Handling missing keys in Python dictionaries

In Python, dictionaries are containers that map one key to its value with access time complexity to be O(1). But in many applications, the user doesn't know all the keys present in the dictionaries. In such instances, if the user tries to access a missing key, an error is popped indicating missing keys. 

Handling Missing Keys in Python Dictionaries

In the example, no key named 'c' in the dictionary popped a runtime error. To avoid such conditions, and to make the aware user that a particular key is absent or to pop a default message in that place, there are several methods to handle missing keys.

# Python code to demonstrate Dictionary and
# missing value error
d = { 'a' : 1 , 'b' : 2 }

# trying to output value of absent key 
print ("The value associated with 'c' is : ")
print (d['c'])

Error : 

Traceback (most recent call last):
  File "46a9aac96614587f5b794e451a8f4f5f.py", line 9, in 
    print (d['c'])
KeyError: 'c'

Handling Missing Keys in Python Dictionaries

There are the methods to handle missing keys in Python Dictionaries.

• Using key
• Using get()
• Using setdefault()
• Using defaultdict()
• Using try-except block

Python Program to Handling Missing keys in Python Dictionaries Using key

It is the basic way to solve key errors using if-else condition. To check if the key is present or not.

ele = {'a': 5, 'c': 8, 'e': 2}
if "q" in ele:
    print(ele["d"])
else:
    print("Key not found")

Key not found

Python Program to Handling Missing keys in Dictionaries Using get()

get(key,def_val) method is useful when we have to check for the key. If the key is present, the value associated with the key is printed, else the def_value passed in arguments is returned.

country_code = {'India' : '0091',
                'Australia' : '0025',
                'Nepal' : '00977'}

# search dictionary for country code of India
print(country_code.get('India', 'Not Found'))

# search dictionary for country code of Japan
print(country_code.get('Japan', 'Not Found'))

0091
Not Found

Handling Missing keys in Python Dictionaries Using setdefault()

setdefault(key, def_value) works in a similar way as to get(), but the difference is that each time a key is absent, a new key is created with the def_value associated with the key passed in arguments. In this example, we are using setdefault() function to handle the missing keys.

country_code = {'India' : '0091',
                'Australia' : '0025',
                'Nepal' : '00977'}

# Set a default value for Japan
country_code.setdefault('Japan', 'Not Present') 

# search dictionary for country code of India
print(country_code['India'])

# search dictionary for country code of Japan
print(country_code['Japan'])

0091
Not Present

Python Program to Handling Missing keys in Python Dictionaries Using defaultdict

"defaultdict" is a container that is defined in a module named "collections". It takes a function(default factory) as its argument. By default, the default factory is set to "int" i.e 0. If a key is not present in the defaultdict, the default factory value is returned and displayed. It has advantages over get() or setdefault().  

• A default value is set at the declaration. There is no need to invoke the function again and again and pass the similar values as arguments. Hence saving time.
• The implementation of defaultdict is faster than get() or setdefault().

 In this example, we are using defaultdict to check if the key is present or not and set default value "key not found" to the absent keys by using lambda.

# Python code to demonstrate defaultdict
import collections

# declaring defaultdict
# sets default value 'Key Not found' to absent keys
defd = collections.defaultdict(lambda : 'Key Not found')

# initializing values 
defd['a'] = 1

# initializing values 
defd['b'] = 2

# printing value 
print ("The value associated with 'a' is : ",end="")
print (defd['a'])

# printing value associated with 'c'
print ("The value associated with 'c' is : ",end="")
print (defd['c'])

The value associated with 'a' is : 1
The value associated with 'c' is : Key Not found

Handling Missing keys in Python Dictionaries Using the try-except block

This program shows how to handle missing keys in Python dictionaries using a try-except block. In this example, we are using try-except block to check whether the key is present or not.

country_code = {'India': '0091',
                'Australia': '0025',
                'Nepal': '00977'}

try:
    print(country_code['India'])
    print(country_code['USA'])
except KeyError:
    print('Not Found')

0091
Not Found

Time Complexity: The time complexity of the try-except block is O(1).
Space Complexity: The space complexity of the program depends on the size of the dictionary and the message to be printed in the 'except' block

kartik

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• python-dict
• Python dictionary-programs

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