NEP 16 — An abstract base class for identifying “duck arrays”#
Nathaniel J. Smith <firstname.lastname@example.org>
This NEP has been withdrawn in favor of the protocol based approach described in NEP 22
We propose to add an abstract base class
AbstractArray so that
third-party classes can declare their ability to “quack like” an
ndarray, and an
asabstractarray function that performs
asarray except that it passes through
AbstractArray instances unchanged.
Many functions, in NumPy and in third-party packages, start with some code like:
def myfunc(a, b): a = np.asarray(a) b = np.asarray(b) ...
This ensures that
np.ndarray objects, so
myfunc can carry on assuming that they’ll act like ndarrays both
semantically (at the Python level), and also in terms of how they’re
stored in memory (at the C level). But many of these functions only
work with arrays at the Python level, which means that they don’t
ndarray objects per se: they could work just as
well with any Python object that “quacks like” an ndarray, such as
sparse arrays, dask’s lazy arrays, or xarray’s labeled arrays.
However, currently, there’s no way for these libraries to express that
their objects can quack like an ndarray, and there’s no way for
myfunc to express that they’d be happy with
anything that quacks like an ndarray. The purpose of this NEP is to
provide those two features.
Sometimes people suggest using
np.asanyarray for this purpose, but
unfortunately its semantics are exactly backwards: it guarantees that
the object it returns uses the same memory layout as an
but tells you nothing at all about its semantics, which makes it
essentially impossible to use safely in practice. Indeed, the two
ndarray subclasses distributed with NumPy –
np.ma.masked_array – do have incompatible semantics, and if they
were passed to a function like
myfunc that doesn’t check for them
as a special-case, then it may silently return incorrect results.
Declaring that an object can quack like an array#
There are two basic approaches we could use for checking whether an object quacks like an array. We could check for a special attribute on the class:
def quacks_like_array(obj): return bool(getattr(type(obj), "__quacks_like_array__", False))
Or, we could define an abstract base class (ABC):
def quacks_like_array(obj): return isinstance(obj, AbstractArray)
If you look at how ABCs work, this is essentially equivalent to
keeping a global set of types that have been declared to implement the
AbstractArray interface, and then checking it for membership.
Between these, the ABC approach seems to have a number of advantages:
It’s Python’s standard, “one obvious way” of doing this.
ABCs can be introspected (e.g.
help(np.AbstractArray)does something useful).
ABCs can provide useful mixin methods.
ABCs integrate with other features like mypy type-checking,
One obvious thing to check is whether this choice affects speed. Using the attached benchmark script on a CPython 3.7 prerelease (revision c4d77a661138d, self-compiled, no PGO), on a Thinkpad T450s running Linux, we find:
np.asarray(ndarray_obj) 330 ns np.asarray() 1400 ns Attribute check, success 80 ns Attribute check, failure 80 ns ABC, success via subclass 340 ns ABC, success via register() 700 ns ABC, failure 370 ns
The first two lines are included to put the other lines in context.
This used 3.7 because both
getattrand ABCs are receiving substantial optimizations in this release, and it’s more representative of the long-term future of Python. (Failed
getattrdoesn’t necessarily construct an exception object anymore, and ABCs were reimplemented in C.)
The “success” lines refer to cases where
quacks_like_arraywould return True. The “failure” lines are cases where it would return False.
The first measurement for ABCs is subclasses defined like:
class MyArray(AbstractArray): ...
The second is for subclasses defined like:
class MyArray: ... AbstractArray.register(MyArray)
I don’t know why there’s such a large difference between these.
In practice, either way we’d only do the full test after first
checking for well-known types like
list, etc. This
is how NumPy currently checks for other double-underscore attributes
and the same idea applies here to either approach. So these numbers
won’t affect the common case, just the case where we actually have an
AbstractArray, or else another third-party object that will end up
__array_interface__ or end up as an
So in summary, using an ABC will be slightly slower than using an
attribute, but this doesn’t affect the most common paths, and the
magnitude of slowdown is fairly small (~250 ns on an operation that
already takes longer than that). Furthermore, we can potentially
optimize this further (e.g. by keeping a tiny LRU cache of types that
are known to be AbstractArray subclasses, on the assumption that most
code will only use one or two of these types at a time), and it’s very
unclear that this even matters – if the speed of
pass-throughs were a bottleneck that showed up in profiles, then
probably we would have made them faster already! (It would be trivial
to fast-path this, but we don’t.)
Given the semantic and usability advantages of ABCs, this seems like an acceptable trade-off.
AbstractArray, the definition of
asabstractarray is simple:
def asabstractarray(a, dtype=None): if isinstance(a, AbstractArray): if dtype is not None and dtype != a.dtype: return a.astype(dtype) return a return asarray(a, dtype=dtype)
Things to note:
asarrayalso accepts an
order=argument, but we don’t include that here because it’s about details of memory representation, and the whole point of this function is that you use it to declare that you don’t care about details of memory representation.
astypemethod allows the
aobject to decide how to implement casting for its particular type.
