classproperty

class astropy.utils.decorators.classproperty(fget=None, doc=None, lazy=False)

Bases: property

Similar to property, but allows class-level properties. That is, a property whose getter is like a classmethod.

The wrapped method may explicitly use the classmethod decorator (which must become before this decorator), or the classmethod may be omitted (it is implicit through use of this decorator).

Note

classproperty only works for read-only properties. It does not currently allow writeable/deletable properties, due to subtleties of how Python descriptors work. In order to implement such properties on a class a metaclass for that class must be implemented.

Parameters:

fgetpython:callable()

The function that computes the value of this property (in particular, the function when this is used as a decorator) a la property.

docpython:str, optional

The docstring for the property–by default inherited from the getter function.

lazybool, optional

If True, caches the value returned by the first call to the getter function, so that it is only called once (used for lazy evaluation of an attribute). This is analogous to lazyproperty. The lazy argument can also be used when classproperty is used as a decorator (see the third example below). When used in the decorator syntax this must be passed in as a keyword argument.

Examples

>>> class Foo:
...     _bar_internal = 1
...     @classproperty
...     def bar(cls):
...         return cls._bar_internal + 1
...
>>> Foo.bar
2
>>> foo_instance = Foo()
>>> foo_instance.bar
2
>>> foo_instance._bar_internal = 2
>>> foo_instance.bar  # Ignores instance attributes
2

As previously noted, a classproperty is limited to implementing read-only attributes:

>>> class Foo:
...     _bar_internal = 1
...     @classproperty
...     def bar(cls):
...         return cls._bar_internal
...     @bar.setter
...     def bar(cls, value):
...         cls._bar_internal = value
...
Traceback (most recent call last):
...
NotImplementedError: classproperty can only be read-only; use a
metaclass to implement modifiable class-level properties

When the lazy option is used, the getter is only called once:

>>> class Foo:
...     @classproperty(lazy=True)
...     def bar(cls):
...         print("Performing complicated calculation")
...         return 1
...
>>> Foo.bar
Performing complicated calculation
1
>>> Foo.bar
1

If a subclass inherits a lazy classproperty the property is still re-evaluated for the subclass:

>>> class FooSub(Foo):
...     pass
...
>>> FooSub.bar
Performing complicated calculation
1
>>> FooSub.bar
1

Methods Summary

deleter(fdel)	Descriptor to obtain a copy of the property with a different deleter.
getter(fget)	Descriptor to obtain a copy of the property with a different getter.
setter(fset)	Descriptor to obtain a copy of the property with a different setter.

Methods Documentation

deleter(fdel)

Descriptor to obtain a copy of the property with a different deleter.

getter(fget)

Descriptor to obtain a copy of the property with a different getter.

setter(fset)

Descriptor to obtain a copy of the property with a different setter.