FAQ: Quicksort: Python - Quicksort Introduction

This community-built FAQ covers the “Quicksort Introduction” exercise from the lesson “Quicksort: Python”.

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Sorting Algorithms

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Why doesnt my version work, they are the same?

# Define your quicksort function
def quicksort_my(list, start, end):
  print(start, end)
  if (start >= end):
  quicksort_my(list, start+1,end)
colors = ["blue", "red", "green", "purple", "orange"]

def quicksort_1(list, start, end):
  if start >= end:

  start += 1
  quicksort_1(list, start, end)

colors = ["blue", "red", "green", "purple", "orange"]
quicksort_1(colors, 0, len(colors) - 1)

I dont get why quicksort_my fails on the codeacademy IDE and the quicksort_1 doesnt…

1 Like

In the “_1” version, the third argument in the calling statement depends upon the length of the input list. In the “_my” version, the third argument is a number, 4. The grader probably is testing on lists of varying length, not just the one you see.

1 Like

That makes sense, I just didn’t understand the error message that came with the failing test because I was still printing the list elements in the correct order. Thanks for getting back to me!

1 Like

start_of_sub_list = 1
end_of_sub_list = len(my_list) - 2

my_list[start_of_sub_list : end_of_sub_list]

[‘burrito’, ‘sandwich’, ‘salad’]

Shouldn’t it be :end_of_sub_list = len(my_list) - 1
This will give an index value of 4, and then include the middle three elements?


From the exercise:

[“race cars”, “lasers”, “airplanes”]

start = 1
end = 1
start == end
A sub-list starting at index 1 and concluding at index 1
This would actually return [] not [“lasers”]

start = 2
end = 1
start > end
A sub-list start index 2 and concluding at index 1

list[1:1] would return and
list[3,4] ie when start is greater then end will also return

I think the correct check for the base case should have been:
start == end or start == end + 1
the exercise uses start>= end which also works but a different explanation should suffice instead of the one given


I was thinking the same thing as well. i think they made a mistake?

1 Like

list is a reserved keyword. Albeit it is asked to use it as a parameter and do not have significant impact, it is not a good habit to take to use reserved keyword like that!


Not sure if this will be cleared in the future if we go more in depth with pointers, but the base case for our function ( if start >= end )seems problematic:
if start = 1 and end = 1 list[1:1] will be []
if start = 0 and end = -1 list[0:-1] can have any length, not limited to “one or zero elements”

1 Like

Thank you for the interesting question! I did not pay attention why not “if len(list) >= 0”, as usual…

1 Like

Isn’t it bad that we are using list, which is a built-in python function, as a variable name?


When does start > end actually occur?

Not sure if they read the comments on these forums. As a few others pointed out, list is a reserved keyword in Python and should not be used in code because it can produce unexpected outcomes. This is basic. Being an educational platform, they should fix it ASAP. People are already getting the wrong idea.