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Is it possible to use a parent class method as a decorator for a child class method?

I have two classes, Manager and DataManager, simplified in the example below:
import numpy as np
class Manager:
def __init__(self, value, delay_init=True):
self.value = value
self.is_init = False
self.data = None
if not delay_init:
self._initialize()
#staticmethod
def delayed_init(fn):
def wrapped_delayed_init(obj, *args, **kwargs):
if not obj.is_init:
obj.data = np.random.randn(obj.value, obj.value)
obj.is_init = True
return fn(obj, *args, **kwargs)
return wrapped_delayed_init
#delayed_init.__get__(object)
def _initialize(self):
pass
class DataManager(Manager):
def __init__(self, value):
super().__init__(value)
#Manager.delayed_init
def calculate_mean(self):
return np.mean(self.data)
data_manager = DataManager(100)
assert data_manager.data is None
mean = data_manager.calculate_mean()
What my code needs to do is pass the method calculate as an argument to some other function as part of a test suite. In order to do this I need to create an instance of DataManager. What I must avoid is the time incurred by the full instance creation (since it involved downloading data), and so I delegate this task to some function in the parent class called delayed_init. There are a subset of methods belonging to DataManager that require this delayed_init to have been run, and so I choose to decorate them with delayed_init to ensure it is run whenever 1) another method requires it and 2) it has not already been run.
Now my problem: Currently it appears I need to explicitly define the decorator as #Manager.delayed_init, but this can be re-written as #<parent>.delayed_init. I would like to write it this way if possible given that in my opinion it is cleaner to not have to explicitly write out a given type if the type is always the parent. However, I cannot find a way to properly reference the parent class before an instance/object is created. Is it possible to access the parent class without the creation of any instances?
Thank you for the assistance.

Wrapping an object completely without knowing anything about it (python)

I am working with htcondor python bindings (https://htcondor.readthedocs.io/en/latest/apis/python-bindings/index.html)
You don't need to know htcondor, but for reference I am working with the
htcondor.JobEvent class to get the data that I want.
From the description of that object it follows that it behaves like a dictionary,
but it has no __dict__ property.
Basically you can't tell what this object is, because it's translated from C++ into python, hence I want to wrap it with all it's functionalities to add more functionalities.
The way I am solving this atm is:
class HTCJobEventWrapper:
"""
Wrapper for HTCondor JobEvent.
Extracts event number and time_stamp of an event.
The wrapped event can be printed to the terminal for dev purpose.
:param job_event: HTCJobEvent
"""
def __init__(self, job_event: HTCJobEvent):
self.wrapped_class = job_event
self.event_number = job_event.get('EventTypeNumber')
self.time_stamp = date_time.strptime(
job_event.get('EventTime'),
STRP_FORMAT
)
def __getattr__(self, attr):
return getattr(self.wrapped_class, attr)
def get(self, *args, **kwargs):
"""Wraps wrapped_class get function."""
return self.wrapped_class.get(*args, **kwargs)
def items(self):
"""Wraps wrapped_class items method."""
return self.wrapped_class.items()
def keys(self):
"""Wraps wrapped_class keys method."""
return self.wrapped_class.keys()
def values(self):
"""Wraps wrapped_class values method."""
return self.wrapped_class.values()
def to_dict(self):
"""Turns wrapped_class items into a dictionary."""
return dict(self.items())
def __repr__(self):
return json.dumps(
self.to_dict(),
indent=2
)
With this it's possible to get any attribute and to use the methods described in the documentation.
However as you can see HTCJobEventWrapper is not of type htcondor.JobEvent
and is not inheriting from it.
If you try to instatiate a htcondor.JobEvent class it results in the following error: RuntimeError: This class cannot be instantiated from Python.
What I want:
I would like it to be a child class which copies a given htcondor.JobEvent object completely
and adds the functionalities I want and returns a HTCJobEventWrapper object
This kind of relates to this question: completely wrap an object in python
Is there a pythonic way to dynamically call every attribute, function or method on self.wrapped_class ? Just like with getattr ?
But in this case I've tried getattr but it works only for attributes.

Give function scope access to the self where it is called?

