The fastest way to check a key exists in the Python dictionary

Sometimes we might need to check whether a key exists in the Python dictionary. In this article, we will discuss the fastest way to check a key that exists in a Python dictionary.

The has_key() method was removed in Python 3 . So the recommended way is to use the in operator.

Two ways to check whether a key exists in a dictionary.

• Checking the key in the dictionary
• Checking the key in the dictionary keys list

The in operator

The in or not in operator in Python checks the membership. The x in s evaluates to true if x is a member of s and false otherwise.

Syntax

x in s
x not in s

To check whether a key exists in a dictionary:

 user = {"name": "Tom","age" : "24"}

# check name exists in user method 1
if "name" in user:
    print("Name key exits in user")

# check name exists in users method 2
if "name" in user.keys():
    print("Name key exits in user")

Output

Name key exits in user
Name key exits in user

So both methods check the membership of a key, but the question is which method is fastest? Let’s find out.

Check using the in operator and dictionary.

import time

user = {"name": "Tom","age" : "24"}

start = time.time()
for x in range(10000000):
  has_name = "name" in user
stop = time.time()

print(f"Elapsed time: {round(stop-start,4)} sec")

Output

Elapsed time: 0.9452 sec

Check using the in operator and dictionary keys list.

import time

user = {"name": "Tom","age" : "24"}

start = time.time()
for x in range(10000000):
  has_name = "name" in user.keys()
stop = time.time()

print(f"Elapsed time: {round(stop-start,4)} sec")

Output

Elapsed time: 1.6135 sec

We can see that user.keys() method is slower compared to the “name” in user.

As per the time complexity document, the time complexity of in operator as follows:

• dict – O(1), which means it will take constant time does not depend on the size of data.
• list – O(n), proportional to the amount of data.

Elapsed time of operation:

• dict – “name” in user – 0.942 sec
• list – “name” in user.keys() – 1.613 sec

Conclusion

In conclusion, the fastest way to check a key in a Python dictionary is to use the in operator and dictionary ( key in dict ). Read about how to find substring in a Python string.