I'm using django, but this is rather a generic python question.
I have defined a class that I intend to use to extend the ModelForm and Form classes, from django.forms.
The code looks like this:
class FormMixin(object):
def __init__(self, *args, **kwargs):
""" every method ocurrence must call super """
super(FormMixin, self).__init__(*args, **kwargs)
self.new_attr = 'This is an attribute'
class ModelFormAdapter(forms.ModelForm):
""" I use this class so __init__ signatures match """
def __init__(self, *args, **kwargs):
""" every method ocurrence must call super """
super(ModelFormAdapter, self).__init__(*args, **kwargs)
class BaseModelForm(ModelFormAdapter, FormMixin):
def __init__(self, *args, **kwargs):
""" BaseModelForm never gets the attribute new_attr """
super(BaseModelForm, self).__init__(*args, **kwargs)
I have even debugged this and the FormMixin init method is never called. What am I doing wrong? What I want to achieve is to add some attributes to the form and preprocess field labels and css classes
That's because one of ModelFormAdapter's ancestors (BaseForm), doesn't call super, and the chain breaks. Put FormMixin first in the parent list.
Related
What is the disadvantage of doing this way in python
I want to reduce variable that unique to each class but same line for every class.
For example a logging
import abc
import logging
def get_logger(name: str) -> logging.Logger:
return logging.getLogger("myapp.{}".format(name))
class AbstractService(abc.ABC):
def __init__(self, *args, **kwargs):
self._logger = get_logger(self.__class__.__name__)
self._init(*args, **kwargs)
def _init(self, *args, **kwargs):
pass
Then we have the actual class which is extending AbstractService
But here the _init will be call by AbstractService.
class UserService(AbstractService):
def _init(self, user_repository):
self._user_repository = user_repository
self._logger.info("") in UserService will print the correct class as "myapp.UserService" when logging something
This is help me a lot, like sharing db, queue, etc.
This way the _init argument can be focus on what dependency is actually need for the service.
I use dependency-injector https://python-dependency-injector.ets-labs.org/ for the class initiation.
But what is the disadvantage, I haven't tested it if it was deep nested class or a diamond inheritance
Is there a better way to call nested init without writing this
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
on every class ?
I'm writing a library that provides subclasses of each of two existing base classes with extra functionality.
Rather than explain the arrangement in words, here's a diagram:
And minimal code:
class Base0:
pass
class Base1(Base0):
def __init__(self, *args, **kwargs):
super().__init__(*args, **kwargs)
self.foo = something()
class Base2(Base0):
pass
class Mixin:
def __init__(self, bar):
self.bar = bar
# More code
class Child1(Base1, Mixin):
def __init__(self, *args, **kwargs):
Base1.__init__(self, *args, **kwargs)
Mixin.__init__(self, some_function_of(self.foo))
class Child2(Base2, Mixin):
def __init__(self, *args, **kwargs):
Base2.__init__(self, *args, **kwargs)
Mixin.__init__(self, something_else())
The Base classes are outside my control. I wrote the Mixin and Child classes. Users of my library will subclass the Child classes, so it's very important that the inheritance be sane and correct.
What I'd like to do is use super().__init__ in the Child classes rather than explicitly invoking the Base and Mixin initializers. The reason this is nontrivial is that in Child1, the value passed to the Mixin initializer can't be determined until after the Base1 initializer has run.
What is the simplest/sanest way to set this up?
I have a MyModelForm form class for MyModel model class, and I want to generate a random value for a certain field.
The way I see it is either inside init or save functions, I tried using self.fields['randfield'] but it throws an error 'MyModelForm' object has no attribute 'fields'.
How can I access and update a field inside form class so that I can instantiate it with a random value?
Thanks.
