Let's consider the following example:
class SubClass:
def __init__(self):
self._param = None
#property
def param(self):
print('read param')
return self._param
#param.setter
def param(self, value):
print('set param')
self._param = value
class MainClass:
def __init__(self):
self._var = SubClass()
#property
def var(self):
print('read var')
return self._var
#var.setter
def var(self, value):
print('set var')
self._var = value
If I do:
cls = MainClass()
cls.var.param = 3
I obtain:
'read var'
'set param'
How can I make MainClass aware that var.param has changed?
Useful additional info: consider that in my actual code param is not a scalar but an array with hundreds of elements, so I would like to avoid to create a copy and then just compare them. Moreover, param and var are not the only properties.
One approach is to pass a method from MainClass as an argument when instantiating SubClass, and have SubClass call it whenever it changes a variable:
class SubClass:
def __init__(self, changed):
self._param = None
self.changed = changed
#property
def param(self):
print('read param')
return self._param
#param.setter
def param(self, value):
print('set param')
self._param = value
self.changed('param')
class MainClass:
def __init__(self):
self._var = SubClass(self.changed)
def changed(self, name):
print("Subclass changed: ", name, getattr(self._var, name))
#property
def var(self):
print('read var')
return self._var
#var.setter
def var(self, value):
print('set var')
self._var = value
m = MainClass()
m.var.param = 'test'
Related
I have a class like this:
class A:
b = ""
#property
some_property(self):
raise Error()
#some_property.setter
def some_property(self, some_property_var)
self.b = some_property_var
and i want to do A(some_property='A'), but i can't because
TypeError: A() takes no arguments
. Is there any way to make it work?
I think this is a better example of how to demonstrate what you are trying to do;
#set a property in a class in python
class Person:
def __init__(self, name, age):
self.name = name
self.age = age
#property
def name(self):
return self._name
#name.setter
def name(self, name):
self._name = name
#property
def age(self):
return self._age
#age.setter
def age(self, age):
self._age = age
p = Person("John", 36)
So in your case;
class A:
def __init__(self, b):
self.b = b
#property
def b(self):
return self._b
#b.setter
def b(self, b):
self._b = b
a = A(1)
print(a.b)
Based on your comments;
#create a class that takes keyword arguments
class A:
def __init__(self, **kwargs):
for key, value in kwargs.items():
setattr(self, key, value)
a = A(a=1, b=2)
class Polygon():
def __init__(self, side_lengths):
self.side_lengths = side_lengths
def perimeter(self):
return sum(self.side_lengths)
def __str__(self):
side_lengths = len(self.side_lengths)
return 'Polygon with {} sides'.format(side_lengths)
class Triangle(Polygon):
def __init__(self, side_lengths):
super().__init__(side_lengths)
def area(self):
s = sum(self.side_lengths)/2
a = self.side_lengths[0]
b = self.side_lengths[1]
c = self.side_lengths[2]
return float((s*(s-a)*(s-b)*(s-c)) ** 0.5)
def __str__(self):
return 'Triangle, Area: {:.2f} units^2, Perimeter: {:.2f} units'.format(self.area(), self.perimeter())
class Rectangle(Polygon):
def __init__(self, side_lengths):
super().__init__(side_lengths)
def area(self):
return float(self.side_lengths[0]*self.side_lengths[1])
def __str__(self):
return 'Rectangle, Area: {:.2f} units^2, Perimeter: {:.2f} units'.format(self.area(), self.perimeter())
class Square(Rectangle):
def __init__(self, side_length):
self.side_length = side_length
def area(self):
return float(4*side_length)
def __str__(self):
return 'Square, Area: {:.2f} units^2, Perimeter: {:.2f} units'.format(self.area(), self.perimeter())
For this program, I want to calculate the area of the square when the input of side_length is a float instead of a list, but name error occurs. How to deal with such issue?
class Square(Rectangle):
def __init__(self,side_length):
super().__init__(side_length)
I also tried this method, it does not work as well.
