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ufunc and binop overrides and behavior suggestions
Pauli Virtanen edited this page Jul 16, 2015
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31 revisions
This page lists several suggestions on how __numpy_ufunc__, __array_priority__, ndarray subclassing, and Python binary operations should work together in the future.
The discussion is ongoing in gh-5844. As sometimes code speaks more than words, and it is clumsy to revise an evolving suggestion in Github discussion thread, the latest suggestions can be kept here.
Let us assign a name and a revision number for each suggestion. Increment revision number on each edit.
class MyNdarray(object):
_known_classes = (np.ndarray,)
def _can_handle(self, other):
if isinstance(other, _known_classes + (MyNdarray,)):
return True
if hasattr(other, "__numpy_ufunc__"):
return False
return True
def __add__(self, other):
return np.add(self, other)
def __radd__(self, other):
return np.add(other, self)
def __iadd__(self, other):
return np.add(self, other, out=self)
# ... repeat for other binops ...
def __numpy_ufunc__(self, ufunc, method, args, kwargs):
if method == "__call__":
return self._numpy_ufunc_call(ufunc, *args, **kwargs)
elif method == "reduce":
return self._numpy_ufunc_reduce(ufunc, *args, **kwargs)
else:
raise NotImplementedError("it's just an example ok")
def _numpy_ufunc_call(self, ufunc, *args, out=None, **kwargs):
new_args = []
for arg in args:
if not self._can_handle(arg):
return NotImplemented
new_args.append(np.asarray(arg))
args = new_args
if out is not None:
if not isinstance(out, tuple):
out = (out,)
new_out = []
for arg in out:
if not self._can_handle(arg):
return NotImplemented
new_out.append(np.asarray(arg))
out = tuple(new_out)
return ufunc(*args, out=out, **kwargs)
def _numpy_ufunc_reduce(self, a, out=None, **kwargs):
if not self._can_handle(a):
return NotImplemented
else:
a = np.asarray(a)
if out is not None:
if not self._can_handle(out):
return NotImplemented
else:
out = np.asarray(out)
return ufunc.reduce(a, out=out, **kwargs)class MyNdarray(object):
__numpy_binop_override__ = True
_known_classes = (np.ndarray,)
def _can_handle(self, other):
if isinstance(other, self._known_classes + (MyNdarray,)):
return True
if getattr(other, "__numpy_binop_override__", False):
return False
# potential addition/alternative: consider presence of
# __numpy_ufunc__ as an opt-out
return True
def __add__(self, other):
return self.__numpy_ufunc__(self.add, "__call__", (self, other), {})
def __radd__(self, other):
return self.__numpy_ufunc__(self.add, "__call__", (other, self), {})
def __iadd__(self, other):
# IF we want to never give `other` the opportunity to do an out-of-place
# operation:
return np.add(self, other, out=self)
# OTHERWISE:
return self.__numpy_ufunc__(self.add, "__call__", (self, other), out=self)
# ... repeat for every binop ...
# Implementation of __numpy_ufunc__ is the same as in ufunc.1 above
def __numpy_ufunc__(self, ufunc, method, args, kwargs):
if method == "__call__":
return self._numpy_ufunc_call(ufunc, *args, **kwargs)
elif method == "reduce":
return self._numpy_ufunc_reduce(ufunc, *args, **kwargs)
else:
raise NotImplementedError("it's just an example ok")
def _numpy_ufunc_call(self, ufunc, *args, out=None, **kwargs):
new_args = []
for arg in args:
if not self._can_handle(arg):
return NotImplemented
new_args.append(np.asarray(arg))
args = new_args
if out is not None:
if not isinstance(out, tuple):
out = (out,)
new_out = []
for arg in out:
if not self._can_handle(arg):
return NotImplemented
new_out.append(np.asarray(arg))
out = tuple(new_out)
return ufunc(*args, out=out, **kwargs)
def _numpy_ufunc_reduce(self, a, out=None, **kwargs):
if not self._can_handle(a):
return NotImplemented
else:
a = np.asarray(a)
if out is not None:
if not self._can_handle(out):
return NotImplemented
else:
out = np.asarray(out)
return ufunc.reduce(a, out=out, **kwargs)Here, I first try to define how ndarray itself behaves, and then give an example of a container subclass as well as a different array-like (the latter is nearly identical to opt-out above).
