"""
Abstract methods
"""
import types
def abstract_method(f = None, optional = False):
r"""
Abstract methods
INPUT:
- ``f`` -- a function
- ``optional`` -- a boolean; defaults to False
The decorator :obj:`abstract_method` can be used to declare
methods that should be implemented by all concrete derived
classes. This declaration should typically include documentation
for the specification for this method.
The purpose is to enforce a consistent and visual syntax for such
declarations. It is used by the Sage categories for automated
tests (see ``Sets.Parent.test_not_implemented``).
EXAMPLES:
We create a class with an abstract method::
sage: class A(object):
...
... @abstract_method
... def my_method(self):
... '''
... The method :meth:`my_method` computes my_method
...
... EXAMPLES::
...
... '''
... pass
...
sage: A.my_method
<abstract method my_method at ...>
The current policy is that a ``NotImplementedError`` is raised
when accessing the method through an instance, even before the
method is called::
sage: x = A()
sage: x.my_method
Traceback (most recent call last):
...
NotImplementedError: <abstract method my_method at ...>
It is also possible to mark abstract methods as optional::
sage: class A(object):
...
... @abstract_method(optional = True)
... def my_method(self):
... '''
... The method :meth:`my_method` computes my_method
...
... EXAMPLES::
...
... '''
... pass
...
sage: A.my_method
<optional abstract method my_method at ...>
sage: x = A()
sage: x.my_method
NotImplemented
The official mantra for testing whether an optional abstract
method is implemented is::
# Fixme: sage -t complains about indentation below
# sage: if x.my_method is not NotImplemented:
# ... x.my_method()
# ... else:
# ... print "x.my_method not available. Let's use some other trick."
# ...
# x.my_method not available. Let's use some other trick.
.. rubric:: Discussion
The policy details are not yet fixed. The purpose of this first
implementation is to let developers experiment with it and give
feedback on what's most practical.
The advantage of the current policy is that attempts at using a
non implemented methods are caught as early as possible. On the
other hand, one cannot use introspection directly to fetch the
documentation::
sage: x.my_method? # todo: not implemented
Instead one needs to do::
sage: A._my_method? # todo: not implemented
This could probably be fixed in :mod:`sage.misc.sageinspect`.
TODO: what should be the recommended mantra for existence testing from the class?
TODO: should extra information appear in the output? The name of
the class? That of the super class where the abstract method is defined?
TODO: look for similar decorators on the web, and merge
.. rubric:: Implementation details
Technically, an abstract_method is a non-data descriptor (see
Invoking Descriptors in the Python reference manual).
The syntax ``@abstract_method`` w.r.t. @abstract_method(optional = True)
is achieved by a little trick which we test here::
sage: abstract_method(optional = True)
<function <lambda> at ...>
sage: abstract_method(optional = True)(banner)
<optional abstract method banner at ...>
sage: abstract_method(banner, optional = True)
<optional abstract method banner at ...>
"""
if f is None:
return lambda f: AbstractMethod(f, optional = optional)
else:
return AbstractMethod(f, optional)
class AbstractMethod(object):
def __init__(self, f, optional = False):
"""
Constructor for abstract methods
EXAMPLES::
sage: def f(x):
... "doc of f"
... return 1
...
sage: x = abstract_method(f); x
<abstract method f at ...>
sage: x.__doc__
'doc of f'
sage: x.__name__
'f'
sage: x.__module__
'__main__'
"""
assert isinstance(f, types.FunctionType)
assert isinstance(optional, bool)
self._f = f
self._optional = optional
if hasattr(f, "func_doc"):
self.__doc__ = f.func_doc
if hasattr(f, "func_name"):
self.__name__ = f.func_name
else:
self.__name__ = "..."
if hasattr(f, "__module__"):
self.__module__ = f.__module__
def __repr__(self):
"""
EXAMPLES::
sage: abstract_method(banner)
<abstract method banner at ...>
sage: abstract_method(banner, optional = True)
<optional abstract method banner at ...>
"""
return "<" + ("optional " if self._optional else "") + "abstract method %s"%repr(self._f)[10:]
def _sage_src_lines_(self):
"""
Returns the source code location for the wrapped function.
EXAMPLES::
sage: from sage.misc.sageinspect import sage_getsourcelines
sage: g = abstract_method(banner)
sage: (src, lines) = sage_getsourcelines(g)
sage: src[0]
'def banner():\n'
sage: lines
79
"""
from sage.misc.sageinspect import sage_getsourcelines
return sage_getsourcelines(self._f)
def __get__(self, instance, cls):
"""
Implements the attribute access protocol.
EXAMPLES::
sage: class A: pass
sage: def f(x): return 1
...
sage: f = abstract_method(f)
sage: f.__get__(A(), A)
Traceback (most recent call last):
...
NotImplementedError: <abstract method f at ...>
"""
if instance is None:
return self
elif self._optional:
return NotImplemented
else:
raise NotImplementedError(repr(self))
def is_optional(self):
"""
Returns whether an abstract method is optional or not.
EXAMPLES::
sage: class AbstractClass:
... @abstract_method
... def required(): pass
...
... @abstract_method(optional = True)
... def optional(): pass
sage: AbstractClass.required.is_optional()
False
sage: AbstractClass.optional.is_optional()
True
"""
return self._optional
def abstract_methods_of_class(cls):
"""
Returns the required and optional abstract methods of the class
EXAMPLES::
sage: class AbstractClass:
... @abstract_method
... def required1(): pass
...
... @abstract_method(optional = True)
... def optional2(): pass
...
... @abstract_method(optional = True)
... def optional1(): pass
...
... @abstract_method
... def required2(): pass
...
sage: sage.misc.abstract_method.abstract_methods_of_class(AbstractClass)
{'required': ['required1', 'required2'], 'optional': ['optional1', 'optional2']}
"""
result = { "required" : [],
"optional" : []
}
for name in dir(cls):
entry = getattr(cls, name)
if not isinstance(entry, AbstractMethod):
continue
if entry.is_optional():
result["optional"].append(name)
else:
result["required"].append(name)
return result
