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""" Lazy attributes
AUTHORS:
- Nicolas Thiery (2008): Initial version - Nils Bruin (2013-05): Cython version """
#***************************************************************************** # Copyright (C) 2008 Nicolas M. Thiery <nthiery at users.sf.net> # # Distributed under the terms of the GNU General Public License (GPL) # # This code is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU # General Public License for more details. # # The full text of the GPL is available at: # # http://www.gnu.org/licenses/ #***************************************************************************** from __future__ import print_function
cdef class _lazy_attribute(object): """ Cython base class for lazy attributes.
EXAMPLES:
Only Python subclasses of this class are supposed to be instantiated::
sage: from sage.misc.lazy_attribute import _lazy_attribute sage: _lazy_attribute(lambda x:1) Traceback (most recent call last): ... NotImplementedError: Only instantiate wrapper python class
"""
cdef public f cdef public str __name__
def __init__(self, f): r""" Constructor for lazy attributes.
EXAMPLES::
sage: def f(x): ....: "doc of f" ....: return 1 ....: sage: x = lazy_attribute(f); x <sage.misc.lazy_attribute.lazy_attribute object at ...> sage: x.__doc__ 'doc of f' sage: x.__name__ 'f' sage: x.__module__ '__main__'
TESTS:
We check that :trac:`9251` is solved::
sage: Parent.element_class <sage.misc.lazy_attribute.lazy_attribute object at 0x...> sage: Parent.element_class.__doc__[91:147] 'The (default) class for the elements of this parent\n\n ' sage: Parent.element_class.__name__ 'element_class' sage: Parent.element_class.__module__ 'sage.misc.lazy_attribute' """
def _sage_src_lines_(self): r""" Returns the source code location for the wrapped function.
EXAMPLES::
sage: from sage.misc.sageinspect import sage_getsourcelines sage: g = lazy_attribute(banner) sage: (src, lines) = sage_getsourcelines(g) sage: src[0] 'def banner():\n' sage: lines 81 """
def __get__(self, a, cls): """ Implements the attribute access protocol.
EXAMPLES::
sage: class A: pass sage: def f(x): return 1 ... sage: f = lazy_attribute(f) sage: f.__get__(A(), A) 1 """ cdef CM cdef result # __cached_methods is supposed to be a public Cython attribute. # Apparently, these are *not* subject to name mangling. pass # Workaround: we make sure that cls is the class # where the lazy attribute self is actually defined. # This avoids running into an infinite loop # See About descriptor specifications
class lazy_attribute(_lazy_attribute): r""" A lazy attribute for an object is like a usual attribute, except that, instead of being computed when the object is constructed (i.e. in ``__init__``), it is computed on the fly the first time it is accessed.
For constant values attached to an object, lazy attributes provide a shorter syntax and automatic caching (unlike methods), while playing well with inheritance (like methods): a subclass can easily override a given attribute; you don't need to call the super class constructor, etc.
Technically, a :class:`lazy_attribute` is a non-data descriptor (see Invoking Descriptors in the Python reference manual).
EXAMPLES:
We create a class whose instances have a lazy attribute ``x``::
sage: class A(object): ....: def __init__(self): ....: self.a=2 # just to have some data to calculate from ....: ....: @lazy_attribute ....: def x(self): ....: print("calculating x in A") ....: return self.a + 1 ....:
For an instance ``a`` of ``A``, ``a.x`` is calculated the first time it is accessed, and then stored as a usual attribute::
sage: a = A() sage: a.x calculating x in A 3 sage: a.x 3
.. rubric:: Implementation details
We redo the same example, but opening the hood to see what happens to the internal dictionary of the object::
sage: a = A() sage: a.__dict__ {'a': 2} sage: a.x calculating x in A 3 sage: a.__dict__ {'a': 2, 'x': 3} sage: a.x 3 sage: timeit('a.x') # random 625 loops, best of 3: 89.6 ns per loop
This shows that, after the first calculation, the attribute ``x`` becomes a usual attribute; in particular, there is no time penalty to access it.
