Many licenses in the Python world are like: "You can make
changes, but have to leave in my Copyright notice.".

Would it be possible that the original author could not
claim a Copyright anymore when code has been changed?

The following example code is short, so it exaggerates the
effect, but you may get the idea:

# Copyright 1979 John Doe, you may make changes, but leave in this line!
import turtle
turtle.write( 'Hello 80s!' )

, now, a user changes this into:

# Copyright 1979 John Doe, you may make changes, but leave in this line!
print( 'Hello 80s!' )

, and later into:

# Copyright 1979 John Doe, you may make changes, but leave in this line!
print( 'Python is cool!' )

. I am asking because I think such Copyright claims might be
too vague because one has to speculate about such questions.

On 2023-02-02, Stefan Ram <ram@zedat.fu-berlin.de> wrote:
> Many licenses in the Python world are like: "You can make
> changes, but have to leave in my Copyright notice.".
>
> Would it be possible that the original author could not
> claim a Copyright anymore when code has been changed?

No. If you change someone else's code then you have created a derived
work, which requires permission from both the original author and you
to copy. (Unless you change it so much that nothing remains of the
original author's code, of course.)

On 3/02/23 6:38 am, Jon Ribbens wrote:
> If you change someone else's code then you have created a derived
> work, which requires permission from both the original author and you
> to copy. (Unless you change it so much that nothing remains of the
> original author's code, of course.)

"Nothing" is probably a bit extreme; somewhere between "exactly the
same" and "completely different" there will be a borderline case,
although exactly where the border lies would require a court case
to determine.

When in doubt, the sensible and courteous thing would be to include
the original copyright notice as requested, maybe with a "based on
work by..." attribution.

--
Greg

On 2023-02-03, Greg Ewing <greg.ewing@canterbury.ac.nz> wrote:
> On 3/02/23 6:38 am, Jon Ribbens wrote:
>> If you change someone else's code then you have created a derived
>> work, which requires permission from both the original author and you
>> to copy. (Unless you change it so much that nothing remains of the
>> original author's code, of course.)
>
> "Nothing" is probably a bit extreme; somewhere between "exactly the
> same" and "completely different" there will be a borderline case,
> although exactly where the border lies would require a court case
> to determine.

Well yes, technically if you remove so much code that what remains
of the original is so de minimis that it can't be considered
copyrightable then you're good. But that doesn't seem that useful
to know, because if you've removed that much then what remains,
pretty much by definition, isn't going to be useful. You'd be
better off simply starting from scratch and having an unimpeachable
claim to own the entire copyright yourself.