#13949: Make mutability on matrices a simple bint flag
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Reporter: nbruin | Owner: jason, was
Type: enhancement | Status: new
Priority: major | Milestone: sage-5.6
Component: linear algebra | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Comment (by nbruin):
Just some notes here. In order to make usable matrices from just
`Matrix.__new__()`, i.e., without running `__init__` we'd need to move
some things into `Matrix.__cinit__`. The main things there are:
{{{
self._immutability = False
self._cache = None
self._parent = parent
self._base_ring = parent.base_ring()
self._nrows = parent.nrows()
self._ncols = parent.ncols()
}}}
The first 3 instructions are fast and could be moved into
`matrix.__cinit__` (which doesn't exist at the moment), except that some
matrix routines call `__new__` with `parent=NULL`. They set the parent
themselves manually afterwards. We could fix that. However, if they have
to muck about with `_base_ring` and `_nrows,_ncols` anyway, there's not
much to win.
The last 3 instructions are slow, of course, because they require full-
blown method resolutions. Many matrix subclasses can be more efficient
about those, so it's probably good to leave those in the avoidable
`__init__` rather than the unavoidable `__cinit__`.
Note that the first two instructions are not necessary: Those are the
values to which those attributes get initialized by cython already.
The following might be reasonable:
- Change `mutability` to a `bint` flag, as proposed to this ticket. It
means that a mutable matrix doesn't need to touch that flag at all upon
initialization!
- matrix classes can avoid the base class `matrix.__init__` as long as
they set `_parent`, `_base_ring`, `_nrows`, `_ncols` themselves.
This is largely how currently things work, except for
`Matrix_integer_dense`, where the `__cinit__` calls
`Matrix_dense.__init__` already. Hence, if you inherit from
`Matrix_integer_dense`, you cannot get super efficient `__new__` anymore.
This is of course silly, because already setting `self._base_ring` is
something this class could do very efficiently. So, probably this call to
`Matrix_dense` should be removed, setting `self._base_ring` can be done in
`Matrix_integer_dense.__cinit__` directly (because it's just `ZZ`), and
setting `nrows` and `ncols` should probably be done explicitly (in most
cases you KNOW how big the matrix is)
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/13949#comment:6>
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