Check if a given key already exists in a dictionary in Python

Dictionary is an unordered collection of data in the form of key-value pairs separated by commas inside curly brackets. Dictionaries are indexed by keys. They are generally optimized to retrieve values when the key is known. Values (definitions) are mapped to a specific key (words) similar to that of a dictionary in the real world. 

Now our task is to check if a given key exists in a dictionary or not. We can accomplish this through the ways below:

• Using the keys() method
• Using if and in
• Using has_key()

1. Using keys() method:

This inbuilt method returns a list of all the available keys in a given dictionary. We use this along with the ‘in’  keyword.

Example:

def fun_check_key(dict, key):

   if key in dict.keys():
      print("The key is present in dictionary", end ="\n")
      print("Corresponding value is:", dict[key])
   else:
      print("The key is present in the dictionary")

dict = {'One': '1', 'Two': '2', 'Three': 3}

key = 'Two'
fun check key(dict,key)

key = 'Four'
fun_check_key(dict, key)

Output:

2. Using if and in:

Here we won’t necessarily use the keys() method. It just returns a boolean value, True if present else returns False

Example:

def fun_check_key(dict, key):

   if key in dict.keys():
       print("The key is present in dictionary", end ="\n")
       print("Corresponding value is:", dict[key])
   else:
       print("The key is present in the dictionary")

dict = {'One': '1', 'Two': '2', 'Three': 3}

key = 'Two'
fun check key(dict,key)

key = 'Four'
fun_check_key(dict, key)

Output:

3. Using has_key():

This method also returns a boolean value, True if the key is present in the dictionary else False. We use this method along with the if statement as shown below. 

Example:

if dict.has_key(key):

Output:

This method is no longer supported by the versions Python 3 and above. We can only use it in Python 2. The output is much similar to the above mentioned ways.