[Numpy-discussion] problem with assigning to recarrays
Sat Feb 28 00:58:49 CST 2009
On Fri, Feb 27, 2009 at 19:06, Brian Gerke <firstname.lastname@example.org> wrote:
> On Feb 27, 2009, at 4:30 PM, Robert Kern wrote:
>> r[where(r.field1 == 1.)] make a copy. There is no way for us to
>> construct a view onto the original memory for this circumstance given
>> numpy's memory model.
> Many thanks for the quick reply. I assume that this is true only for
> record arrays, not for ordinary arrays? Certainly I can make an
> assignment in this way with a normal array.
Well, you are doing two very different things. Let's back up a bit.
Python gives us two hooks to modify an object in-place with an
assignment: __setitem__ and __setattr__.
x[<item>] = y ==> x.__setitem__(<item>, y)
x.<attr> = y ==> x.__setattr__('<attr>', y)
Now, we don't need to restrict ourselves to just variables for 'x'; we
can have any expression that evaluates to an object.
(<expr>)[<item>] = y ==> (<expr>).__setitem__(<item>, y)
(<expr>).<attr> = y ==> (<expr>).__setattr__('<attr>', y)
The key here is that the (<expr>) on the LHS is evaluated just like
any expression appearing anywhere else in your code. The only special
in-place behavior is restricted to the *outermost* [<item>] or
So when you do this:
r[where(r.field1 == 1.)].field2 = 1.0
it translates to something like this:
tmp = r.__getitem__(where(r.field1 == 1.0)) # Makes a copy!
Note that the first line is a __getitem__, not a __setitem__ which can
modify r in-place.
> Also, if it is truly impossible to change this behavior, or to have it
> raise an error--then are there any best-practice suggestions for how
> to remember and avoid running into this non-obvious behavior? If one
> thinks of record arrays as inheriting from numpy arrays, then this
> problem is certainly unexpected.
It's a natural consequence of the preceding rules. This a Python
thing, not a difference between numpy arrays and record arrays. Just
keep those rules in mind.
> Also, I've just found that the following syntax does do what is
> (r.field2)[where(field1 == 1.)] = 1.
> It is at least a little aesthetically displeasing that the syntax
> works one way but not the other. Perhaps my best bet is to stick with
> this syntax and forget that the other exists? A less-than-satisfying
> solution, but workable.
If you drop the extraneous bits, it becomes a fair bit more readable:
r.field2[r.field1 == 1] = 1
This is idiomatic; you'll see it all over the place where record
arrays are used. The reason that this form modifies r in-place is
because r.__getattr__('field2') is able to return a view rather than a
"I have come to believe that the whole world is an enigma, a harmless
enigma that is made terrible by our own mad attempt to interpret it as
though it had an underlying truth."
-- Umberto Eco
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