(update 2: correct MaskedArray) (update 3: mention iadd special treatment) (update 4: let output argument handle all inplace stuff)
class ndarray():
def _can_handle(self, other):
# to ensure that ndarray subclasses can handle ndarray by default:
# we're a subclass (or the same), so no need to convert.
if isinstance(self, other):
return True
# Strict subclasses of us and objects defining `__numpy_ufunc__` get treated identically.
if isinstance(other, type(self)) or hasattr(other, '__numpy_ufunc__'):
return False
# backwards compatibility
if getattr(other, '__array_priority__', 0) > 0:
return False
return True
def __add__(self, other):
return self.__numpy_ufunc__(np.add, "__call__", 0, (self, other), {})
def __radd__(self, other):
return self.__numpy_ufunc__(np.add, "__call__", 1, (other, self), {})
def __iadd__(self, other):
# Here, we explicitly stick with python operator logic, which means that if
# self doesn't know what to do with other, but other does know what to do
# with self, this will not happen in-place.
return self.__numpy_ufunc__(np.add, "__call__", 0, (self, other), out=self)
# ... repeat for every binop ...
def __numpy_ufunc__(self, ufunc, method, i, args, **kwargs):
new_args = []
for arg in args:
if self._can_handle(other):
new_args.append(np.asarray(other))
else:
try:
new_args.append(other.__array__())
except:
return NotImplemented
args = new_args
# The next check is not strictly necessary, since one should only be able to
# get here with `self not in out`.
out = kwargs.get('out', None)
if out is not None:
if not instance(out, tuple):
out = (out,)
if self not in out or any(type(out) is not type(self)):
raise TypeError("Can only handle in-place to arrays of own class")
# Note that we are guaranteed to work here, since all arguments are ndarray,
# so we do not have to worry about catching possible TypeError.
return getattr(ufunc, method)(*args, **kwargs)
class ufunc():
def __init__(self, ndarray_only_routine):
# the ndarray_only_routine is the basic code that can only handle ndarray, i.e.,
# does not do any coercing any more.
self.ndarray_only_routine = ndarray_only_routine
def __call__(self, *args, **kwargs):
return self.execute('__call__', *args, **kwargs)
def execute(self, method, *args, **kwargs):
#
<some code that ensures that args only contains inputs and kwargs everything else>
#
out = kwargs.get('out', ())
if isinstance(out, tuple):
if any(type(o) is not type(out[0]) for o in out[1:]):
raise TypeError("All outputs must be of the same type.")
else:
out = (out,)
if all(type(arg) is ndarray for arg in args + out):
return self.ndarray_only_routine(method, *args, **kwargs)
# we may have non-ndarray instances; try __numpy_ufunc__ until something succeeds.
result = None
for i, arg in enumerate(args + out):
if hasattr(arg, '__numpy_ufunc__'):
# If outputs are given, an argument should only be tried if it is one of
# them, since inputs cannot be expected to coerce output to the right shape.
if out != () and arg not in out:
continue
# Could add subclass check here as well to speed things up.
result = arg.__numpy_ufunc__(self, method, i, args, **kwargs)
if result is not NotImplemented:
return result
if result is None and out == ():