EDIT: After using self.fields['randint'].initial I am getting a KeyError. The code is
Okay, here goes:
def __init__(self, instance=None, *args, **kwargs):
_fields = ('username', 'email')
_initial = model_to_dict(instance.user, _fields) if instance is not None else {}
super(UserDetailsForm, self).__init__(initial=_initial, instance=instance, *args, **kwargs)
self.fields.update(fields_for_model(User, _fields))
self.fields['randint'].initial = '987654321'
Use something like this:
class RandomValueForm(ModelForm):
myfield = models.IntegerField(default=0)
def __init__(self, *args, **kwargs):
super(RandomValueForm, self).__init__(*args, **kwargs)
self.fields['myfield'].initial = my_random_generator()
You got this error because you would have tried accessing fields on self without calling the __init__ of superclass. So, first you need to call superclass __init__ i.e __init__ of ModelForm and then you can access fields.
class MyModelForm(ModelForm):
def __init__(self, *args, **kwargs):
super(MyModelForm, self).__init__(*args, **kwargs)
self.fields['myfield'].initial = my_random_number()
I'm trying to find the best way to create a class decorator that does the following:
Injects a few functions into the decorated class
Forces a call to one of these functions AFTER the decorated class' __init__ is called
Currently, I'm just saving off a reference to the 'original' __init__ method and replacing it with my __init__ that calls the original and my additional function. It looks similar to this:
orig_init = cls.__init__
def new_init(self, *args, **kwargs):
"""
'Extend' wrapped class' __init__ so we can attach to all signals
automatically
"""
orig_init(self, *args, **kwargs)
self._debugSignals()
cls.__init__ = new_init
Is there a better way to 'augment' the original __init__ or inject my call somewhere else? All I really need is for my self._debugSignals() to be called sometime after the object is created. I also want it happen automatically, which is why I thought after __init__ was a good place.
Extra misc. decorator notes
It might be worth mentioning some background on this decorator. You can find the full code here. The point of the decorator is to automatically attach to any PyQt signals and print when they are emitted. The decorator works fine when I decorate my own subclasses of QtCore.QObject, however I've been recently trying to automatically decorate all QObject children.
I'd like to have a 'debug' mode in the application where I can automatically print ALL signals just to make sure things are doing what I expect. I'm sure this will result in TONS of debug, but I'd still like to see what's happening.
The problem is my current version of the decorator is causing a segfault when replacing QtCore.QObject.__init__. I've tried to debug this, but the code is all SIP generated, which I don't have much experience with.
So, I was wondering if there was a safer, more pythonic way to inject a function call AFTER the __init__ and hopefully avoid the segfault.
Based on this post and this answer, an alternative way to do this is through a custom metaclass. This would work as follows (tested in Python 2.7):
# define a new metaclass which overrides the "__call__" function
class NewInitCaller(type):
def __call__(cls, *args, **kwargs):
"""Called when you call MyNewClass() """
obj = type.__call__(cls, *args, **kwargs)
obj.new_init()
return obj
# then create a new class with the __metaclass__ set as our custom metaclass
class MyNewClass(object):
__metaclass__ = NewInitCaller
def __init__(self):
print "Init class"
def new_init(self):
print "New init!!"
# when you create an instance
a = MyNewClass()
>>> Init class
>>> New init!!
The basic idea is that:
when you call MyNewClass() it searches for the metaclass, finds that you have defined NewInitCaller
The metaclass __call__ function is called.
This function creates the MyNewClass instance using type,
The instance runs its own __init__ (printing "Init class").
The meta class then calls the new_init function of the instance.
Here is the solution for Python 3.x, based on this post's accepted answer. Also see PEP 3115 for reference, I think the rationale is an interesting read.
Changes in the example above are shown with comments; the only real change is the way the metaclass is defined, all other are trivial 2to3 modifications.
