key=functools.cmp_to_key

The `functools.cmp_to_key` function is a utility function in Python's `functools` module that helps convert a comparison function to a key function.

In Python, sorting a list of objects requires a way to compare the objects. This is done by providing a comparison function that takes two objects as input and returns `-1`, `0`, or `1` depending on whether the first object is less than, equal to, or greater than the second object.

However, some sorting functions in Python (such as the `sorted` function) require a key function instead of a comparison function. A key function takes a single object as input and returns a value that can be used for sorting.

The `cmp_to_key` function helps convert a comparison function to a key function by returning a new function that takes a single object as input and returns a tuple `(comparison_result, object)` where `comparison_result` is the result of calling the original comparison function with the input object and another object, and `object` is the input object itself. This tuple can then be used for sorting.

Here's an example of how to use `cmp_to_key`:

from functools import cmp_to_key

def compare_length(str1, str2):
    if len(str1) < len(str2):
        return -1
    elif len(str1) > len(str2):
        return 1
    else:
        return 0
        
strings = ['cat', 'dog', 'elephant', 'a', 'zebra']
sorted_strings = sorted(strings, key=cmp_to_key(compare_length))
print(sorted_strings)
# Output: ['a', 'cat', 'dog', 'zebra', 'elephant']

In this example, we define a comparison function `compare_length` that compares the lengths of two strings. We then use `cmp_to_key` to convert this comparison function to a key function, and pass it to the `sorted` function to sort a list of strings by length. The resulting sorted list is `['a', 'cat', 'dog', 'zebra', 'elephant']`.