Functions related to dundermethods¶
Working with dundermethods can be tricky, because when python searches
for a dundermethod, it doesn’t use the same mechanism as it does for
normal attribute access. Normal attribute access like obj.attr
will
look for attr
in the instance’s namespace (i.e. obj.__dict__
)
and the class’s namespace (i.e. type(obj).__dict__
plus the __dict__
of every parent class). Dundermethods, on the other hand, are only
searched for in the class namespace - not the instance namespace.
Defining a dundermethod in the instance namespace won’t work:
class Demo:
pass
obj = Demo()
obj.__len__ = lambda self: 0
print(len(obj)) # throws TypeError: object of type 'Demo' has no len()
And neither will defining a dundermethod in a metaclass:
class DemoMeta(type):
def __len__(cls):
return 0
class Demo(metaclass=DemoMeta):
pass
obj = Demo()
print(len(obj)) # throws TypeError: object of type 'Demo' has no len()
So if you wanted to implement your own len
function, you wouldn’t have an
easy way of accessing the relevant __len__
method - obj.__len__
would
be incorrect because it would search the instance namespace, and type(obj).__len__
would be incorrect because it would search the metaclass namespace. That’s where
these functions come in - get_bound_dundermethod(obj)
or get_class_dundermethod(type(obj))
would do the work for you.
-
introspection.
DUNDERMETHOD_NAMES
= {'__abs__', '__add__', '__aenter__', '__aexit__', '__aiter__', '__and__', '__anext__', '__await__', '__bool__', '__bytes__', '__call__', '__ceil__', '__class_getitem__', '__complex__', '__contains__', '__del__', '__delattr__', '__delete__', '__delitem__', '__delslice__', '__dir__', '__div__', '__divmod__', '__enter__', '__eq__', '__exit__', '__float__', '__floor__', '__floordiv__', '__format__', '__fspath__', '__ge__', '__get__', '__getattr__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__iand__', '__ifloordiv__', '__ilshift__', '__imatmul__', '__imod__', '__imul__', '__index__', '__init__', '__init_subclass__', '__instancecheck__', '__int__', '__invert__', '__ior__', '__ipow__', '__irshift__', '__isub__', '__iter__', '__itruediv__', '__ixor__', '__le__', '__len__', '__length_hint__', '__lshift__', '__lt__', '__matmul__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__next__', '__or__', '__pos__', '__pow__', '__prepare__', '__radd__', '__rand__', '__rdiv__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rfloordiv__', '__rlshift__', '__rmatmul__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__set__', '__set_name__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__sub__', '__subclasscheck__', '__subclasses__', '__truediv__', '__trunc__', '__xor__'}¶ A set containing the names of all dundermethods.
-
introspection.
AUGMENTED_ASSIGNMENT_DUNDERMETHOD_NAMES
= {'__iadd__', '__iand__', '__ifloordiv__', '__ilshift__', '__imatmul__', '__imod__', '__imul__', '__ior__', '__ipow__', '__irshift__', '__isub__', '__itruediv__', '__ixor__'}¶ A set containing the names of all augmented assignment dundermethods.
New in version 1.1.
-
introspection.
iter_class_dundermethods
(cls, bound=None)¶ Yields all dundermethods implemented by the given class as
(method_name, method)
tuples.(For the purpose of this function, “implemented” simply means “exists”. Even if the method’s value is
None
or anything else, it will still be yielded.)If multiple classes in the MRO implement the same dundermethod, both methods will be yielded. Methods implemented by subclasses will always be yielded before methods implemented by parent classes.
You can cause the iteration to stop early by passing in a class as the upper
bound
. The MRO will only be iterated up to thebound
, excluding thebound
class itself. This is useful for excluding dundermethods implemented inobject
.- Parameters
cls – The class whose dundermethods to yield
bound – Where to stop iterating through the class’s MRO
- Returns
An iterator yielding
(method_name, method)
tuples- Raises
TypeError – If
cls
is not a class
-
introspection.
class_implements_dundermethod
(cls, method_name, bound=None)¶ Checks whether the given class implements a certain dundermethod.
The method is considered implemented if any of the classes in the MRO have an entry for
method_name
in their__dict__
. The only exception is that__hash__
methods are considered not implemented if their value isNone
.Note that
object
implements various dundermethods, including some unexpected ones like__lt__
. Remember to pass inbound=object
if you wish to exclude these.- Parameters
cls – A class
method_name – The name of a dundermethod
bound – Where to stop searching through the class’s MRO
- Returns
A boolean indicating whether the class implements that dundermethod
- Raises
TypeError – If
cls
is not a class
-
introspection.
class_implements_any_dundermethod
(cls, methods, bound=None)¶ Checks whether the given class implements at least one of the given dundermethods.
- Parameters
cls – A class
methods – The names of a bunch of dundermethods
bound – Where to stop searching through the class’s MRO
- Returns
A boolean indicating whether the class implements any of those dundermethods
- Raises
TypeError – If
cls
is not a class
-
introspection.
class_implements_dundermethods
(cls, methods, bound=None)¶ Checks whether the given class implements all given dundermethods.
- Parameters
cls – A class
methods – The names of a bunch of dundermethods
bound – Where to stop searching through the class’s MRO
- Returns
A boolean indicating whether the class implements all those dundermethods
- Raises
TypeError – If
cls
is not a class
-
introspection.
collect_class_dundermethods
(cls, bound=None)¶ Generates a dict of the form
{method_name: method}
containing all dundermethods implemented by the given class.If multiple classes in the MRO implement the same dundermethod, only the first implementation is included in the result.
- Parameters
cls – The class whose dundermethods to collect
bound – Where to stop iterating through the class’s MRO
- Returns
A
{method_name: method}
dict- Raises
TypeError – If
cls
is not a class
-
introspection.
get_class_dundermethod
(cls, method_name, bound=None)¶ Retrieves a class’s implementation of the given dundermethod.
- Parameters
cls – A class
method_name – The name of a dundermethod
bound – Where to stop searching through the class’s MRO
- Returns
The function object for the given
method_name
- Raises
TypeError – If
cls
is not a classAttributeError – If
cls
does not implement that dundermethod
-
introspection.
get_bound_dundermethod
(instance, method_name, bound=None)¶ Retrieves a class’s implementation of the given dundermethod.
New in version 1.1.
- Parameters
instance – Any object
method_name – The name of a dundermethod
bound – Where to stop searching through the class’s MRO
- Returns
A bound method for the given
method_name
- Raises
TypeError – If
cls
is not a classAttributeError – If
cls
does not implement that dundermethod