I'm having trouble understanding nested classes and have a specific problem
I have a model "Pair" which has an attribute "name." I'd like to have a nested class called "History" that passes variables "start" and "end" as well as its parent (Pair) variable's "name". A #property decorated function would then return the standard deviation of that list.
To call the standard deviation I'd be looking for something like:
pair = Pair(base_currency='USD', quote_currency='GBP')
print(pair.History(granularity='1D', start=start_date, end=end_date).std_dev
>>> 634
I think it should look something like the below, but cant figure out how to inherit the outer class attribute "name"
class Pair:
def __init__(self, base_currency, quote_currency):
self.base_currency = base_currency
self.quote_currency = quote_currency
self.name = '{base}-{quote}'.format(base=base_currency, quote=quote_currency)
self.history = self.History()
class History:
def __init__(self, granularity, start, end):
self.name = HOW DO I GET THE PARENT NAME HERE
self.granularity = granularity
self.start = start
self.end = end
def data(self):
return retrieve_history(product=self.name, granularity=self.granularity, start=self.start, end=self.end)
#property
def std_dev(self):
return stdev(self.data)
Related
I found a workaround for my issue but I need to know why the first above case doesn't work.
I need to pass a parameter (reman_pk) to my view but when I try :
class RepairCreateView(LoginRequiredMixin, CreateView):
#property
def reman_pk(self):
return int(self.kwargs['reman_pk'])
[...]
success_url = reverse_lazy(
'reman:update-reman', kwargs={'pk': reman_pk})
[...]
... I got an error
django.urls.exceptions.NoReverseMatch: Reverse for 'update-reman' with keyword arguments '{'pk': <property object at 0x10c20bbd0>}' not found. 1 pattern(s) tried: ['reman/update/(?P[0-9]+)/$']
But when in the same class based view I use :
def get_success_url(self, **kwargs):
if kwargs != None:
return reverse_lazy('reman:update-reman', kwargs={'pk': self.reman_pk})
... it's OK : an int is well passed in my URL.
I tried to pass int(reman_pk) in the first method ... not better.
I've already use #property in the past and always got a value (int/str) and not property object.
EDIT (FULL views.py)
success_url = reverse_lazy...is commented. I must use def get_success_url( ... instead. Otherwise I get the above mentioned error.
class RepairCreateView(LoginRequiredMixin, CreateView):
#property
def reman_pk(self):
return int(self.kwargs['reman_pk'])
# success_url = reverse_lazy(
# 'reman:repairs-list', kwargs={'pk': reman_pk})
success_message = "Nouvelle réparation créée"
form_class = RepairCreateForm
template_name = 'reman/repair_create_form.html'
def get_context_data(self, *args, **kwargs):
context = super(RepairCreateView, self).get_context_data(
*args, **kwargs)
context['title'] = 'Nouveau Repair'
context['pk'] = self.reman_pk
return context
def get_initial(self):
reman = Reman.objects.get(pk=self.reman_pk)
return {'reman': reman}
def get_success_url(self, **kwargs):
return reverse_lazy('reman:repairs-list', kwargs={'pk': self.reman_pk})
This is not related to django, this is related to python in general. When you want to access a class property within the class you always have to call self before!
class Tree:
fruits = 5
#property
def leafes(self):
return self.fruits * 5
def show_tree(self):
print(self.fruits)
print(self.leafes)
print(leafes) # THIS LINE WOULD ERROR
Edit after comment of OP
I don't know how to phrase this properly. Anyhow this keeps being a problem related to python and not to django. The reason is how classes work.
You probably know the def __init__(self): function. That is called when the class gets instanciated. After that function got called your class can use all the self attributes (class attributes). But class attributes like my fruits = 5 get assigned even before that def __init__(self) method is called. So all your assignments directly inside the body of the class do not have self yet.
class Tree:
fruits = 5
def __init__(self):
self.twigs = 10
self.weight = self.twigs + self.fruits # THIS WORKS
class Tree:
fruits = 5
weight = self.twigs + fruits # THIS DOES NOT WORK
def __init__(self):
self.twigs = 10
Last example does not work because at the moment you want to assign weight = self.twigs + fruits your class's __init__ function was not called yet. So you can not use self at that place.
