How to use a custom comparator functions in python to sort a 2D list?

Question

I am trying to solve a problem: https://hack.codingblocks.com/app/practice/1/176/problem

The problem is to sort a list of lists on the basis of the 2nd element. For any two lists if the second element is the same then sort on the basis of the first element. The input is like this:

79
4
Eve 78
Bob 99
Suzy 86
Alice 86

And the output should be:

Bob 99
Alice 86
Suzy 86

This is the Python code that I have written.

 def compare(item1, item2):
    if item1[1] == item2[1]:
         return item1[0] < item2[0]
    else:
         return item1[1] < item2[1]

 ts = int(input())
 n = int(input())
 m = []
 for i in range(n):
      l = list(input().split())
      l[1] = int(l[1])
      m.append(l)
 sorted(m, key=compare, reverse=True) # or m.sort(key=compare, reverse=True)

 for i in range(n):
      if m[i][1] >= ts:
          print(m[i][0], m[i][1])

For both approaches sort/sorted, there is an error:

For sorted:

TypeError: compare() missing 1 required positional argument: 'item2'
For sort:

TypeError: compare() missing 1 required positional argument: 'item2'

The error is the same for both approaches.

You are confusing two concepts: "key" and "compare". Python standard functions make a distinction between those two. In your code you define a "compare" function, then use it as "key" for sorted, and this doesn't work the way you expect it. I'll post a more detailed answer. — Stef
– Stef, Commented Jan 25, 2021 at 10:44
From the documentation: key specifies a function of one argument that is used to extract a comparison key from each element in iterable. So it should do access to the item properties which should be used for comparison (it will do key_function(item) for each element and compare the result in a default comparison way. — astentx
– astentx, Commented Jan 25, 2021 at 10:54
Please, correct the question and remove first two elements since they are not subjects to be sorted. The question should be clear without a need to register in some service to read the entire description. — astentx
– astentx, Commented Jan 25, 2021 at 11:17

Stef · Accepted Answer · 2021-01-25 11:03:30Z

You are confusing two concepts: a key function and a compare function. Although they can both be used as helpers for a sort, those are different types of functions.

Links to relevant documentation:

A compare function inputs two items, and returns true or false to tell which one is smaller. A key function inputs one item, and returns a value that can easily be compared to the values of other items. Imagine if you're in a store and want to sort items by price: the key function you'll need is the one that returns the price of an item.

Here you want to sort according to the pair (item[1], item[0]). So you can do exactly that:

unsorted_list = [('Eve', 78), ('Bob', 99), ('Suzy', 86), ('Alice', 86) ]

def the_key(item):
  return (item[1], item[0])

sorted_list = sorted(unsorted_list, key=the_key)

If you're not planning on reusing function the_key later in your code, you might find this a bit tedious and want to avoid using the def keyword to define a named function. In that case, you can use lambda instead of def, to define an anonymous function:

unsorted_list = [('Eve', 78), ('Bob', 99), ('Suzy', 86), ('Alice', 86) ]

sorted_list = sorted(unsorted_list, key=lambda item: (item[1], item[0]))

Finally, since sorting according to some fields of a list is very common, rather than defined the key yourself with def or lambda, you can use the pre-existing functions from module operator:

from operator import itemgetter

unsorted_list = [('Eve', 78), ('Bob', 99), ('Suzy', 86), ('Alice', 86) ]

sorted_list = sorted(unsorted_list, key=itemgetter(1,0))

Further fitting your input example

In your input example, it appears some of your list only have one element, instead of two. Trying to refer to "the second element" of a list which has only one element is bound to crash the python interpreter. You could define a custom key function which handles one-element lists separately:

unsorted_list = [[79], [4], ['The answer is', 42, 'but this list is too long'], ['Eve', 78], ['Bob', 99], ['Suzy', 86], ['Alice', 86]]

def the_key(item):
  if len(item) >= 2:
    return (item[1], item[0])
  else:
    return (item[0], '')

sorted_list = sorted(unsorted_list, key=the_key)

# or with lambda
sorted_list = sorted(unsorted_list, key=lambda item: (item[1], item[0]) if len(item) >= 2 else (item[0], ''))

Note that this will still crash if some of the items are empty lists. If that might be the case, you should add a third case in the_key to handle empty lists, if you want these empty lists kept in the sorted list; or you can filter the list first, using a list comprehension or filter, to remove the empty items.

I'm sorry, the first numbers are not sortable input, it is some auxiliary variables that not a part of the (sorting) question. I've had to register to find out how do they want to handle that empty-named values, but turnd out they are not intended for sorting. I'll add the comment on this to the original question. So I've deleted my comment on empty-named items and after you've fixed tuples with list it became entirely irrelevant

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