Hacker News Reader

Comment by tuxcanfly | Hacker News Reader

But don't the alternatives also depend on list slicing? Heck, the title is `coping a list`. If you have a different datatype, it is the business of that type to specify how to copy using __copy__, isn't it?

View on HN

Imagine a case like this:

  def example(input_data):
    l = list(input_data)
    # code that uses list-specific stuff and returns a result

The function 'example' doesn't want to touch the original input data, so it needs to make a copy of it. It also contains code that assumes operation on a list, so the copied value needs to support list-like operators. If you assume input_data is a list, you can use [:] or copy() to copy just fine, but if input_data is NOT a list then you cannot feed example a generator or some other list-like object or iterable and know for sure that it is going to work. By explicitly converting to list, you can take anything that implements __iter__, and then safely assume that the rest of your code will be working with lists. This adds a pretty bit of extra flexibility to the function and can make it much easier and/or cleaner to use.

Obviously as with anything the choice of list copy method is situation-dependent. Using [:] makes sense if you can guarantee the input is a list and you need maximal speed. Using copy() makes sense if you just want a copy of the input object and don't specifically care that the copy is itself a list. Using list() makes sense if you want to be able to take in all kinds of input values and be assured that the copy is a list. Use what is best for the situation at hand.

Reply on news.ycombinator.com