Python dictionary find key by value (4 methods)

In this article, I will explain how in Python dictionary find key by value using various methods using demonstrative examples. We will also see how to find key with maximum value in Python Dictionary.

In Python, a dictionary is a built-in data type that can be used to store a mutable, unordered collection of items. Each item stored in a dictionary has a key and value, making it a common choice for data retrieval.

While dictionaries allow for fast and easy access to data based on the key, there may be situations where you need to find a key based on a value. This article will explore various methods to find a key by its value in a Python dictionary.

Methos for Python dictionary find key by value

There are several methods to find key by value in Python dictionary.

Using for loop with dict.item() function
Using List comprehension
Using list.index() function
Using lambda and filter() functions

Let’s see them one by one using illustrative examples:

Method 1: Python find key by value in dictionary through for loop with item() function.

In Python, finding a key by a value in a dictionary using a for loop and the items() method involves iterating over all the key-value pairs in the dictionary and checking if the current value matches the desired value.

If a match is found, the corresponding key is returned. If the loop completes without finding a match, None (or some other default value) is returned to indicate that no key was found for the given value.

Example: Let’s use a scenario where we have a dictionary that represents the capital cities of various states in the USA. We’ll try to find the state given its capital.

You’re given the task of designing a simple system that takes in the name of a capital and returns the corresponding state. For simplicity, we’ll consider a few states and their capitals:

state_capitals = {
    'California': 'Sacramento',
    'Texas': 'Austin',
    'New York': 'Albany',
    'Florida': 'Tallahassee',
    'Illinois': 'Springfield'
}
def find_state_by_capital(capitals_dict, capital):
    for state, cap in capitals_dict.items():
        if cap == capital:
            return state
    return None

capital_input = 'Austin'
state_name = find_state_by_capital(state_capitals, capital_input)
print(f"The state with the capital {capital_input} is {state_name}.")

Output: We define a function find_state_by_capital that takes in the Python dictionary and a capital name. We use a for loop to iterate over the dictionary items. For each iteration, we compare the current capital (cap) with the given capital. If there’s a match, we immediately return the corresponding state. If the loop finishes without finding a match, we return None.

The state with the capital Austin is Texas.

This way we can use the for loop with dict.item() function to iterate over the Python dictionary to find a key by value.

Method 2: Using List comprehension to get key from value Python Dictionary

List comprehensions provide a concise and readable way to create lists in Python. They can also be used for searching a Python dictionary to find a key based on a given value. To find a key based on a value in a dictionary using list comprehension, we can iterate over the dictionary items and return the key where the value matches the desired value.

For example: Imagine we are creating a program where we have a dictionary of books and their authors in Python, and we need to find the authors of specific books in the Python list.

authors_dict = {
    'To Kill a Mockingbird': 'Harper Lee',
    '1984': 'George Orwell',
    'Moby Dick': 'Herman Melville',
    'Pride and Prejudice': 'Jane Austen',
    'The Great Gatsby': 'F. Scott Fitzgerald',
}
book_titles = ['1984', 'The Great Gatsby']
author_names = [author for book, author in authors_dict.items() if book in book_titles]
print(author_names)

Output: We iterate through the authors_dict Python Dictionary, using the items() method which provides each book title (book) and its author (author). Then, we use the condition if book in book_titles to check if the current book exists in our Python list of book_titles. If the condition is met, we add the author name to our result list author_names.

['George Orwell', 'F. Scott Fitzgerald']

We can use list comprehension to extract the author names based on the book titles in Python.

Method 3: Python get key from value from dictionary using list.index() function

In Python, we can get a key from a value in a dictionary by using the list.index() function, although it’s a bit indirect. This approach involves converting the Python dictionary values to a list and finding the index of the desired value using the list.index() function. This index corresponds to the key in the list of keys from the Python dictionary.

