Master Python List Sorting: sort() vs sorted() and Custom Comparisons

Python's sort() method sorts a list in place, while the sorted() function returns a new sorted list without modifying the original.

1. Basic Usage

Lists have a built‑in sort() method that rearranges the elements directly; tuples cannot use it because they are immutable.

x = [4, 6, 2, 1, 7, 9]
x.sort()
print(x)  # [1, 2, 4, 6, 7, 9]

2. Getting a Sorted Copy

To keep the original list unchanged, create a shallow copy with slicing and sort the copy:

x = [4, 6, 2, 1, 7, 9]
y = x[:]
y.sort()
print(y)  # [1, 2, 4, 6, 7, 9]
print(x)  # [4, 6, 2, 1, 7, 9]

Note: y = x[:] copies the list, whereas y = x would only create another reference to the same list.

Alternatively, use the sorted() function, which works on any iterable and always returns a new list:

x = [4, 6, 2, 1, 7, 9]
y = sorted(x)
print(y)  # [1, 2, 4, 6, 7, 9]
print(x)  # [4, 6, 2, 1, 7, 9]

Even strings can be sorted, producing a list of characters:

print(sorted('Python'))  # ['P', 'h', 'n', 'o', 't', 'y']

3. Custom Comparison Function

You can define your own comparator and pass it to sort() (Python 2 style):

def comp(x, y):
    if x < y:
        return 1
    elif x > y:
        return -1
    else:
        return 0

nums = [3, 2, 8, 0, 1]
nums.sort(comp)
print(nums)  # descending order: [8, 3, 2, 1, 0]

4. Optional Parameters: key and reverse

The key argument specifies a function that extracts a comparison key from each element:

x = ['mmm', 'mm', 'm', 'mm']
x.sort(key=len)
print(x)  # ['m', 'mm', 'mm', 'mmm']

The reverse flag sorts in descending order when set to True:

y = [3, 2, 8, 0, 1]
y.sort(reverse=True)
print(y)  # [8, 3, 2, 1, 0]

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