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# Lazy attributes

Lazy attributes

AUTHORS:

- Nicolas Thiery (2008): Initial version
- Nils Bruin (2013-05): Cython version

class `sage.misc.lazy_attribute.``lazy_attribute`(f)

  Bases: `sage.misc.lazy_attribute._lazy_attribute`

  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 `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

  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 ...>

  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

  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

  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:

  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

  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 ticket #11115, extension classes derived from `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'>

  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

class `sage.misc.lazy_attribute.``lazy_class_attribute`(f)

  Bases: `sage.misc.lazy_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 `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

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