value pair. Key-Value is provided in the dictionary to make it more optimized.
Dict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
print(Dict)
Output:
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
In Python, a dictionary can be created by placing a sequence of elements within curly {} braces, separated by ‘comma’. Dictionary holds pairs of values, one being the Key and the other corresponding pair element being its Key:value. Values in a dictionary can be of any data type and can be duplicated, whereas keys can’t be repeated and must be immutable.
Note – Dictionary keys are case sensitive, the same name but different cases of Key will be treated distinctly.
# Creating a Dictionary
# with Integer Keys
Dict = {1: 'Geeks', 2: 'For', 3: 'Geeks'}
print("\nDictionary with the use of Integer Keys: ")
print(Dict)
# Creating a Dictionary
# with Mixed keys
Dict = {'Name': 'Geeks', 1: [1, 2, 3, 4]}
print("\nDictionary with the use of Mixed Keys: ")
print(Dict)
Output:
Dictionary with the use of Integer Keys:
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Dictionary with the use of Mixed Keys:
{'Name': 'Geeks', 1: [1, 2, 3, 4]}
Dictionary can also be created by the built-in function dict(). An empty dictionary can be created by just placing to curly braces{}.
# Creating an empty Dictionary
Dict = {}
print("Empty Dictionary: ")
print(Dict)
# Creating a Dictionary with dict() method
Dict = dict({1: 'Geeks', 2: 'For', 3: 'Geeks'})
print("\nDictionary with the use of dict(): ")
print(Dict)
# Creating a Dictionary with each item as a Pair
Dict = dict([(1, 'Geeks'), (2, 'For')])
print("\nDictionary with each item as a pair: ")
print(Dict)
Output:
Empty Dictionary:
{}
Dictionary with the use of dict():
{1: 'Geeks', 2: 'For', 3: 'Geeks'}
Dictionary with each item as a pair:
{1: 'Geeks', 2: 'For'}
Complexities for Creating a Dictionary:
Time complexity: O(len(dict))
Space complexity: O(n)
# Creating a Nested Dictionary
# as shown in the below image
Dict = {1: 'Geeks', 2: 'For',
3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}
print(Dict)
Output:
{1: 'Geeks', 2: 'For', 3: {'A': 'Welcome', 'B': 'To', 'C': 'Geeks'}}
Addition of elements can be done in multiple ways. One value at a time can be added to a Dictionary by defining value along with the key e.g. Dict[Key] = ‘Value’. Updating an existing value in a Dictionary can be done by using the built-in update() method. Nested key values can also be added to an existing Dictionary.
Note- While adding a value, if the key-value already exists, the value gets updated otherwise a new Key with the value is added to the Dictionary.
# Creating an empty Dictionary
Dict = {}
print("Empty Dictionary: ")
print(Dict)
# Adding elements one at a time
Dict[0] = 'Geeks'
Dict[2] = 'For'
Dict[3] = 1
print("\nDictionary after adding 3 elements: ")
print(Dict)
# Adding set of values
# to a single Key
Dict['Value_set'] = 2, 3, 4
print("\nDictionary after adding 3 elements: ")
print(Dict)
# Updating existing Key's Value
Dict[2] = 'Welcome'
print("\nUpdated key value: ")
print(Dict)
# Adding Nested Key value to Dictionary
Dict[5] = {'Nested': {'1': 'Life', '2': 'Geeks'}}
print("\nAdding a Nested Key: ")
print(Dict)
Output:
Empty Dictionary:
{}
Dictionary after adding 3 elements:
{0: 'Geeks', 2: 'For', 3: 1}
Dictionary after adding 3 elements:
{0: 'Geeks', 2: 'For', 3: 1, 'Value_set': (2, 3, 4)}
Updated key value:
{0: 'Geeks', 2: 'Welcome', 3: 1, 'Value_set': (2, 3, 4)}
Adding a Nested Key:
{0: 'Geeks', 2: 'Welcome', 3: 1, 'Value_set': (2, 3, 4), 5:
{'Nested': {'1': 'Life', '2': 'Geeks'}}}
Complexities for Adding elements in a Dictionary:
Time complexity: O(1)/O(n)
Space complexity: O(1)
Adding an item to the dictionary is done by using a new index key and assigning a value to it:
Example
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict["color"] = "red"
print(thisdict)
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.
Example Add a color item to the dictionary by using the update() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.update({"color": "red"})
There are several methods to remove items from a dictionary:
Example
The pop() method removes the item with the specified key name:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.pop("model")
print(thisdict)
Example
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)
Example
The del keyword removes the item with the specified key name:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
del thisdict["model"]
print(thisdict)
Example
The del keyword can also delete the dictionary completely:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
del thisdict
print(thisdict) #this will cause an error because "thisdict" no longer exists.
Example
The clear() method empties the dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.clear()
print(thisdict)
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.
