I want to build a class, that is handling and processing some data. So I want property to handle all the data processing silently when new data is passed And I want to overload init with classmethod, to have flexibility on parameters passed to instance creation. So I came up with the following solution :
class Cooper():
def __init__(self):
...create all 'private' attributes
#classmethod
def Dougie(cls,data,datatype):
inst = cls.__new__(cls)
setattr(inst,datatype,data)
return inst
#property
def datatype1(self):
return self._datatype1
#datatype1.setter
def datatype1(self,newdata):
self._datatype1,self._datatype2,... = updatedata1(newdata)
#property
def datatype2(self):
return self._datatype2
#datatype2.setter
def datatype2(self,newdata):
self._datatype1,self._datatype2,... = updatedata2(newdata)
... to be continued...
Is this a pythonic way ? Or shoud I really create a metaclass (I get a little fir afraid there) ? What are the caveats ? Is there a better way ?
Related
The issue
I would like to be able to re-use methods by implementing them with a decorator, while preserving my IDE's ability to type-hint the methods added, such that:
#methods(methods=[implement_foo, implement_bar])
class K:
pass
# OR
#method(methods[Foo, Bar])
class K:
pass
k = K()
#### THE ISSUE
k. <- # IDE should recognize the methods .foo() or bar(), but does not.
My issue is much like How to create a class decorator that can add multiple methods to a class?, but as mentioned, while preserving the type-hint and only use one decorator.
What I have tried
I can make it work with one decorator, but not with multiple.
Example with one decorator called implement_method
def implement_method(cls):
class Inner(cls):
def __init__(self, *args, **kargs):
super(Inner, self).__init__(*args, **kargs)
def method(self):
pass
return Inner
#implement_method
class K:
pass
And type hint works for a new instance of K:
I imagine that one of the issues is using a loop, but I am unable to come up with a different solution. The following is my best attempt:
def methods(methods):
def wrapper(cls):
for method in methods:
cls = method(cls)
return cls
return wrapper
class Bar:
def bar(self):
pass
#methods(methods=[Bar])
class K:
pass
k = K()
k. # <- not finding bar()
Since your question is a 2 part one:
I have an answer for your first part and I am quite stuck on the second. You can modify signatures of functions using the inspect module, but I have not found anything similar for classes and I am not sure if it is possible. So for my answer I will focus on your first part:
One decorator for multiple functions:
Let's look at the decorator first:
def add_methods(*methods):
def wrapper(cls):
for method in methods:
setattr(cls, method.__name__, staticmethod(method))
return cls
return wrapper
We use *methods as a parameter so that we can add as many methods as we want as arguments.
Then we define a wrapper for the class and in it iterate over all methods we want to add using setattr to add the method to the class. Notice the staticmethod wrapping the original method. You can leave this out if you want the methods to receive the argument self.
Then we return from the wrapper returning the class and return from the decorator returning the wrapper.
Let's write some simple methods next:
def method_a():
print("I am a banana!")
def method_b():
print("I am an apple!")
Now we create a simple class using our decorator:
#add_methods(method_a, method_b)
class MyClass:
def i_was_here_before(self):
print("Hah!")
And finally test it:
my_instance = MyClass()
my_instance.i_was_here_before()
my_instance.method_a()
my_instance.method_b()
Our output:
Hah!
I am a banana!
I am an apple!
A word of caution
Ususally it is not advised to change the signature of functions or classes without a good reason (and sometimes even with a good reason).
Alternate Solution
Given that you will need to apply the decorator to each class anyway, you could also just use a superclass like this:
class Parent:
#staticmethod
def method_a():
print("I am a banana!")
#staticmethod
def method_b():
print("I am an apple!")
class MyClass(Parent):
def i_was_here_before(self):
print("Hah!")
my_instance = MyClass()
my_instance.i_was_here_before()
my_instance.method_a()
my_instance.method_b()
Since python supports multiple inheritance this should work better and it also gives you the correct hints.
Complete working example:
def add_methods(*methods):
def wrapper(cls):
for method in methods:
setattr(cls, method.__name__, staticmethod(method))
return cls
return wrapper
def method_a():
print("I am a banana!")
def method_b():
print("I am an apple!")
#add_methods(method_a, method_b)
class MyClass:
def i_was_here_before(self):
print("Hah!")
my_instance = MyClass()
my_instance.i_was_here_before()
my_instance.method_a()
my_instance.method_b()
Is it possible to pass additional arguments when getting a descriptor?