Below, I show a simplified example of a more complicated code, but nonetheless, it fully represents the issue that I have encountered.
Part 1: this works fine, no issues:
class Animal():
def __init__(self, animal_name = "no name given"):
self.set_name(animal_name)
def get_name(self):
return self._animal_name
def set_name(self, animal_name):
self._animal_name = animal_name
class Dog(Animal):
def __init__(self, dog_breed = "no breed", dog_name = "no name given"):
self._dog_breed = dog_breed
super().__init__(dog_name)
def get_breed(self):
print(self._dog_breed)
x = Dog('Greyhound', 'Rich')
Part 2: after introducing getter & setter decorators, the code stops working:
class Animal():
def __init__(self, animal_name = "no name given"):
#THE LINE BELOW SEEMS TO CAUSE AN ISSUE
self.name(animal_name)
#property
def name(self):
return self._animal_name
#name.setter
def name(self, animal_name):
self._animal_name = animal_name
class Dog(Animal):
def __init__(self, dog_breed = "no breed", dog_name = "no name given"):
self._dog_breed = dog_breed
super().__init__(dog_name)
def get_breed(self):
print(self._dog_breed)
x = Dog('Greyhound', 'Rich')
Output: AttributeError: 'Dog' object has no attribute '_animal_name'
When I keep the decorators in Part 2 but change the constructor in the Animal class to:
class Animal():
def __init__(self, animal_name = "no name given"):
self._animal_name=animal_name
It works.
I am just curious why it doesn't work in the example above in Part 2?
Short answer:
The line
self.name(animal_name)
can be split in two parts:
tmp = self.name
tmp(animal_name)
First, self.name calls the getter and the result is treated as a function. The getter uses return self._animal_name and since the setter has never been called, the respective error occurs.
Long answer:
Let's take the following class:
class Animal:
def __init__(self, animal_name):
self.name(animal_name)
#property
def name(self):
return self._animal_name
#name.setter
def name(self, animal_name):
self._animal_name = animal_name
To understand what the line
self.name(animal_name)
actually does, you first need to understand decorators.
The code
#dec
def func(a, b, ...):
[...]
is equivalent to
def func_impl(a, b, ...):
[...]
func = dec(func_impl)
(except that you can not call func_impl directly). See, for example, PEP 318 for more information.
This means that you can write the Animal class from above without using decorators:
class Animal:
def __init__(self, animal_name):
self.name(animal_name)
def get_name(self):
return self._animal_name
name = property(get_name)
def set_name(self, animal_name):
self._animal_name = animal_name
name = name.setter(set_name)
In order to understand this code, you need to understand the builtin property, which is a class. See the python docs for detailed information.
The line name = property(get_name) creates an object of type property. When retrieving the value of the property, get_name is called.
The line name = name.setter(set_name) first calls name.setter(set_name), which creates a copy of the property, and then overwrites name with this copy. When assigning a value to the copy, set_name is called.
All in all, name is an object of type property that uses get_name as getter and set_name as setter.
How does this help?
You need to understand this: name is not a function. It is a property. It is not callable.
The problematic line
self.name(animal_name)
is actually equivalent to
self.get_name()(animal_name)
which this explains the error message: The constructor calls the getter, which tries to use return self._animal_name. But since the setter has not been called, yet, self._animal_name has not been set.