Example Print all key names in the dictionary, one by one:
for x in thisdict:
print(x)
Example Print all values in the dictionary, one by one:
for x in thisdict:
print(thisdict[x])
Example You can also use the values() method to return values of a dictionary:
for x in thisdict.values():
print(x)
Example You can use the keys() method to return the keys of a dictionary:
for x in thisdict.keys():
print(x)
Example Loop through both keys and values, by using the items() method:
for x, y in thisdict.items():
print(x, y)
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().
Example Make a copy of a dictionary with the copy() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = thisdict.copy()
print(mydict)
Another way to make a copy is to use the built-in function dict().
Example Make a copy of a dictionary with the dict() function:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
mydict = dict(thisdict)
print(mydict)
In order to access the items of a dictionary refer to its key name. Key can be used inside square brackets.
# Python program to demonstrate
# accessing a element from a Dictionary
# Creating a Dictionary
Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}
# accessing a element using key
print("Accessing a element using key:")
print(Dict['name'])
# accessing a element using key
print("Accessing a element using key:")
print(Dict[1])
Output:
Accessing a element using key:
For
Accessing a element using key:
Geeks
There is also a method called get() that will also help in accessing the element from a dictionary.This method accepts key as argument and returns the value.
Complexities for Accessing elements in a Dictionary:
Time complexity: O(1)
Space complexity: O(1)
# Creating a Dictionary
Dict = {1: 'Geeks', 'name': 'For', 3: 'Geeks'}
# accessing a element using get()
# method
print("Accessing a element using get:")
print(Dict.get(3))
Output:
Accessing a element using get:
Geeks
You can access the items of a dictionary by referring to its key name, inside square brackets:
Example Get the value of the “model” key:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = thisdict["model"]
There is also a method called get() that will give you the same result:
Get the value of the “model” key:
x = thisdict.get("model")
The keys() method will return a list of all the keys in the dictionary.
Get a list of the keys:
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.
Add a new item to the original dictionary, and see that the keys list gets updated as well:
car = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
x = car.keys()
print(x) #before the change
car["color"] = "white"
print(x) #after the change
The values() method will return a list of all the values in the dictionary.
Get a list of the values:
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.
Example Make a change in 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["year"] = 2020
print(x) #after the change
Example 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
The items() method will return each item in a dictionary, as tuples in a list.
Example Get a list of the key:value pairs
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.
Make a change in 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["year"] = 2020
print(x) #after the change
Example 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
To determine if a specified key is present in a dictionary use the in keyword:
Check if “model” is present in the dictionary:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
if "model" in thisdict:
print("Yes, 'model' is one of the keys in the thisdict dictionary")
In order to access the value of any key in the nested dictionary, use indexing [] syntax.
# Creating a Dictionary
Dict = {'Dict1': {1: 'Geeks'},
'Dict2': {'Name': 'For'}}
# Accessing element using key
print(Dict['Dict1'])
print(Dict['Dict1'][1])
print(Dict['Dict2']['Name'])
Output:
{1: 'Geeks'}
Geeks
For
You can change the value of a specific item by referring to its key name:
Example Change the “year” to 2018:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict["year"] = 2018
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.
Example Update the “year” of the car by using the update() method:
thisdict = {
"brand": "Ford",
"model": "Mustang",
"year": 1964
}
thisdict.update({"year": 2020})
Method|description
–|–
clear() |Remove all the elements from the dictionary
copy() |Returns a copy of the dictionary
get() |Returns the value of specified key
items() |Returns a list containing a tuple for each key value pair
keys() |Returns a list containing dictionary’s keys
pop() |Remove the element with specified key
popitem() |Removes the last inserted key-value pair
update() |Updates dictionary with specified key-value pairs
values() |Returns a list of all the values of dictionary
fromkeys() |Returns a dictionary with the specified keys and value
setdefault() |Returns the value of the specified key. If the key does not exist: insert the key, with the specified value
# demo for all dictionary methods
dict1 = {1: "Python", 2: "Java", 3: "Ruby", 4: "Scala"}
# copy() method
dict2 = dict1.copy()
print(dict2)
# clear() method
dict1.clear()
print(dict1)
# get() method
print(dict2.get(1))
# items() method
print(dict2.items())
# keys() method
print(dict2.keys())
# pop() method
dict2.pop(4)
print(dict2)
# popitem() method
dict2.popitem()
print(dict2)
# update() method
dict2.update({3: "Scala"})
print(dict2)
# values() method
print(dict2.values())
Output:
{1: 'Python', 2: 'Java', 3: 'Ruby', 4: 'Scala'}
{}
Python
dict_items([(1, 'Python'), (2, 'Java'), (3, 'Ruby'), (4, 'Scala')])
dict_keys([1, 2, 3, 4])
{1: 'Python', 2: 'Java', 3: 'Ruby'}
{1: 'Python', 2: 'Java'}
{1: 'Python', 2: 'Java', 3: 'Scala'}
dict_values(['Python', 'Java', 'Scala'])