Understanding Python's list.sort() Method and sorted() Function

Python provides two primary ways to sort collections: the list method list.sort() , which sorts the list in place, and the built‑in function sorted() , which returns a new sorted list from any iterable.

list.sort() Function

The list.sort() method modifies the original list and accepts optional cmp (removed in Python 3), key , and reverse parameters. It returns None because the sorting is performed in place.

<code>list.sort(cmp=None, key=None, reverse=False)
# cmp parameter existed in Python 2.0
# removed in Python 3.0</code>

Common parameters:

key : a function that extracts a comparison key from each element.

reverse : True for descending order, False (default) for ascending.

Examples

<code># Ascending order
aList = [5, 4, 1, 3, 6, 2]
aList.sort()  # [1, 2, 3, 4, 5, 6]

# Descending order
aList = [5, 4, 1, 3, 6, 2]
aList.sort(reverse=True)
# aList => [6, 5, 4, 3, 2, 1]

# Sort by a specific element (e.g., second element of a tuple)
def takeSecond(elem):
    return elem[1]
random = [(2, 2), (3, 4), (4, 1), (1, 3)]
random.sort(key=takeSecond)
# random => [(4, 1), (2, 2), (1, 3), (3, 4)]

# Sort by length of strings
x = ['a', 'bbb', 'cc']
x.sort(key=len)
# x => ['a', 'cc', 'bbb']

# Custom compare function (Python 2 only)
def comp(x, y):
    if x < y:
        return 1
    elif x > y:
        return -1
    else:
        return 0

aList = [5, 4, 1, 3, 6, 2]
aList.sort(comp)  # works in Python 2 for descending order</code>

Other Tips

To keep the original list unchanged, create a copy before sorting or use the sorted() function.

<code># Method 1: copy then sort
aList = [5, 4, 1, 3, 6, 2]
bList = aList[:]  # deep copy
bList.sort()
print(aList)  # [5, 4, 1, 3, 6, 2]
print(bList)  # [1, 2, 3, 4, 5, 6]

# Method 2: use sorted()
aList = [5, 4, 1, 3, 6, 2]
bList = sorted(aList)
# bList => [1, 2, 3, 4, 5, 6]
</code>

sorted() Function

The built‑in sorted() function works on any iterable, returns a new list, and accepts the same key and reverse arguments.

<code>sorted(iterable, key=None, reverse=False)</code>

Parameters:

iterable : the collection to be sorted.

key : a function to extract a comparison key.

reverse : True for descending order.

Examples

<code># Simple list sorting
a = [5, 2, 3, 1, 4]
sorted(a)  # [1, 2, 3, 4, 5]

# Sorting dictionary keys
b = {1: 'D', 2: 'B', 3: 'B', 4: 'E', 5: 'A'}
sorted(b)  # [1, 2, 3, 4, 5]

# Sorting dictionary items by key
sorted(b.items(), key=lambda x: x[0])

# Sorting dictionary items by value length
sorted(b.items(), key=lambda x: len(x[1]))
</code>

Other Use Cases

Complex data can be sorted by multiple criteria, as shown in the medal‑ranking example where country data is parsed and sorted by medal counts.

<code>s = "Germany 10 11 16\nItaly 10 10 20\n..."
stodata = s.split('\n', -1)
para = {}
for line in range(len(stodata)):
    data = stodata[line].split(' ')
    para[data[0]] = [int(i) for i in data[1:]]
new_para = sorted(para.items(), key=lambda x: (x[1], x[0]))
</code>

These examples illustrate how to choose between in‑place sorting with list.sort() and creating a new sorted list with sorted() , depending on whether the original data should be preserved.