A lazy attribute may be set as usual, even before its first access, in which case the lazy calculation is completely ignored::
sage: a = A() sage: a.x = 4 sage: a.x 4 sage: a.__dict__ {'a': 2, 'x': 4}
Class binding results in the lazy attribute itself::
sage: A.x <sage.misc.lazy_attribute.lazy_attribute object at ...>
.. rubric:: Conditional definitions
The function calculating the attribute may return NotImplemented to declare that, after all, it is not able to do it. In that case, the attribute lookup proceeds in the super class hierarchy::
sage: class B(A): ....: @lazy_attribute ....: def x(self): ....: if hasattr(self, "y"): ....: print("calculating x from y in B") ....: return self.y ....: else: ....: print("y not there; B does not define x") ....: return NotImplemented ....: sage: b = B() sage: b.x y not there; B does not define x calculating x in A 3 sage: b = B() sage: b.y = 1 sage: b.x calculating x from y in B 1
.. rubric:: Attribute existence testing
Testing for the existence of an attribute with hasattr currently always triggers its full calculation, which may not be desirable when the calculation is expensive::
sage: a = A() sage: hasattr(a, "x") calculating x in A True
It would be great if we could take over the control somehow, if at all possible without a special implementation of hasattr, so as to allow for something like::
sage: class A (object): ....: @lazy_attribute ....: def x(self, existence_only=False): ....: if existence_only: ....: print("testing for x existence") ....: return True ....: else: ....: print("calculating x in A") ....: return 3 ....: sage: a = A() sage: hasattr(a, "x") # todo: not implemented testing for x existence sage: a.x calculating x in A 3 sage: a.x 3
Here is a full featured example, with both conditional definition and existence testing::
sage: class B(A): ....: @lazy_attribute ....: def x(self, existence_only=False): ....: if hasattr(self, "y"): ....: if existence_only: ....: print("testing for x existence in B") ....: return True ....: else: ....: print("calculating x from y in B") ....: return self.y ....: else: ....: print("y not there; B does not define x") ....: return NotImplemented ....: sage: b = B() sage: hasattr(b, "x") # todo: not implemented y not there; B does not define x testing for x existence True sage: b.x y not there; B does not define x calculating x in A 3 sage: b = B() sage: b.y = 1 sage: hasattr(b, "x") # todo: not implemented testing for x existence in B True sage: b.x calculating x from y in B 1
.. rubric:: lazy attributes and introspection
.. TODO::
Make the following work nicely::
sage: b.x? # todo: not implemented sage: b.x?? # todo: not implemented
Right now, the first one includes the doc of this class, and the second one brings up the code of this class, both being not very useful.
TESTS:
.. rubric:: Partial support for old style classes
Old style and new style classes play a bit differently with @property and attribute setting::
sage: class A: ....: @property ....: def x(self): ....: print("calculating x") ....: return 3 ....: sage: a = A() sage: a.x = 4 sage: a.__dict__ {'x': 4} sage: a.x 4 sage: a.__dict__['x']=5 sage: a.x 5
sage: class A (object): ....: @property ....: def x(self): ....: print("calculating x") ....: return 3 ....: sage: a = A() sage: a.x = 4 Traceback (most recent call last): ... AttributeError: can't set attribute sage: a.__dict__ {} sage: a.x calculating x 3 sage: a.__dict__['x']=5 sage: a.x calculating x 3
In particular, lazy_attributes need to be implemented as non-data descriptors for new style classes, so as to leave access to setattr. We now check that this implementation also works for old style classes (conditional definition does not work yet)::
sage: class A: ....: def __init__(self): ....: self.a=2 # just to have some data to calculate from ....: ....: @lazy_attribute ....: def x(self): ....: print("calculating x") ....: return self.a + 1 ....: sage: a = A() sage: a.__dict__ {'a': 2} sage: a.x calculating x 3 sage: a.__dict__ {'a': 2, 'x': 3} sage: a.x 3 sage: timeit('a.x') # random 625 loops, best of 3: 115 ns per loop
sage: a = A() sage: a.x = 4 sage: a.x 4 sage: a.__dict__ {'a': 2, 'x': 4}
sage: class B(A): ....: @lazy_attribute ....: def x(self): ....: if hasattr(self, "y"): ....: print("calculating x from y in B") ....: return self.y ....: else: ....: print("y not there; B does not define x") ....: return NotImplemented ....: sage: b = B() sage: b.x # todo: not implemented y not there; B does not define x calculating x in A 3 sage: b = B() sage: b.y = 1 sage: b.x calculating x from y in B 1
.. rubric:: Lazy attributes and Cython
This attempts to check that lazy attributes work with built-in functions like cpdef methods::
sage: class A: ....: def __len__(x): ....: return int(5) ....: len = lazy_attribute(len) ....: sage: A().len 5
Since :trac:`11115`, extension classes derived from :class:`~sage.structure.parent.Parent` can inherit a lazy attribute, such as ``element_class``::
sage: cython_code = ["from sage.structure.parent cimport Parent", ....: "from sage.structure.element cimport Element", ....: "cdef class MyElement(Element): pass", ....: "cdef class MyParent(Parent):", ....: " Element = MyElement"] sage: cython('\n'.join(cython_code)) sage: P = MyParent(category=Rings()) sage: P.element_class # indirect doctest <type '...MyElement'>
.. rubric:: About descriptor specifications
The specifications of descriptors (see 3.4.2.3 Invoking Descriptors in the Python reference manual) are incomplete w.r.t. inheritance, and maybe even ill-implemented. We illustrate this on a simple class hierarchy, with an instrumented descriptor::
sage: class descriptor(object): ....: def __get__(self, obj, cls): ....: print(cls) ....: return 1 sage: class A(object): ....: x = descriptor() sage: class B(A): ....: pass ....:
This is fine::
sage: A.x <class '__main__.A'> 1
The behaviour for the following case is not specified (see Instance Binding) when ``x`` is not in the dictionary of ``B`` but in that of some super category::
sage: B().x <class '__main__.B'> 1
It would seem more natural (and practical!) to get ``A`` rather than ``B``.
