Is the sum of 2 numbers in list k
You are using one explicit pass over the list (for i in list), and one implicit pass over the list (k - i in list).  This means your algorithm is \$O(N^2)\$.

Note: You can reduce your implementation to one line:

return any(k - i in list for i in list)


But you have a bug.   is_sum_of_2nums_in_list_k([5], 10) returns True.

A one pass algorithm


starts with an empty set,
takes each number in the list, 


tests if k-i is in the set,


return true if it is,

adds i to the set. 

returns false