lets say we have class A and it has one instance - x. How to make a child class of class A where I would be able to pass x as an argument and get all its parameters and pass it to child class object. precisely speaking I want to do something like this.
class A:
def __init__(self, parameter1, parameter2):
self.parameter1 = parameter1
self.parameter2 = parameter2
class B(A):
def __init__(self, Ainstance, someParameter):
super().__init__(**Ainstance.__dict__)
self.someParameter = someParameter
x = A(parameter1='1', parameter2='2')
x = B(x, someParameter='3')
print(x.parameter1)
print(x.parameter2)
print(x.someParameter)
the goal is to create a class where I would be able to get all the parameters of parent class object, and add my own attributes. The problem in the code above is I won't be able to do that with all classes because not all of them has __dict__ attribute.
I have this example code which I use to remind myself how to construct a proxy.
#soProxyPattern
class Example:
def __init__(self):
self.tag_name = 'name'
def foo(self):
return 'foo'
def bar(self, param):
return param
class Container:
def __init__(self, contained):
self.contained = contained
self.user_name = 'username'
def zoo(self):
return 0
def __getattr__(self, item):
if hasattr(self.contained, item):
return getattr(self.contained,item)
#raise item
c = Container(Example())
print(c.zoo())
print(c.foo())
print(c.bar('BAR'))
print(c.tag_name)
print(c.user_name)
The output is:
0
foo
BAR
name
username
This shows that Container can have its own attributes (methods or variables) which you can access over and above all of the attributes of the contained instance.
Instead of dict you could use the dir and getattr like this:
class A:
def __init__(self, parameter1, parameter2):
self.parameter1 = parameter1
self.parameter2 = parameter2
class B(A):
def __init__(self, Ainstance, someParameter):
parameters = {param: getattr(Ainstance, param) for param in dir(Ainstance) if not param.startswith("__")}
super().__init__(**parameters)
self.someParameter = someParameter
For a more detailed explanation see: Get all object attributes in Python?
Related
Let's assume I have a class A and a class B.
class A:
def __init__(self):
self.variable1 = 1
self.variable2 = 'sometext'
class B:
def __init__(self, inst):
self = inst
print(self.__dict__.keys(), inst.__dict__.keys())
The print function returns
b = B(inst)
dict_keys(['variable1', 'variable2']) dict_keys(['variable1', 'variable2'])
However when I try
b.variable1
It returns the following error
AttributeError: 'B' object has no attribute 'variable1'
In my more complex code I need almost all variable from class A in class B.
I tried using class inheritance however I couldn't make it work with class methods and constructors.
Is there a reason why the above method doesn't work?
Thx
You're trying to overwrite self, but that only works while you're in the init. Instead, try assigning the inst to a variable of the class B:
class B:
def __init__(self, inst):
self.inst = inst
print(self.__dict__.keys(), inst.__dict__.keys())
Now you can access the variables of class A via:
inst = A()
b = B(inst)
b.inst.variable1
Not sure what you're trying to achieve here exactly, but you could also initiate the class A object inside the init of class B instead of passing the object to class B.
To use variable from class A in B you have to access to class A from B. Then execute class B
class A:
variable1 = 1
variable2 = 'sometext'
class B:
def __init__(self, inst=None):
self.f1 = A().variable1
self.f2 = A().variable2
def get_var(self):
print (self.f1)
B().get_var()
I have a class Parent:
class Parent:
def __init__(self, foo):
self.foo = foo
I then have another class Child which extends Parent. But I want Child to take a pre-existing instance of parent and use this as the parent to inherit from (instead of creating a new instance of Parent with the same constructor parameters).
class Child(Parent):
def __init__(self, parent_instance):
""" Do something with parent_instance to set this as the parent instance """
def get_foo(self):
return self.foo
Then I would ideally be able to do:
p = Parent("bar")
c = Child(p)
print(c.get_foo()) # prints "bar"
You could copy the content of the parents's __dict__ to the child's. You can use vars() builtin function to do so, and the dictionary's update() method.
class Child(Parent):
def __init__(self, parent_instance):
vars(self).update(vars(parent_instance))
def get_foo(self):
return self.foo
p = Parent("bar")
c = Child(p)
print(c.get_foo())
# prints "bar"
You can use your own constructor - provide a classmethod that takes an instance of a parent.
class Parent:
def __init__(self, foo):
self.foo = foo
class Child(Parent):
def get_foo(self):
return self.foo
#classmethod
def from_parent(cls, parent_instance):
return cls(parent_instance.foo)
p = Parent('bar')
c = Child.from_parent(p)
c.get_foo()
I'm not sure inheritance is the right solution here as it breaks the LSP in the __init__ method.
