#12978: conditionally_defined decorator for methods and other attributes
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Reporter: nthiery | Owner: jason
Type: enhancement | Status: new
Priority: major | Milestone: sage-
Component: misc | wishlist
Keywords: categories, | Resolution:
conditionally_defined | Merged in:
Authors: | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: #15056 | Stopgaps:
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Comment (by SimonKing):
By the way, here is yet another example. This time, it is for a special
Python method (`__len__`).
Scenario: We have a class A, with `len` being implemented. But the
instances are mutable. in a sub-class B, some instances are immutable (but
not all). If an instance is immutable, we want that the length is cached
(which is possible by #12601).
This can be done as follows:
{{{
sage: class A(object):
....: def __init__(self, n):
....: self.data = range(n)
....: def __len__(self):
....: return len(self.data)
sage: class B(A):
....: def __init__(self, n, immutable):
....: A.__init__(self, n)
....: if immutable:
....: self.is_immutable = True
....: @conditionally_defined("is_immutable")
....: @cached_method
....: def __len__(self):
....: return super(B,self).__len__()
}}}
First of all, the length works for both mutable and "immutable" instances
of B:
{{{
sage: a = B(3,False)
sage: b = B(5,True)
sage: len(a)
3
sage: len(b)
5
}}}
Of course, one can alter both instances, but we see that the length of b
is cached, in contrast to the length of a:
{{{
sage: a.data.append(4)
sage: b.data.append(5)
sage: len(a)
4
sage: len(b)
5
sage: len(b.data)
6
}}}
Note that the cached length function is very fast. Compare with this:
{{{
sage: class C(object):
....: def __len__(self):
....: return int(5)
....:
sage: c = C()
sage: len(c)
5
sage: len(b)
5
sage: %timeit len(c)
1000000 loops, best of 3: 793 ns per loop
sage: %timeit len(b)
1000000 loops, best of 3: 377 ns per loop
}}}
This is because a `CachedMethodCallerNoArgs` has Cython speed.
--
Ticket URL: <http://trac.sagemath.org/ticket/12978#comment:23>
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