For example, I'd like to have:
class Element(object):
def __init__(self, text='initial'):
self.text = text
def __get__(self, instance, owner, extra_text=''):
print self.text + instance.name + extra_text
And then be able to use it like:
class MyClass(object):
elem1 = Element()
elem2 = Element(text='override')
def __init__(self, name):
self.name = name
def print_elems(self):
self.elem1
self.elem1(extra_text='extra')
self.elem2
self.elem2(extra_text='extra')
MyClass('name').print_elems()
And then get:
initialname
initialnameextra
overridename
overridenameextra
Is there any way to make this work? I've even tried calling elem1.__get__(self, self.__class__, extra_text='extra) and making extra_text a required param, but couldn't figure out any way to actually provide it?
If not with descriptors, is there an alternative way to achieve this syntax?
Thanks!
In order to call get() with anything but the automatic arguments, you need to access the instance of the descriptor. To do that...
Step 1: add as the first line to get():
if instance is None:
return self
This is a fairly common addition to the method.
Step 2: When accessing the descriptor, get it from the class instead of the instance:
MyClass.elem1
MyClass.elem1
or
type(my_inst).elem1
type(my_inst).elem1
Step 3: Call get () explicitly, passing in the desired argument.
Ugly and verbose; avoid as long as you can.
I've got a book released about descriptors, which can be found at http://www.lulu.com/spotlight/jacobz_20
It can really help you wrap your head around this if you're confused.
I feel like I have a pretty good grasp on using decorators when dealing with regular functions, but between using methods of base classes for decorators in derived classes, and passing parameters to said decorators, I cannot figure out what to do next.
Here is a snippet of code.
class ValidatedObject:
...
def apply_validation(self, field_name, code):
def wrap(self, f):
self._validations.append(Validation(field_name, code, f))
return f
return wrap
class test(ValidatedObject):
....
#apply_validation("_name", "oh no!")
def name_validation(self, name):
return name == "jacob"
If I try this as is, I get an "apply_validation" is not found.
If I try it with #self.apply_validation I get a "self" isn't found.
I've also been messing around with making apply_validation a class method without success.
Would someone please explain what I'm doing wrong, and the best way to fix this? Thank you.
The issue you're having is that apply_validation is a method, which means you need to call it on an instance of ValidatedObject. Unfortunately, at the time it is being called (during the definition of the test class), there is no appropriate instance available. You need a different approach.
The most obvious one is to use a metaclass that searches through its instance dictionaries (which are really class dictionaries) and sets up the _validations variable based on what it finds. You can still use a decorator, but it probably should be a global function, or perhaps a static method, and it will need to work differently. Here's some code, that uses a metaclass and a decorator that adds function attributes:
class ValidatedMeta(type):
def __new__(meta, name, bases, dct):
validations = [Validation(f._validation_field_name, f._validation_code, f)
for f in dct.values if hasattr(f._validation_field_name)]
dct["_validations"] = validations
super(ValidatedMeta, meta).__new__(meta, name, bases, dct)
def apply_validation(field_name, code):
def decorator(f):
f._validation_field_name = field_name
f._validation_code = code
return f
return decorator
def ValidatedObject(metaclass=ValidatedMeta):
pass
class test(ValidatedObject):
#apply_validation("_name", "oh no!")
def name_validation(self, name):
return name == "jacob"
After this code runs, test._validations will be [Validation("_name", "oh no!", test.name_validation)]. Note that the method that is be passed to Validation is unbound, so you'll need to pass it a self argument yourself when you call it (or perhaps drop the self argument and change the decorator created in apply_validation to return staticmethod(f)).
This code may not do what you want if you have validation methods defined at several levels of an inheritance hierarchy. The metaclass as written above only checks the immediate class's dict for methods with the appropriate attributes. If you need it include inherited methods in _validations too, you may need to modify the logic in ValidatedMeta.__new__. Probably the easiest way to go is to look for _validations attributes in the bases and concatenate the lists together.
Just an example for using decorators on class method:
from functools import wraps
def VALIDATE(dec):
#wraps(dec)
def _apply_validation(self, name):
self.validate(name)
return dec(self, name)
return _apply_validation
class A:
def validate(self, name):
if name != "aamir":
raise Exception, 'Invalid name "%s"' % name
class B(A):
#VALIDATE
def name_validation(self, name):
return name
b = B()
b.name_validation('jacob') # should raise exception
I'm using Python 3.
I know about the #classmethod decorator. Also, I know that classmethods can be called from instances.
class HappyClass(object):
#classmethod
def say_hello():
print('hello')
HappyClass.say_hello() # hello
HappyClass().say_hello() # hello
However, I don't seem to be able to create class methods dynamically AND let them be called from instances. Let's say I want something like
class SadClass(object):
def __init__(self, *args, **kwargs):
# create a class method say_dynamic
SadClass.say_dynamic() # prints "dynamic!"
