Python | Get key from value in Dictionary

Let’s see how to get the key by value in Python Dictionary. 

Example: One Liner Code

Python3

# creating a new dictionary
my_dict ={"Java":100, "Python":112, "C":11}

# one-liner
print("One line Code Key value: ", list(my_dict.keys())
      [list(my_dict.values()).index(100)])

Output:

Java

Extract Key from Python Dictionary using Value

Method 1: Get the key by value using list comprehension

A list comprehension consists of brackets containing the expression, which is executed for each element along with the for loop to iterate over each element in the Python list to get the key from a value in Dictionary.

Python3

dic ={"geeks": "A","for":"B","geeks":"C"}

value = {i for i in dic if dic[i]=="B"}
print("key by value:",value)

Output:

key by value: {'for'}

Time Complexity: O(n)
Auxiliary Space: O(1)

Method 2: Get the key by value using a list.index()

The index() method returns the index of the corresponding value in a list. The approach used here is to find two separate lists of keys and values. Then fetch the key using the position of the value in the val_list. As key at any position N in key_list will have a corresponding value at position N in val_list. 

Python3

# creating a new dictionary
my_dict ={"java":100, "python":112, "c":11}

# list out keys and values separately
key_list = list(my_dict.keys())
val_list = list(my_dict.values())

# print key with val 100
position = val_list.index(100)
print(key_list[position])

Output:

java

Time complexity: O(1)
Auxiliary space: O(1)

Method 3: Get the key by value using dict.item()

We can also fetch the key from a value by matching all the values using the dict.item() and then print the corresponding key to the given value. 

Python3

# function to return key for any value
def get_key(val):
    for key, value in my_dict.items():
        if val == value:
            return key

    return "key doesn't exist"


# Driver Code
my_dict = {"Java": 100, "Python": 112, "C": 11}

print(get_key(100))
print(get_key(11))

Output:

Java
C

Time complexity: O(n), where n is the number of key-value pairs in the dictionary.
Auxiliary space: O(1), as the space used by the function does not depend on the size of the input dictionary.

Method 4: Using lambda and filter()

Here is an example of using the filter() function to get the key corresponding to a value in a dictionary:

Python3

my_dict = {"Java": 100, "Python": 112, "C": 11}

# Get the key corresponding to value 100
key = list(filter(lambda x: my_dict[x] == 100, my_dict))[0]
print(key)
#This code is contributed by Edula Vinay Kumar Reddy

Output

Java

In this example, the filter() function is used to create a list of keys from my_dict where the value is equal to 100. The resulting list is then indexed at position 0 to get the first element, which is the key corresponding to the value 100.

The time complexity of this approach is O(n), as the filter() function needs to iterate through the entire dictionary to create the list of keys. The space complexity is O(n), as the list of keys created by filter() has a size equal to the number of elements in the dictionary.