On Wed, Sep 9, 2020 at 6:18 PM Nicholas Cole <nicholas.c...@gmail.com> wro > > On Wed, Sep 9, 2020 at 8:52 AM Chris Angelico <ros...@gmail.com> wrote: > > [snip] > > And if you absolutely have to mutate in place: > > > > items[:] = [i for i in items if i not in "bcd"] > > How does that work to mutate in place?
Technically it constructs a new filtered list, and then replaces the contents of the original list with the filtered one. That's effectively an in-place mutation; any other reference to that same list will see the change. Compare: a = list(range(20)) b = a a = [n for n in a if n % 3] print(a) print(b) You'll see that a and b now differ. But if you use slice assignment: a[:] = [n for n in a if n % 3] you'll see that the two are still the same list (and "a is b" will still be True). It's replacing the contents, rather than rebinding the name. ChrisA -- https://mail.python.org/mailman/listinfo/python-list
