The syntax of setdefault() is as follows:

dict.setdefault(key1, default=None)  

key1 -- This is key to be searched.

Default -- if key1 is not found, then the message will be returned and added to the dictionary. Let's see the following example:

 >>> port1.setdefault(23, "Unknown")
 'Telnet'
 >>> port1
 {80: 'http', 22: 'SSH', 23: 'Telnet'}
 >>> port1.setdefault(19, "Unknown")
 'Unknown'
 >>> port1
 {80: 'http', 19: 'Unknown', 22: 'SSH', 23: 'Telnet'}

If the message has been not been set, then it returns and adds a default value, None. See the following example:

 >>> port1.setdefault(18)
 >>> port1
 {80: 'http', 18: None, 19: 'Unknown', 22: 'SSH', 23: 'Telnet'}
 >>>

To avoid KeyError, we can use the get() method, but we can add one more check to avoid KeyError. The has_key() method facilitates you to check whether the given key exists or not.