Maybe parents and children just share a common interface.
I'd prefer something like (python3.8):
from typing import Protocol
class FoeAware(Protocol):
#property
def foe(self):
...
class Parent:
def __init__(self, foe):
self._foe = foe
#property
def foe(self):
return self._foe
class Child:
def __init__(self, parent: FoeAware):
self.parent = parent
#property
def foe(self):
return self.parent.foe
p = Parent("bar")
c = Child(p)
c.foe # bar
The key point is that it takes advantage of polymorphism with a common interface FoeAware, which is preferable to an inheritance tree.
Using getattr() to fetch the attribute from the parent instance
class Parent:
def __init__(self, foo):
self.foo = foo
class Child(Parent):
def __init__(self, parent_instance):
self.parent_instance = parent_instance
def get_foo(self):
return self.foo
def __getattr__(self, attr):
return getattr(self.parent_instance, attr)
par = Parent("bar")
ch = Child(par)
print(ch.get_foo())
#prints bar
I have a Cash class (derived from the Instrument class) which has a .manager class attribute. This class attribute (an instance of the Manager class), when it is initialized, needs to receive the class to which it is attached, as per below.
class Manager:
def __init__(self, instrument):
self.instrument = instrument #instrument is a class (a type)
def get(self, *args, **kwargs):
return self.instrument(30.99) #return an instance (e.g. ORM-like operation)
class Instrument:
pass
class Cash(Instrument):
manager = Manager(Cash) #this fails... as Cash isn't defined yet
def __init__(self, amount):
self.amount = amount
How do we achieve this, i.e. pass one's class to one of its own class attributes at definition?
If you have circular class/attributes, you can use functions that you call after instantiation, in my example add_A and add_B :
class A:
def __init__(self):
self.foo = 1
def add_B(self, b):
self.b = b
class B:
def __init__(self):
self.bar = 2
def add_A(self, a):
self.a = a
a = A()
b = B()
a.add_B(b)
print(a.b.bar)
# 2
b.add_A(a)
print(b.a.foo)
# 1
I'm working on a small personal project where I created many classes. Each class has its own register containing all its instances:
class IterReg(type):
# Register of all objects for iteration
def __iter__(cls):
return iter(cls._reg.values())
The "template" which I use for each class is
class Point(object):
# Point object
__metaclass__ = IterReg
_reg = dict()
counter = 0
# Initialize
def __init__(self, name):
self.name = name
self._reg[self.name] = self
Point.counter += 1
# Return register keys
#staticmethod
def get_reg_keys(cls):
return cls._reg.keys()
And some other (not important for a MWE) stuff.
If I initialize, e.g., a = Point("Point A"), I can then call:
>>> a.get_reg_keys()
['Point A']
which is (obviously) the same result that I get from
Point.get_reg_keys()
Now my question: is there a way to define this method once for all classes (i.e. somehow in the metaclass).
I first thought something like this had worked:
class IterReg(type):
# Register of all objects for iteration
def __iter__(cls):
return iter(cls._reg.values())
def get_reg_keys(cls):
return cls._reg.keys()
But then I can just call get_reg_keys() from the class (i.e. Point.get_reg_keys()) and not from any instance of it.
You can define a generic class that has .get_reg_keys() and subclass it when you want to use that method. In the below example, any class that inherits from Generic will have the .get_reg_keys() method.
class IterReg(type):
# Register of all objects for iteration
def __iter__(cls):
return iter(cls._reg.values())
class Generic(object):
#classmethod
def get_reg_keys(cls):
return cls._reg.keys()
class Point(Generic):
# Point object
__metaclass__ = IterReg
_reg = dict()
counter = 0
# Initialize
def __init__(self, name):
self.name = name
self._reg[self.name] = self
Point.counter += 1
a = Point('Point A')
a.get_reg_keys()
# returns:
['Point A']
I have a Parent class and a inherited child class, I would like to know how to access the child class variable in my Parent class..
