Is there a way to wrap all methods of a superclass, if I can't change its code?
As a minimal working example, consider this base class Base
, which has many methods that return a new instance of itself, and the descendent class Child
class Base:
def __init__(self, val):
self.val = val
def newinst_addseven(self):
return Base(self.val + 7)
def newinst_timestwo(self):
return Base(self.val * 2)
# ...
class Child(Base):
@property
def sqrt(self):
return math.sqrt(self.val)
The issue here is that calling childinstance.newinst_addseven()
returns an instance of Base
, instead of Child
.
Is there a way to wrap the Base
class's methods to force a return value of the type Child
?
With something like this wrapper:
def force_child_i(result):
"""Turn Base instance into Child instance."""
if type(result) is Base:
return Child(result.val)
return result
def force_child_f(fun):
"""Turn from Base- to Child-instance-returning function."""
def wrapper(*args, **kwargs):
result = fun(*args, **kwargs)
return force_child_i(result)
return wrapper
Many thanks!
PS: What I currently do, is look at Base
's source code and add the methods to Child
directly, which is not very mainainable:
Child.newinst_addseven = force_child_f(Base.newinst_addseven)
Child.newinst_timestwo = force_child_f(Base.newinst_timestwo)
One option is to use a metaclass:
class ChildMeta(type):
def __new__(cls, name, bases, dct):
child = super().__new__(cls, name, bases, dct)
for base in bases:
for field_name, field in base.__dict__.items():
if callable(field):
setattr(child, field_name, force_child(field))
return child
class Child(Base, metaclass=ChildMeta):
pass
It will automatically wrap all the Base
s methods with your force_child
decorator.
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