# None of the arguments were array-like (e.g., np.add(1., 1.); so nothing has been tried
# yet. Use ndarray to see if the arguments can be converted.
result = ndarray.__numpy_ufunc__(self, method, None, *args, **kwargs)
if result is not NotImplemented:
return result
raise TypeError
add = ufunc(ndarray_only_add)
class MaskedArray(ndarray):
# A container class that defers everything except masks to ndarray
def __numpy_ufunc__(self, ufunc, method, i, args, **kwargs):
new_args = []
masks = []
for arg in args:
if hasattr(arg, 'mask'): # Duck-typing
masks.append(arg.mask)
new_args.append(arg.data)
else:
masks.append(None)
new_args.append(arg)
args = new_args
out = kwargs.pop('out', None)
if out is not None:
if not isinstance(out, tuple):
out = (out,)
if self not in out or any(type(out) is not type(self)):
raise TypeError("Can only handle in-place to arrays of own class")
kwargs['out'] = tuple(arg.data for arg in out)
# All arguments are now guaranteed not to be type(self) any more, so we call
# the ufunc to deal with possible other types (e.g., if self.data defined
# __numpy_ufunc__, or one of the other arguments had one). If this failes,
# we return NotImplemented, since possibly the other argument can handle
# MaskedArray even though it cannot handle our content (seems unlikely, but
# let's be proper).
try:
result = getattr(ufunc, method)(*args, **kwargs)
except TypeError:
return NotImplemented
# Alternatively (possibly more logical), we just let our content try.
result = self.data.__numpy_ufunc__(ufunc, method, i, args, **kwargs)
if result is NotImplemented:
return NotImplemented
# ignore multiple outputs here for this example's sake.
if result is not out:
out = result.view(type(self))
out.mask = self.combine_masks(masks)
return out
class MyNdarray(object):
_known_classes = (np.ndarray,)
def _can_handle(self, other):
if isinstance(other, self._known_classes + (self.__class__,)):
return True
if hasattr(other, "__numpy_ufunc__"):
return False
return True
# These methods are defined just like for `ndarray`
def __add__(self, other):
return self.__numpy_ufunc__(np.add, "__call__", (self, other), {})
def __radd__(self, other):
return self.__numpy_ufunc__(np.add, "__call__", (other, self), {})
def __iadd__(self, other):
# See note on ndarray above.
return self.__numpy_ufunc__(np.add, "__call__", (self, other), out=self)
# ... repeat for every binop ...
# Implementation of __numpy_ufunc__ inspired by ufunc.1 above. This non-container
# class insists it has to know how to deal with other classes rather than change itself into
# an ndarray and let other classes try.
def __numpy_ufunc__(self, ufunc, method, i, args, **kwargs):
new_args = []
for arg in args:
if self._can_handle(arg):
# this class is rather boring, it just turns itself into an array.
new_args.append(np.asarray(arg))
else:
try:
new_args.append(arg.__array__())
except:
return NotImplemented
args = new_args
out = kwargs.pop('out', None)
if out is not None:
if not isinstance(out, tuple):
out = (out,)
# We can only get here is type(out) is type(self).
out = tuple(np.asarray(arg) for arg in out)
result = getattr(ufunc, method)(*args, out=out, **kwargs)
<possible stuff to turn result into type(self)>
return resultclass UfuncWrapper:
def __init__(self, values):
# values should be an array-like object
self.values = values
def __numpy_ufunc__(self, ufunc, method, i, inputs, **kwargs):
# replace self with self.values before calling the ufunc again
inputs = tuple(x.values if x is self else x for x in inputs)
if out is not None:
if not isinstance(out, tuple):
out = (out,)
out = tuple(x.values if x is self else x for x in out)
# do the computation on unwrapped arrays
result = getattr(ufunc, method)(*inputs, **kwargs)
# now wrap the result
return type(self)(result)
# binary ops are defined by calling ufuncs
def __add__(self, other):
return np.add(self, other)
def __radd__(self, other):
return np.add(other, self)
def __iadd__(self, other):
return np.add(self, other, out=self)