For instance: Let’s take another scenario where we have a dictionary in Python that contains the names and ages of people, where the keys are the names of people, and the values are their ages. In this example, we’ll find the name of a person given their age using the list.index() function.

people = {
    'Alice': 30,
    'Bob': 25,
    'Charlie': 35,
    'Diana': 40,
    'Eve': 22,
}
age_to_find = 35
ages = list(people.values())
names = list(people.keys())
index = ages.index(age_to_find)
name = names[index]
print(f"The name of the person with the age {age_to_find} is {name}")

Output: We first converted the ages and names to two separate Python lists, ages and names. We then found the index of the age 35 in the ages list in Python using the index() function. This index was used to get the name from the names Python list. Finally, we printed out the output, showing that the name of the person with the age 35 is “Charlie“.

The name of the person with the age 35 is Charlie

This way we can use list.index() function to get the key by value from dictionary in Python.

Method 4: How to get a key from a value in Python Dictionary using lambda and filter() function

To find a key by its value in a Python dictionary, we can use the filter() function in combination with a lambda function.

Name	Description
Lambda Function	In Python, a lambda function is an anonymous function that is defined with the lambda keyword. It can have any number of arguments but can only have one expression.
Filter Function	The filter() function takes two arguments: a function and an iterable. It applies the function to every item in the iterable and returns an iterator for the items where the function evaluates to True.
Combining filter() and lambda functions	We can use a lambda function as the first argument to the filter() function. This lambda function will compare each item’s value in the dictionary to the desired value. The filter() function will then return an iterator containing the items that match the condition set by the lambda function.

List of functions to be used for finding key by value in Python dictionary.

For instance: Let’s consider a scenario based on a company’s employee data in the USA. Suppose we have a Python dictionary that contains employee IDs as keys and their respective salaries as values. We want to find all employee IDs who have a specific salary.

We can use the filter() function along with a lambda function to filter out the items (employee IDs and salaries) where the salary is 75000.

employee_data = {
    'EMP123': 50000,
    'EMP124': 75000,
    'EMP125': 60000,
    'EMP126': 75000,
    'EMP127': 80000
}
salary = 75000
result = filter(lambda item: item[1] == salary, employee_data.items())
matched_items = list(result)
matched_employee_ids = [item[0] for item in matched_items]
print(f"The employee IDs for the salary {salary} are {matched_employee_ids}")

Output: This example demonstrates how to use the filter() and lambda functions to find keys (in this case, employee IDs) by their associated values (in this case, salaries) in a Python dictionary. In the example output, the employee IDs for employees with a salary of 75000 are ‘EMP124’ and ‘EMP126’.

The employee IDs for the salary 75000 are ['EMP124', 'EMP126']

This way we can use lambda and filter() functions to get key by value Python dictionary.

How to find key with maximum value in Python Dictionary?

In Python, we can easily find the key associated with the maximum value in a dictionary by using various approaches.

One of the most concise and readable ways to find the key with the maximum value in a dictionary in Python is to use the max() function. We can use the key parameter of the max() function to specify a function of one argument to extract a comparison key from each input element.

Example: Let’s consider another scenario related to a US based e-commerce platform to find out which product has the highest number of sales.

Imagine that an e-commerce platform wants to determine which product has the highest number of sales. They have a Python dictionary where the keys are product IDs and the values are the number of units sold for each product.

product_sales = {
    'P12345': 1200,
    'P54321': 2500,
    'P11222': 1750,
    'P23456': 3200,
    'P67890': 1500
}
best_selling_product = max(product_sales, key=product_sales.get)
print(f"The product with the highest sales is {best_selling_product}.")

Output: The result, best_selling_product, is then printed out, indicating the product with the highest number of sales.

The product with the highest sales is P23456.

This way we can find the key with maximum value in the Python dictionary using max() function.

Conclusion

This tutorial explains how in Python dictionary find key by value using four different methods such as the for loop with dict.item() function, the List comprehension, the list.index() function, and the lambda and filter() function with demonstrative examples.

While each method has its own advantages and disadvantages, the choice of method will depend on the specific requirements of our task.