I tried this and it fails -
class Parent(object):
def __init__(self):
print x
class Child(Parent):
x = 1;
x = Child();
Error:-
NameError: global name 'x' is not defined
This question is in relation to Django forms where we inherit the form class
and declare some class variables.
For example:-
My form looks like this
from django import forms
class EmployeeForm(forms.Form):
fname = forms.CharField(max_length=100)
lname = forms.CharField(max_length=100)
I believe the form fields are considered as class variable and somehow passed to the parent class..
Django does this with metaclasses. (Relevant Django source)
Here's a distilled example of the relevant code:
class Field(object):
def __init__(self, *args):
self.args = args
def __repr__(self):
return "Form(%s)" % (', '.join(map(repr, self.args)),)
class Meta(type):
def __new__(mcs, name, bases, attrs):
field_list = []
for k,v in attrs.items():
if isinstance(v, Field):
field_list.append(v)
cls = type.__new__(mcs, name, bases, attrs)
cls.fields = field_list
return cls
class Form(object):
__metaclass__ = Meta
class MyForm(Form):
fe1 = Field("Field1", "Vars1")
fe2 = Field("Field2", "Vars2")
x = "This won't appear"
form_fields = MyForm.fields
print(form_fields)
There are many questions on here about Python metaclasses (example), so I won't try to re-explain the concept.
In this case, when you create the class MyForm, each of the class attributes are checked for being instances of Field. If they are, they're added to a list (field_list).
The class is created, then an attribute .fields is added to the class, which is field_list, the list of Field elements.
You can then access the form fields through <FormSubclass>.fields or in the case of this example, MyForm.fields.
Edit:
It's worth noting that you can accomplish very similar functionality, without the metaclass syntactic sugar with something like:
class Field(object):
def __init__(self, *args):
self.args = args
def __repr__(self):
return "Form(%s)" % (', '.join(map(repr, self.args)),)
class Form(object):
def __init__(self):
self._fields = None
def fields(self):
if self._fields is None:
field_list = []
for k in dir(self):
v = getattr(self, k)
if isinstance(v, Field):
field_list.append(v)
self._fields = field_list
return self._fields
class MyForm(Form):
def __init__(self):
Form.__init__(self)
self.fe1 = Field("Field1", "Vars1")
self.fe2 = Field("Field2", "Vars2")
self.x = "This won't appear"
form_fields = MyForm().fields()
print(form_fields) # [Form('Field1', 'Vars1'), Form('Field2', 'Vars2')]
Short answer : you dont access subclasse's attributes from a parent class - because the parent class can not know what attributes a child class might have.
Long answer : ... unless the parent class defines a protocol allowing subclasses to let the parent class knows about at least part of it's own attributes.
Django's form framework (as well as django's orm FWIW) use such a protocol: the base Form class has a custom metaclass that collects the form.fields declared in a subclass - and do quite some black magic. FWIW, Django is oss so you could have answered the question yourself just reading the source code: https://github.com/django/django/blob/master/django/forms/forms.py
You need to refer to self.x to access Child class variables:
class Parent(object):
def __init__(self):
print(self.x)
class Child(Parent):
x = 1
if __name__ == '__main__':
child_instance = Child()
This might not help you in regards to Django Forms, but another alternative is to work with abstract classes. You would exchange attributes with methods/properties. It also prevents you from using the parent class by itself.
from abc import ABC, abstractmethod
class Parent(ABC):
#property
#abstractmethod
def x(self):
pass
def __init__(self):
print(self.x)
class Child(Parent):
#property
def x(self):
return 1
if __name__ == '__main__':
child_instance = Child() # prints "1"
parent_instance = Parent() # fails
Well, if I got you right... Maybe you're thinking of getting a field from the child class to work on the parent class. Well, that's polymorphism and it's done by overriding the parent class.
Let's assume you have :
A parent has x, now to increase x from the child and make it reflect in the parent, check the code below to get it.
class Parent:
def __init__(self, x):
self.x = x
def Print(self):
print(f"{self.x}")
class Child(Parent):
def __init__(self, x):
Parent.__init__(self, x)
x += 1
self.x = x
""""""
c1 = Child(2)
c1.Print()
#output: 3
""""""
c2 = Child(8)
c2.Print()
#output: 9