# define a new metaclass which overrides the "__call__" function
class NewInitCaller(type):
def __call__(cls, *args, **kwargs):
"""Called when you call MyNewClass() """
obj = type.__call__(cls, *args, **kwargs)
obj.new_init()
return obj
# then create a new class with the metaclass passed as an argument
class MyNewClass(object, metaclass=NewInitCaller): # added argument
# __metaclass__ = NewInitCaller this line is removed; would not have effect
def __init__(self):
print("Init class") # function, not command
def new_init(self):
print("New init!!") # function, not command
# when you create an instance
a = MyNewClass()
>>> Init class
>>> New init!!
Here's a generalized form of jake77's example which implements __post_init__ on a non-dataclass. This enables a subclass's configure() to be automatically invoked in correct sequence after the base & subclass __init__s have completed.
# define a new metaclass which overrides the "__call__" function
class PostInitCaller(type):
def __call__(cls, *args, **kwargs):
"""Called when you call BaseClass() """
print(f"{__class__.__name__}.__call__({args}, {kwargs})")
obj = type.__call__(cls, *args, **kwargs)
obj.__post_init__(*args, **kwargs)
return obj
# then create a new class with the metaclass passed as an argument
class BaseClass(object, metaclass=PostInitCaller):
def __init__(self, *args, **kwargs):
print(f"{__class__.__name__}.__init__({args}, {kwargs})")
super().__init__()
def __post_init__(self, *args, **kwargs):
print(f"{__class__.__name__}.__post_init__({args}, {kwargs})")
self.configure(*args, **kwargs)
def configure(self, *args, **kwargs):
print(f"{__class__.__name__}.configure({args}, {kwargs})")
class SubClass(BaseClass):
def __init__(self, *args, **kwargs):
print(f"{__class__.__name__}.__init__({args}, {kwargs})")
super().__init__(*args, **kwargs)
def configure(self, *args, **kwargs):
print(f"{__class__.__name__}.configure({args}, {kwargs})")
super().configure(*args, **kwargs)
# when you create an instance
a = SubClass('a', b='b')
running gives:
PostInitCaller.__call__(('a',), {'b': 'b'})
SubClass.__init__(('a',), {'b': 'b'})
BaseClass.__init__(('a',), {'b': 'b'})
BaseClass.__post_init__(('a',), {'b': 'b'})
SubClass.configure(('a',), {'b': 'b'})
BaseClass.configure(('a',), {'b': 'b'})
I know that the metaclass approach is the Pro way, but I've a more readable and easy proposal using #staticmethod:
class Invites(TimestampModel, db.Model):
id = db.Column(db.Integer, primary_key=True, autoincrement=True)
invitee_email = db.Column(db.String(128), nullable=False)
def __init__(self, invitee_email):
invitee_email = invitee_email
#staticmethod
def create_invitation(invitee_email):
"""
Create an invitation
saves it and fetches it because the id
is being generated in the DB
"""
invitation = Invites(invitee_email)
db.session.save(invitation)
db.session.commit()
return Invites.query.filter(
PartnerInvites.invitee_email == invitee_email
).one_or_none()
So I could use it this way:
invitation = Invites.create_invitation("jim#mail.com")
print(invitation.id, invitation.invitee_email)
>>>> 1 jim#mail.com
In django, I have a form being called from the view, which is passed an extra object that popped in the form init. I want to use this object data (person) in the clean def's outside of init. How can I fix the scope of this passed information? Thanks!
class RegForm(forms.Form):
first = forms.CharField(min_length=5)
def __init__(self, *args, **kwargs):
person = kwargs.pop("person")
super(CompleteRegistrationForm, self).__init__(*args, **kwargs)
def clean_first(self):
if not self.cleaned_data['first'] == person.first:
raise forms.ValidationError(_("This information does not match records."))
else:
return self.cleaned_data['first']
person should be an instance variable:
def __init__(self, *args, **kwargs):
self.person = kwargs.pop("person")
super(CompleteRegistrationForm, self).__init__(*args, **kwargs)
Then, in other methods, refer to it as self.person (not just person).
You should assign it to self:
self.person = kwargs.pop("person")
This is fairly basic Python - you would probably benefit from doing a tutorial.