From the specifications for Super Binding, it would be expected to get ``A`` and not ``B`` as cls parameter::
sage: super(B, B()).x <class '__main__.B'> 1
Due to this, the natural implementation runs into an infinite loop in the following example::
sage: class A(object): ....: @lazy_attribute ....: def unimplemented_A(self): ....: return NotImplemented ....: @lazy_attribute ....: def unimplemented_AB(self): ....: return NotImplemented ....: @lazy_attribute ....: def unimplemented_B_implemented_A(self): ....: return 1 ....: sage: class B(A): ....: @lazy_attribute ....: def unimplemented_B(self): ....: return NotImplemented ....: @lazy_attribute ....: def unimplemented_AB(self): ....: return NotImplemented ....: @lazy_attribute ....: def unimplemented_B_implemented_A(self): ....: return NotImplemented ....: sage: class C(B): ....: pass ....:
This is the simplest case where, without workaround, we get an infinite loop::
sage: hasattr(B(), "unimplemented_A") # todo: not implemented False
.. TODO::
Improve the error message::
sage: B().unimplemented_A # todo: not implemented Traceback (most recent call last): ... AttributeError: 'super' object has no attribute 'unimplemented_A'
We now make some systematic checks::
sage: B().unimplemented_A Traceback (most recent call last): ... AttributeError: '...' object has no attribute 'unimplemented_A' sage: B().unimplemented_B Traceback (most recent call last): ... AttributeError: '...' object has no attribute 'unimplemented_B' sage: B().unimplemented_AB Traceback (most recent call last): ... AttributeError: '...' object has no attribute 'unimplemented_AB' sage: B().unimplemented_B_implemented_A 1
sage: C().unimplemented_A() Traceback (most recent call last): ... AttributeError: '...' object has no attribute 'unimplemented_A' sage: C().unimplemented_B() Traceback (most recent call last): ... AttributeError: '...' object has no attribute 'unimplemented_B' sage: C().unimplemented_AB() Traceback (most recent call last): ... AttributeError: '...' object has no attribute 'unimplemented_AB' sage: C().unimplemented_B_implemented_A # todo: not implemented 1 """ def __init__(self, f): """ Initialize ``self``.
EXAMPLES::
sage: def f(x): ....: "doc of f" ....: return 1 ....: sage: x = lazy_attribute(f); x <sage.misc.lazy_attribute.lazy_attribute object at ...> sage: x.__doc__ 'doc of f' sage: x.__name__ 'f' sage: x.__module__ '__main__' """ elif hasattr(f, "__doc__"): # Needed to handle Cython methods self.__doc__ = f.__doc__ elif hasattr(f, "__name__"): # Needed to handle Cython methods self.__name__ = f.__name__
class lazy_class_attribute(lazy_attribute): """ A lazy class attribute for an class is like a usual class attribute, except that, instead of being computed when the class is constructed, it is computed on the fly the first time it is accessed, either through the class itself or trough on of its objects.
This is very similar to :class:`lazy_attribute` except that the attribute is a class attribute. More precisely, once computed, the lazy class attribute is stored in the class rather than in the object. The lazy class attribute is only computed once for all the objects::
sage: class Cl(object): ....: @lazy_class_attribute ....: def x(cls): ....: print("computing x") ....: return 1 sage: Cl.x computing x 1 sage: Cl.x 1
As for a any usual class attribute it is also possible to access it from an object::
sage: b = Cl() sage: b.x 1
First access from an object also porperly triggers the computation::
sage: class Cl1(object): ....: @lazy_class_attribute ....: def x(cls): ....: print("computing x") ....: return 1 sage: Cl1().x computing x 1 sage: Cl1().x 1
.. WARNING::
The behavior of lazy class attributes with respect to inheritance is not specified. It currently depends on the evaluation order::
sage: class A(object): ....: @lazy_class_attribute ....: def x(cls): ....: print("computing x") ....: return str(cls) ....: @lazy_class_attribute ....: def y(cls): ....: print("computing y") ....: return str(cls) sage: class B(A): ....: pass
sage: A.x computing x "<class '__main__.A'>" sage: B.x "<class '__main__.A'>"
sage: B.y computing y "<class '__main__.B'>" sage: A.y computing y "<class '__main__.A'>" sage: B.y "<class '__main__.B'>"
TESTS::
sage: "x" in b.__dict__ False """ def __get__(self, _, cls): """ Implements the attribute access protocol.
EXAMPLES::
sage: class A: pass sage: def f(x): return 1 ....: sage: f = lazy_class_attribute(f) sage: f.__get__(A(), A) 1 """ return getattr(super(cls, cls),self.__name__)
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