# repeat for all binary ops... (could use a standard mixin)class OptOutWrapper:
# __init__ and __numpy_ufunc__ are defined as on UfuncWrapper
# add any necessary opt-out flags
def __init__(self, values):
# values should be an array-like object
self.values = values
def __numpy_ufunc__(self, ufunc, method, i, inputs, out=None, **kwargs):
# replace self with self.values before calling the ufunc again
inputs = tuple(x.values if x is self else x for x in inputs)
if out is not None:
if not isinstance(out, tuple):
out = (out,)
out = tuple(x.values if x is self else x for x in out)
# do the computation on unwrapped arrays
result = getattr(ufunc, method)(*inputs, out=out, **kwargs)
# now wrap the result
return type(self)(result)
def _binary_op(self, op, other, reflexive=False):
# we need to write another wrapper for binary operations to properly handle
# passing off to the wrapped object
if reflexive:
result = op(other, self.values)
else:
result = op(self.values, other)
return type(self)(result)
# binary ops use special _binary_op wrapper
def __add__(self, other):
return self._binary_op(operator.add, other)
def __iadd__(self, other):
return self._binary_op(operator.iadd, other)
def __radd__(self, other):
return self._binary_op(operator.add, other, reflexive=True)
# repeat for all binary ops...Example restricting numpy-like binop handling within an interface:
class MyNdarray(object):
# This can be a separate flag [Option 4],
# or replaced with getattr(other, `__numpy_ufunc__`, True) [Option 2]
__numpy_binop__ = True
_known_classes = (SomeNonArrayLikeClass,)
# The default value of the flag for numpy.ndarray is likely fixed to True:
_default_value = True
# However, new classes might want to be better citizens and do instead:
_default_value = False
_known_classes += (float, int, np.generic)
# The argument can be made that automatic cast of buffers etc to ndarrays
# should not be made in binops --- if you want to use a buffer like an array,
# cast it to an array.
# Note that removing edges from a valid dispatch graph cannot break
# validity of the graph, so changing the default is fine.
def __add__(self, other):
if getattr(other, "__numpy_binop__", _default_value):
return np.add(self, other)
elif isinstance(other, self._known_classes):
return np.add(self, np.asarray(other)) # for example
else:
return NotImplemented
def __radd__(self, other):
if getattr(other, "__numpy_binop__", _default_value):
return np.add(other, self)
elif isinstance(other, self._known_classes):
return np.add(self, np.asarray(other)) # for example
else:
return NotImplemented
def __iadd__(self, other):
# IF we want to enforce += allows only array-likes
return np.add(self, other, out=self)
# OTHERWISE:
if getattr(other, "__numpy_binop__", _default_value) or isinstance(other, self._known_classes):
return np.add(self, other, out=self)
else:
return NotImplemented
# ... repeat for every binop ...
def _can_handle(self, other):
if isinstance(other, self._known_classes + (self.__class__,)):
return True
if hasattr(other, "__numpy_ufunc__"):
return False
return True
# Implementation of __numpy_ufunc__ is the same as in ufunc.1 above
def __numpy_ufunc__(self, ufunc, method, args, kwargs):
if method == "__call__":
return self._numpy_ufunc_call(ufunc, *args, **kwargs)
elif method == "reduce":
return self._numpy_ufunc_reduce(ufunc, *args, **kwargs)
else:
raise NotImplementedError("it's just an example ok")
def _numpy_ufunc_call(self, ufunc, *args, out=None, **kwargs):
new_args = []
for arg in args:
if not self._can_handle(arg):
return NotImplemented
new_args.append(np.asarray(arg))
args = new_args
if out is not None:
if not isinstance(out, tuple):
out = (out,)
new_out = []
for arg in out:
if not self._can_handle(arg):
return NotImplemented
new_out.append(np.asarray(arg))
out = tuple(new_out)
return ufunc(*args, out=out, **kwargs)
def _numpy_ufunc_reduce(self, a, out=None, **kwargs):
if not self._can_handle(a):
return NotImplemented
else:
a = np.asarray(a)
if out is not None:
if not self._can_handle(out):
return NotImplemented
else:
out = np.asarray(out)
return ufunc.reduce(a, out=out, **kwargs)