For strict compatibility with
asarray, we skip calling
astypewhen the dtype is already correct. Compare:
>>> a = np.arange(10) # astype() always returns a view: >>> a.astype(a.dtype) is a False # asarray() returns the original object if possible: >>> np.asarray(a, dtype=a.dtype) is a True
What exactly are you promising if you inherit from
This will presumably be refined over time. The ideal of course is that
your class should be indistinguishable from a real
nothing enforces that except the expectations of users. In practice,
declaring that your class implements the
simply means that it will start passing through
and so by subclassing it you’re saying that if some code works for
ndarrays but breaks for your class, then you’re willing to accept
bug reports on that.
To start with, we should declare
__array_ufunc__ to be an abstract
method, and add the
NDArrayOperatorsMixin methods as mixin
astype as an
@abstractmethod probably makes sense as
well, since it’s used by
asabstractarray. We might also want to go
ahead and add some basic attributes like
Adding new abstract methods will be a bit tricky, because ABCs enforce these at subclass time; therefore, simply adding a new @abstractmethod will be a backwards compatibility break. If this becomes a problem then we can use some hacks to implement an @upcoming_abstractmethod decorator that only issues a warning if the method is missing, and treat it like a regular deprecation cycle. (In this case, the thing we’d be deprecating is “support for abstract arrays that are missing feature X”.)
The name of the ABC doesn’t matter too much, because it will only be
referenced rarely and in relatively specialized situations. The name
of the function matters a lot, because most existing instances of
asarray should be replaced by this, and in the future it’s what
everyone should be reaching for by default unless they have a specific
reason to use
asarray instead. This suggests that its name really
should be shorter and more memorable than
is difficult. I’ve used
asabstractarray in this draft, but I’m not
really happy with it, because it’s too long and people are unlikely to
start using it by habit without endless exhortations.
One option would be to actually change
asarray's semantics so
that it passes through
AbstractArray objects unchanged. But I’m
worried that there may be a lot of code out there that calls
asarray and then passes the result into some C function that
doesn’t do any further type checking (because it knows that its caller
has already used
asarray). If we allow
asarray to return
AbstractArray objects, and then someone calls one of these C
wrappers and passes it an
AbstractArray object like a sparse
array, then they’ll get a segfault. Right now, in the same situation,
asarray will instead invoke the object’s
__array__ method, or
use the buffer interface to make a view, or pass through an array with
object dtype, or raise an error, or similar. Probably none of these
outcomes are actually desirable in most cases, so maybe making it a
segfault instead would be OK? But it’s dangerous given that we don’t
know how common such code is. OTOH, if we were starting from scratch
then this would probably be the ideal solution.
We can’t use
array, since those are already
Any other ideas?
AbstractArray(leaving behind an alias for backwards compatibility) and make it an ABC.
asabstractarray(or whatever we end up calling it), and probably a C API equivalent.
Begin migrating NumPy internal functions to using
This is purely a new feature, so there are no compatibility issues.
(Unless we decide to change the semantics of
One suggestion that has come up is to define multiple abstract classes for different subsets of the array interface. Nothing in this proposal stops either NumPy or third-parties from doing this in the future, but it’s very difficult to guess ahead of time which subsets would be useful. Also, “the full ndarray interface” is something that existing libraries are written to expect (because they work with actual ndarrays) and test (because they test with actual ndarrays), so it’s by far the easiest place to start.
Appendix: Benchmark script#
import perf import abc import numpy as np class NotArray: pass class AttrArray: __array_implementer__ = True class ArrayBase(abc.ABC): pass class ABCArray1(ArrayBase): pass class ABCArray2: pass ArrayBase.register(ABCArray2) not_array = NotArray() attr_array = AttrArray() abc_array_1 = ABCArray1() abc_array_2 = ABCArray2() # Make sure ABC cache is primed isinstance(not_array, ArrayBase) isinstance(abc_array_1, ArrayBase) isinstance(abc_array_2, ArrayBase) runner = perf.Runner() def t(name, statement): runner.timeit(name, statement, globals=globals()) t("np.asarray()", "np.asarray()") arrobj = np.array() t("np.asarray(arrobj)", "np.asarray(arrobj)") t("attr, False", "getattr(not_array, '__array_implementer__', False)") t("attr, True", "getattr(attr_array, '__array_implementer__', False)") t("ABC, False", "isinstance(not_array, ArrayBase)") t("ABC, True, via inheritance", "isinstance(abc_array_1, ArrayBase)") t("ABC, True, via register", "isinstance(abc_array_2, ArrayBase)")
This document has been placed in the public domain.