Python list permutations in lexicographic order

Question

This code is about Lexicographic Ordering algorithms and its works but how can I change the style of the code to professional coding style?

from typing import List    



def swap(_list: List, index1: int, index2: int):
        _list[index1], _list[index2] = _list[index2], _list[index1]
        return _list
    

def lexico_graphic(items: List):
    paths = [items]
    while True:
        # step 1
        largest_i = None
        for i in range(len(items) - 1):
            if items[i] < items[i + 1]:
                largest_i = i
        if largest_i is None:
            return paths

        # step 2
        largest_j = 0
        for j in range(len(items)):
            if items[largest_i] < items[j]:
                largest_j = j

        # step 3
        items = swap(items, largest_i, largest_j)

        # step 4
        items = items[:largest_i + 1] + items[:largest_i:-1]
        paths.append(items)

For me, the token ‘items: List’ causes compile-time error. — coderodde
– coderodde, Commented Nov 3, 2022 at 15:09

QuasiStellar · Accepted Answer · 2022-11-02 15:34:11Z

In Python objects are passed into functions by creating a reference that points to the same value, meaning that when you invoke your function swap() on list items, local _list refers to the exact same items object you passed in there. When you mutate _list in the next line (_list[index1], _list[index2] = _list[index2], _list[index1]) the underlying object changes, which means your function is no longer pure: you were trying to return a new list with 2 items swapped, but instead modified the original list.

If you don't want this function to mutate the original list you can write it like this:

def swap(_list: List, index1: int, index2: int):
    new_list = _list[:]  # equivalent to `_list.copy()`
    new_list[index1], new_list[index2] = new_list[index2], new_list[index1]
    return new_list

As you can see the original list never changes.

If you instead wanted the function to mutate your list, you can get rid of the return statement:

def swap(_list: List, index1: int, index2: int):
    _list[index1], _list[index2] = _list[index2], _list[index1]

and use it without the assignment:

swap(items, largest_i, largest_j)

# instead of

items = swap(items, largest_i, largest_j)

The algorithm looks fine, but naming may need some adjustments:
_list - underscores in python are used as access modifiers, this parameter can be named items for example
lexico_graphic - it's unclear what this function does by looking at its name

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