I have the following code:
class Parent():
def __init__(self):
self.variable1 = self.method1()
self.variable2 = self.method2()
self.variable3 = self.method3()
self.variable4 = self.method4()
#.........(for example I have 100 variables here)
def method1(self):
return 100
def method2(self):
return 200
def method3(self):
return 300
def method4(self):
return 400
class Third():
def __init__(self):
a = 1
def call(self):
value = self.variable3 + 1
return value
class Child(Parent):
def __init__(self):
super().__init__()
self.getanswer = self.method11()
def method11(self):
value_count = Third().call()
return value_count
obj = Child()
It throwed the following Error:
AttributeError: 'Third' object has no attribute 'variable3'
Here I wanted to send all the values of parent to Third Class through Child class. How can I achieve it? I know that I can pass Parent class variables seperately in directly as a parameter in the class Third as following:
value_count = Third(self.variable3).call()
and change the Third Class constructor accordingly. But I don't want to do it as my Parent class has some time taking operations to do.
Also I don't want to make class Third as child to the class Parent.
So How can I recognize the Parent class variables(variable1, variable2, variable3, variable4, ....) in Third Class through Child Class ?
for a specific framework i work with, i need to define object attributes as special classes, for example, instead of writing this:
class A:
def __init__(self):
self.some_int = 2
i would need to write:
class A:
def __init__(self):
self.some_int = SpecialIntWrapper(name = "some_int", value = 2)
I would like to somehow override operators/methods so that typing the first code (self.some_int = 2) will call SpecialIntWrapper behind the scenes, with the attribute name and value.
is this possible?
Basically there are two ways - via a #property decorator (preferable unless you want to affect arbitrary names)
class MyClass:
def __init__(self):
self.some_int = 2
# if you know the name of the property define it as a property - a getter
#property
def some_int(self):
return self._some_int
# and a setter
#some_int.setter
def some_int(self, value):
self._some_int = SpecialIntWrapper("some_int", value)
or overloading the __setattr__ magic method
class MyClass:
def __init__(self):
self.some_int = 2
def __setattr__(self, name, value):
# in general if you dont know the names of the properties
# beforehand you can somehow filter them here
if name == "some_int":
super().__setattr__(name, SpecialIntWrapper(name=name, value=value))
else:
# to use the setattr in a default way, just call it via super(Python 3)
super().__setattr__(name, value)
either way the some_int will be initialized to the SpecialIntWrapper instance
>>>print(MyClass().some_int)
<__main__.SpecialIntWrapper object at 0x03721810>
Something like this
class SpecialIntWrapper:
def __init__(self, name, value):
pass
class MyClass:
def __init__(self):
self.some_int = 3
def __setattr__(self, key, value):
if key == 'some_int':
self.__dict__[key] = SpecialIntWrapper(key, value)
else:
self.__dict__[key] = value
print(MyClass().some_int)
# >>> <__main__.SpecialIntWrapper object at 0x1076f1748>
So i have a problem, that i don't quite understand why it's happening. I get a (Name Error global variable "value" is not defined) when it should be on my weapons class.
from items import *
class weapons(Item):
def __init__(self, name, attack_damage, lifesteal = 0):
super(weapons,self).__init__(name, value, quantity=1)
self.attack_damage = attack_damage
self.lifesteal = lifesteal
Here is the class that weapons is getting it from that already has value defined.
class Item(object):
def __init__(self, name, value, quantity=1):
self.name = name
self.raw = name.replace(" ","").lower()
self.quantity = quantity
self.value = value
self.netValue = quantity * value
def recalc(self):
self.netValue = self.quantity * self.value
I already have a piece of code similar to this that is working, but for some reason this value error is happening. I'm just going to include it.
from character import*
class player(character):
def __init__(self,name,hp,maxhp,attack_damage,ability_power):
super(player,self).__init__(name, hp, maxhp)
self.attack_damage = attack_damage
self.ability_power = ability_power
and the class that player is getting its stuff from
class character(object):
def __init__(self,name,hp,maxhp):
self.name = name
self.hp = hp
self.maxhp = maxhp
def attack(self,other):
pass
as you can see i did it here and this piece of code works when i call a player.
You need to add the value argument to the __init__ constructor of the weapons class.
super needs a parameter value but you did not pass it into the init