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A sorting function that mimics Python 2's behavior of grouping incomparable types separately and sorting within each group, rather than raising a TypeError when comparing incompatible types.

Purpose

This function provides backward compatibility with Python 2's sorting behavior where mixed-type collections could be sorted by grouping incomparable types separately. In Python 3, comparing incompatible types (e.g., strings and integers) raises a TypeError. This function handles such cases by creating separate groups for incomparable items, sorting each group independently, and then chaining them together. This is useful when dealing with heterogeneous data that may contain mixed types that cannot be directly compared.

Source Code

def python2sort(x,key=None):
    if len(x) == 0: return x
    it = iter(x)
    groups = [[next(it)]]
    for item in it:
        for group in groups:
            try:
                item_precedence = item if key is None else key(item)
                group_precedence = group[0] if key is None else key(group[0])
                item_precedence < group_precedence  # exception if not comparable
                group.append(item)
                break
            except TypeError:
                continue
        else:  # did not break, make new group
            groups.append([item])
    return itertools.chain.from_iterable(sorted(group, key=key) for group in groups)

Parameters

Parameter Details

x: An iterable collection to be sorted. Can contain mixed types including incomparable types (e.g., mix of strings, integers, None values). Must be iterable and have at least one element to produce meaningful results.

key: Optional function that extracts a comparison key from each element. If None (default), elements are compared directly. If provided, should be a callable that takes one argument and returns a value to use for comparisons. The key function is applied before attempting comparisons to determine grouping and for sorting within groups.

Return Value

Returns an itertools.chain object that lazily yields elements from the sorted groups. The chain contains all elements from the input iterable, organized such that comparable items are grouped together and sorted within their groups. The order of groups depends on the order in which incomparable types were first encountered. To get a concrete list, wrap the result in list().

Dependencies

• itertools

Required Imports

import itertools

Usage Example

import itertools

def python2sort(x, key=None):
    if len(x) == 0: return x
    it = iter(x)
    groups = [[next(it)]]
    for item in it:
        for group in groups:
            try:
                item_precedence = item if key is None else key(item)
                group_precedence = group[0] if key is None else key(group[0])
                item_precedence < group_precedence
                group.append(item)
                break
            except TypeError:
                continue
        else:
            groups.append([item])
    return itertools.chain.from_iterable(sorted(group, key=key) for group in groups)

# Example 1: Mixed types (strings and integers)
mixed_data = [3, 'apple', 1, 'banana', 2, 'cherry']
result = list(python2sort(mixed_data))
print(result)  # [1, 2, 3, 'apple', 'banana', 'cherry']

# Example 2: With key function
data = [('a', 3), ('b', 1), (1, 5), ('c', 2)]
result = list(python2sort(data, key=lambda x: x[1]))
print(result)  # Sorted by second element within comparable groups

# Example 3: Empty list
empty = []
result = list(python2sort(empty))
print(result)  # []

Best Practices

• Wrap the returned itertools.chain object in list() if you need a concrete list rather than a lazy iterator
• Be aware that the order of groups for incomparable types depends on the order they first appear in the input
• This function is primarily useful for legacy code or data migration scenarios where Python 2 sorting behavior is needed
• For homogeneous data types, use Python's built-in sorted() function instead for better performance
• The key function should return comparable values; if key functions return incomparable types, those will also be grouped separately
• Empty iterables are returned as-is without modification
• The function creates groups in memory, so very large datasets with many incomparable type groups may consume significant memory

Tags

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