Re-using copyrighted code

[Chris Angelico]

On Sun, Jun 9, 2013 at 11:21 PM, Malte Forkel <malte.forkel at berlin.de> wrote:
> At least partially, my confusion seems to be caused by the dichotomy of
> the concepts of copyright and license. How do these relate to each other?

Ah, that one's easy enough to answer!

When you create something, you own it. That is what copyright
supports. A copyright line is a declaration "Hi, I created this, it's
mine". (Copyright can be transferred, so the "I created this" part
isn't always true, but the "it's mine" part is the point here.) Now,
just as with anything else you own, you have the right to choose who
uses it. I might say "This is my car, but I'll let you drive it on
condition you return it with a full tank of fuel". I'm not allowed to
put that sort of restriction on something that isn't mine, but because
it's mine, I can. The same applies to copyright; "This is my software,
but you're allowed to use it as long as you keep it free" (that's the
gist of the GPL) or "This is my software; go ahead, use it, do what
you like with it, only don't sue me" (the MIT license) are both
granting permissions on the basis of ownership.

Effectively, copyright is a declaration of a closed door, and the
license specifically opens it again. With something that's completely
unowned (aka "public domain"), nobody can place any restrictions on
it; otherwise, it's up to the owner to set the rules. Fortunately for
the open source world, lots and LOTS of owners are prepared to give
fairly generous rules regarding the use of their material!

When you make a significant change to something, you have copyright in
the changes. Otherwise, copyright remains with the original holder,
and you should acknowledge that. So if you take one entire source file
and make little or no changes to it, and then you link that with
source files of your own creation, your files would have your
copyright notice, and sre_parse.py would have the original copyright
notice (with an annotation indicating your changes, if you've made any
- see clause 3 of the original license). Things are a little messier
if it's hard to distinguish your code from the rest, but sometimes
you'll see copyright/license blocks saying things like "This
incorporates code from X and from Y and from Z", then follows up with
their original copyright lines and license texts.

tl;dr version: Copyright says "This is mine". License says "This is
how you may use my stuff".

ChrisA