Python | Find indices with None values in given list

Many times while working in data science domain we need to get the list of all the indices which are None, so that they can be easily be prepossessed. This is quite a popular problem and solution to it comes quite handy. Let’s discuss certain ways in which this can be done.

Method #1 : Using list comprehension + range()
In this method we just check for each index using the range function and store the index if we find that index is None.

# Python3 code to demonstrate 
# finding None indices in list  
# using list comprehension + enumerate 
  
# initializing list  
test_list = [1, None, 4, None, None, 5] 
  
# printing original list 
print("The original list : " + str(test_list)) 
  
# using list comprehension + enumerate 
# finding None indices in list  
res = [i for i in range(len(test_list)) if test_list[i] == None] 
  
# print result 
print("The None indices list is : " + str(res)) 

Output :

The original list : [1, None, 4, None, None, 5]
The None indices list is : [1, 3, 4]

Method #2 : Using list comprehension + enumerate()
The enumerate function can be used to iterate together the key and value pair and list comprehension can be used to bind all this logic in one line.

# Python3 code to demonstrate 
# finding None indices in list  
# using list comprehension + enumerate 
  
# initializing list  
test_list = [1, None, 4, None, None, 5] 
  
# printing original list 
print("The original list : " + str(test_list)) 
  
# using list comprehension + enumerate 
# finding None indices in list  
res = [i for i, val in enumerate(test_list) if val == None] 
  
# print result 
print("The None indices list is : " + str(res)) 

Output :

The original list : [1, None, 4, None, None, 5]
The None indices list is : [1, 3, 4]

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