Difference between reverse and [::-1]

Regarding reversing list in python, the 3 main ways:

• the built-in function reversed(seq) that will return a reverse iterator which is an object representing the stream of data that will return successive items of this steam. Generating this reverse iterator is O(1) in time/space complexity and using it to iterate through the elements of a list will be O(N), O(1) in time/space complexity where N is the length of the list. This is the approach you want to go for if you simply want to iterate on the reversed list without modifying it. This approach gives the best performances in this case.

• p.reverse() reverses the list in-place. The operation is O(N), O(1) in time/space complexity as it will have to go through half of the elements of the list to reverse them and it doesn’t store the results in a new list.
  p.reverse() will return None and the elements will be directly reversed in p after this instruction. This approach is good if you do NOT need to keep the original list and you have several passes/operations to do on the elements of the reversed list and if you have to save the processed reversed list.

• Using slicing [::-1] creates a new object of the list/a copy in reversed order. The operation is O(N), O(N) in space/time complexity as you have to copy all the elements of the list in the new one and this new list will also consume the same amount of space as the original list. In terms of reference, you will have a new object created so even if print(p == x) returns True, print(p is x) will return False as the two objects even if they share the same values have different references. This approach is great if you need to keep the original list and have a reversed copy of it stored in a different object for further processing.

To summarize, depending on your use case you will have to use one of those 3 approaches that have slightly different objectives and completely different performances.

In Python, both the reverse() method of lists and the [::-1] slice notation can be used to reverse a list. However, the reverse() method is generally faster than the [::-1] slice notation, especially for larger lists.

Here is an example of how to use the reverse() method to reverse a list:

my_list = [1, 2, 3, 4, 5]
my_list.reverse()
print(my_list)  # Output: [5, 4, 3, 2, 1]

And here is an example of how to use the [::-1] slice notation to reverse a list:

my_list = [1, 2, 3, 4, 5]
my_list = my_list[::-1]
print(my_list)  # Output: [5, 4, 3, 2, 1]

To compare the performance of the two methods, you can use the timeit module to measure the execution time of each method. Here is an example of how to use the timeit module to compare the performance of the reverse() method and the [::-1] slice notation:

import timeit

# Set up the test lists
test_list_1 = list(range(10000))
test_list_2 = list(range(10000))

# Test the reverse() method
timeit.timeit(lambda: test_list_1.reverse(), number=10000)

# Test the [::-1] slice notation
timeit.timeit(lambda: test_list_2[::-1], number=10000)

This will execute each method 10000 times and measure the execution time. You can then compare the execution times to see which method is faster.