Q: lambda basics

lambda x: x * 2 creates an anonymous function that takes one argument and returns x * 2. Assigning it to double makes it callable as double(7). Lambdas are single-expression functions - they cannot contain statements (no if/else blocks, no for loops, no assignments). The equivalent def: def double(x): return x * 2. Use lambda for short inline functions; use def for anything more complex.

Q: list.sort() returns None

list.sort() sorts the list in place and returns None, not the sorted list. Assigning the return value of .sort() is one of the most common Python mistakes - you silently get None while your data disappears. The list itself (nums) is sorted correctly to [1, 1, 3, 4, 5]. If you need the sorted result as a value you can pass around, use sorted(nums) instead.

Question 1

sorted() vs list.sort()

Accepted Answer

sorted() is a built-in function that returns a new sorted list, leaving the original unchanged. It accepts any iterable (tuples, sets, generators). list.sort() mutates the list in place and returns None - a common bug is assigning the result: my_list = my_list.sort() gives None. Prefer sorted() when you need the original preserved; use .sort() when in-place is fine and you want to avoid the memory cost of a copy.

Question 2

sorted() with key=

Accepted Answer

key=len tells sorted() to sort by the length of each string. fig(3) < kiwi(4) < apple(5) < banana(6). The key function is applied to each element once before comparison - it does not change the elements themselves, only the sort order. key= is how you sort by any attribute or derived value without writing a custom comparator.

Question 3

lambda basics

Accepted Answer

lambda x: x * 2 creates an anonymous function that takes one argument and returns x * 2. Assigning it to double makes it callable as double(7). Lambdas are single-expression functions - they cannot contain statements (no if/else blocks, no for loops, no assignments). The equivalent def: def double(x): return x * 2. Use lambda for short inline functions; use def for anything more complex.

Question 4

list.sort() returns None

Accepted Answer

list.sort() sorts the list in place and returns None, not the sorted list. Assigning the return value of .sort() is one of the most common Python mistakes - you silently get None while your data disappears. The list itself (nums) is sorted correctly to [1, 1, 3, 4, 5]. If you need the sorted result as a value you can pass around, use sorted(nums) instead.