Python 3.x不支持cmp(list1,list2)函数（Python2.x支持），有无替代方案？

Absolutely! Since Python 3 removed the cmp() function that was available in Python 2, there are several straightforward alternatives depending on your use case:

1. Direct Value Comparison with Operators #

For most basic comparisons (checking equality, which value is larger/smaller), you can use Python's built-in comparison operators directly—==, !=, <, >, <=, >=. These work seamlessly with all standard Python types (lists, strings, numbers, etc.):

list1 = [1, 2, 3]
list2 = [1, 2, 4]

if list1 == list2:
    print("Lists are equal")
elif list1 < list2:
    print("list1 is smaller than list2")
else:
    print("list1 is larger than list2")

2. Emulate cmp()'s Return Value (-1, 0, 1) #

If you specifically need the same integer return style as Python 2's cmp() (where -1 means a < b, 0 means equal, 1 means a > b), you can create a tiny, reusable custom function:

def cmp(a, b):
    return (a > b) - (a < b)

# Example usage
print(cmp(5, 3))   # Output: 1
print(cmp(3, 5))   # Output: -1
print(cmp(4, 4))   # Output: 0

This works because in Python, boolean values (True/False) are subclasses of integers—True equals 1 and False equals 0, so the subtraction gives exactly the result we need.

3. Custom Sorting Logic #

In Python 2, you could pass a cmp parameter to sort() or sorted() for custom sorting. Python 3 replaced this with the more efficient key parameter, which is the preferred approach for most cases:

# Sort a list of tuples by the second element (Python 3 style)
my_tuples = [(1, 3), (2, 1), (3, 2)]
my_tuples.sort(key=lambda x: x[1])
# Result: [(2, 1), (3, 2), (1, 3)]

If you have complex sorting logic that can't be easily expressed with a key function, use functools.cmp_to_key() to convert a Python 2-style comparison function into a compatible key function:

from functools import cmp_to_key

# Custom comparison: sort strings first by length, then alphabetically
def compare_strings(a, b):
    if len(a) != len(b):
        return len(a) - len(b)
    return (a > b) - (a < b)

words = ["cat", "apple", "dog", "banana"]
sorted_words = sorted(words, key=cmp_to_key(compare_strings))
# Result: ["cat", "dog", "apple", "banana"]

内容的提问来源于stack exchange，提问作者Edward795