How to access the variables of the outer class in the Inner Class?
class Student:
def __init__(self,Name,rollNumber):
self.Name=Name
self.rollNumber=rollNumber
self.lap=self.Laptop()
def Show(self):
print(self.Name)
print(self.lap.show())
class Laptop:
def __init__(self):
self.brand = "Mac"
self.cpu = "i9"
self.ram = 16
def show(self):
return self.brand
#staticmethod
def Show():
return s1.Name
s1=Student("Teja",2)
print(s1.Name,s1.rollNumber)
s1.Show()
print(s1.lap.brand)
system=s1.lap
print(system.brand)
print(system.cpu)
print(system.show())
print(system.Show())
It happens that I have two property implementations. but one works and the other does not. That is, that in one it does not even enter the setters.
I gave myself the task of wandering online, and this happens to me is very rare, both are supposed to work.
Do you know why this happens?
Thank you
class QuickTasks():
def __init__(self, name=None, value=None):
self.name = name
self.value = value
#property
def name(self):
return self._name
#name.setter
def name(self, value):
if isinstance(value, str):
self._name = value
else:
raise TypeError("name must be str")
#property
def value(self):
return self._value
#value.setter
def value(self, value):
if isinstance(value, int):
self._value = value
else:
raise TypeError("value must be int")
obj = QuickTasks(name=4, value='j')
print(obj.name)
obj.name = 5
print(obj.name)
################################################################
class TreeNode(object):
def __init__(self, value = None):
self.value = value
self._left_node = None
self._right_node = None
#property
def value(self):
return self._value
#value.setter
def value(self, value):
if isinstance(value, int):
self._value = 8
else:
raise TypeError("value must be int")
def main():
tree_node = TreeNode(3)
#tree_node.value = 3
print (tree_node.value)
if __name__ == '__main__':
print("")
main()
When you hit the line:
obj = QuickTasks(name=4, value='j')
and it assigns to self.name in the QuickTasks initializer, it raises a TypeError, which you don't catch. That bypasses the rest of your code entirely, skipping all uses of the value property (and of TreeNode entirely).
What is the simplest / most pythonic way to override only the setter of an abstract property in Python 3? Variant 3 seems to mean the least effort for the derived class implementor. Is it correct? Does it have disadvantages?
import abc
class A1(metaclass=abc.ABCMeta):
def __init__(self, x, **kwargs):
super().__init__(**kwargs)
self._x = x
#property
def x(self):
return self._x
#x.setter
#abc.abstractmethod
def x(self, value):
self._x = value
class B1(A1):
#property
def x(self):
return super().x
#x.setter
def x(self, value):
print("B1 setter")
super(B1, self.__class__).x.fset(self, value)
b1 = B1(x=1)
b1.x = 3
print(b1.x)
class A2(metaclass=abc.ABCMeta):
def __init__(self, x, **kwargs):
super().__init__(**kwargs)
self._x = x
#abc.abstractmethod
def _get_x(self):
return self._x
#abc.abstractmethod
def _set_x(self, value):
self._x = value
x = property(_get_x, _set_x)
class B2(A2):
def _get_x(self):
return super()._get_x()
def _set_x(self, value):
print("B2 setter")
super()._set_x(value)
x = property(_get_x, _set_x)
b2 = B2(x=1)
b2.x = 3
print(b2.x)
class A3(metaclass=abc.ABCMeta):
def __init__(self, x, **kwargs):
super().__init__(**kwargs)
self._x = x
def _get_x(self):
return self._x
#abc.abstractmethod
def _set_x(self, value):
self._x = value
x = property(
lambda self: self._get_x(),
lambda self, value: self._set_x(value))
class B3(A3):
def _set_x(self, value):
print("B3 setter")
super()._set_x(value)
b3 = B3(x=1)
b3.x = 3
print(b3.x)
So, yes, you listed a lot of ways in there - and although the one that requires more code is your variant 3, the most straighforard, least surprising way to do it is your variant 1 -
It just works, and is perfectly readable, no surprises - and there seems to be no simpler way than calling fget explicitly there.