SadClass().say_dynamic() # prints "dynamic!"
I've played with cls.__dict__ (which produces exceptions), and with setattr(cls, 'say_dynamic', blahblah) (which only makes the thingie callable from the class and not the instance).
If you ask me why, I wanted to make a lazy class property. But it cannot be called from instances.
#classmethod
def search_url(cls):
if hasattr(cls, '_search_url'):
setattr(cls, '_search_url', reverse('%s-search' % cls._meta.model_name))
return cls._search_url
Maybe because the property hasn't been called from the class yet...
In summary, I want to add a lazy, class method that can be called from the instance... Can this be achieved in an elegant (nottoomanylines) way?
Any thoughts?
How I achieved it
Sorry, my examples were very bad ones :\
Anyway, in the end I did it like this...
#classmethod
def search_url(cls):
if not hasattr(cls, '_search_url'):
setattr(cls, '_search_url', reverse('%s-search' % cls._meta.model_name))
return cls._search_url
And the setattr does work, but I had made a mistake when testing it...
You can add a function to a class at any point, a practice known as monkey-patching:
class SadClass:
pass
#classmethod
def say_dynamic(cls):
print('hello')
SadClass.say_dynamic = say_dynamic
>>> SadClass.say_dynamic()
hello
>>> SadClass().say_dynamic()
hello
Note that you are using the classmethod decorator, but your function accepts no arguments, which indicates that it's designed to be a static method. Did you mean to use staticmethod instead?
If you want to create class methods, do not create them in the __init__ function as it is then recreated for each instance creation. However, following works:
class SadClass(object):
pass
def say_dynamic(cls):
print("dynamic")
SadClass.say_dynamic = classmethod(say_dynamic)
# or
setattr(SadClass, 'say_dynamic', classmethod(say_dynamic))
SadClass.say_dynamic() # prints "dynamic!"
SadClass().say_dynamic() # prints "dynamic!"
Of course, in the __init__ method the self argument is an instance, and not the class: to put the method in the class there, you can hack something like
class SadClass(object):
def __init__(self, *args, **kwargs):
#classmethod
def say_dynamic(cls):
print("dynamic!")
setattr(self.__class__, 'say_dynamic', say_dynamic)
But it will again reset the method for each instance creation, possibly needlessly. And notice that your code most probably fails because you are calling the SadClass.say_dynamic() before any instances are created, and thus before the class method is injected.
Also, notice that a classmethod gets the implicit class argument cls; if you do want your function to be called without any arguments, use the staticmethod decorator.
As a side note, you can just use an instance attribute to hold a function:
>>> class Test:
... pass
...
>>> t=Test()
>>> t.monkey_patch=lambda s: print(s)
>>> t.monkey_patch('Hello from the monkey patch')
Hello from the monkey patch
How I achieved it:
#classmethod
def search_url(cls):
if not hasattr(cls, '_search_url'):
setattr(cls, '_search_url', reverse('%s-search' % cls._meta.model_name))
return cls._search_url
In formsets.py, you find this code snippet
class BaseFormSet(StrAndUnicode):
"""
A collection of instances of the same Form class.
"""
def __init__(self, data=None, files=None, auto_id='id_%s', prefix=None,
initial=None, error_class=ErrorList):
...
self.prefix = prefix or self.get_default_prefix() # Note the self.get_default_prefix
...
...
#classmethod # Note the #classmethod
def get_default_prefix(cls):
return 'form'
Why is get_default_prefix declared this way and then called with self.? Is there something gained by doing it this way? get_default_prefix has another definition in BaseInlineFormSet (forms/models.py)
class BaseInlineFormSet(BaseModelFormSet):
...
#classmethod
def get_default_prefix(cls):
from django.db.models.fields.related import RelatedObject
return RelatedObject(cls.fk.rel.to, cls.model, cls.fk).get_accessor_name().replace('+','')
and another in BaseGenericInlineFormset again using the #classmethod, so it doesn't appear to be a typo. I just don't understand why it would be done this way and then called with self.
The only clue I see (which I don't understand) is that the admin seems to call it with FormSet.get_default_prefix()
I'm wondering if there is something I'm just not understanding about python.
Calling a class method from an instance is perfectly legal, as you can see in the code. A related stackoverflow post said there was no benefit, (and it is bad practice) to call from an instance; because if you are only calling from instance your method should probably not be a classmethod.
I think you answer your own question, though. If django is calling FormSet.get_default_prefix() from somewhere, then they probably didn't want to instantiate a formset object