I have a class "Wrapper". Wrapper inherits from object, and it's primary responsibility is to hook method calls to selenium during a test. It's __init__ method signature is
__init__(self, object_to_wrap, actions={}, **kwargs)
Part of it's functionality is to take a dictionary as an argument in the init method, and to expose the keys as attributes on the object_to_wrap object by defining the __getattr__ magic method
(signature: __getattr__(self, item):)
and checking self.actions's keys for item. If it is found, the method is invoked.
In the test code, the initialization would look similar to:
def navigate(scoped_self, to=''):
self.driver.switch_to_default_content()
self.driver.switch_to.frame(to)
scoped_self.navigations.append(to)
# the navigate method is scoped in an
# instance method of the test class, so it has access to self
So, my question is, how can I make the scope, or self in the above method, be the scope of my Wrapper class, and not the test class?
For clarity, if the solution I am looking for in this question was found, the navigate implementation would change to be:
def navigate(scoped_self, to=''):
self.object_to_wrap.switch_to_default_content()
self.object_to_wrap.switch_to.frame(to)
Also for clarity, I'm pretty sure what I'm looking for is exactly what Javascripts Function.prototype.bind accomplishes.
UPDATE: Defining the methods, such as navigate, inside the Wrapper class is not an option, as the Wrapper class cannot have test-specific logic. The test suite is done on n > 1 DOM, that are completely unrelated. For example, one of the tests requires the method "alert_handlers" (to overwrite the window.alert function and return the alerts presented as a string), another might require the navigate method, and a third might require both.
UPDATE #2: Thanks to the answer below from c17r, I realized that without the getattribute code included in my example, that it would appear as if I was asking for something we had already accomplished. What I am looking for is the ability, in the above navigate method, is for scoped_self to be the instance of Wrapper.
Furthermore, I am specifically looking for a way to "dynamically" pass scoped_self to the functions (the way that Function.prototype.bind"dynamically" sets this where this is myEventListener.bind(window); if you're unfamiliar with javascript, make an event listener on the body and console.log this without binding, and then with binding, to see the difference)
Furthermore, I figured it might help to give an example implementation without the solution I am looking for. This is currently working as expected:
class Wrapper(object):
def __init__(self, wrapped, actions={}):
self.wrapped = wrapped
self.actions = actions
self.navigations = [] # EXAMPLE, SEE THE TEST CLASS CODE
def __getattr__(self, item):
if item in self.actions:
return self.actions[item]
# do other fancy stuff here
# UPDATE #2: added for clarity. this is the current implementation
orig_attr = self.wrapped.__getattribute__(item)
if callable(orig_attr):
def hooked(*args, **kwargs):
self.pre(item, *args, **kwargs)
self.err = False
try:
result = orig_attr(*args, **kwargs)
except Exception as e:
#logs
self.post(*args, **kwargs)
raise
if type(self.wrapped) == type(result):
return self
return result
return hooked
else:
return orig_attr
class SomeTest():
#blah blah init stuff, set self.driver = selenium.webdriver.Phantomjs
def spawn_actions(self):
def navigate(scoped_self, to=''):
self.driver.switch_to_default_content()
self.driver.switch_to.frame(to)
scoped_self.navigations.append(to) # <--- appended to wrapper.navigations
return {'navigate': navigate}
def run(self):
driver = Wrapper(self.driver, self.spawn_actions())
driver.get(url)
driver.navigate('New Request')
# fun tests!

If I understand you correctly, 3 things:
Wrapper would need to pass any unknown functions down to the wrapped item, otherwise driver.get() won't work properly.
The navigate function needs self as the first parameter, like it would if it was actually defined on Wrapper
Wrapper needs to pass self into the dict-based function. This is a little tricky since __getattr__ doesn't actually call the function, so you need to return a function that calls the underlying function properly, similar to how decorators work.
Like this:
class Driver(object):
def get(self, url):
print('get')
print(repr(self))
print(repr(url))
print('--')
class Wrapper(object):
def __init__(self, wrapped, actions={}):
self.wrapped = wrapped
self.actions = actions
def __getattr__(self, item):
if item in self.actions:
def unwrap(*args, **kwargs):
return self.actions[item](self, *args, **kwargs)
return unwrap
else:
return getattr(self.wrapped, item)
class Test(object):
def __init__(self):
self.driver = Driver();
def spawn_actions(self):
def navigate(self, to=''):
print('navigate')
print(repr(self))
print(repr(to))
print(repr(self.wrapped))
print('--')
return {
'navigate': navigate
}
def run(self):
driver = Wrapper(self.driver, self.spawn_actions())
driver.get('url')
driver.navigate('thing')
Now calling:
t = Test()
t.run()
Outputs:
get
<__main__.Driver object at 0x104008630>
'url'
--
navigate
<__main__.Wrapper object at 0x104008ba8>
'thing'
<__main__.Driver object at 0x104008630>
--
EDIT
You can also dynamically bind the methods to the instance instead of __getattr__ returning the unwrap function:
import types
class Wrapper(object):
def __init__(self, wrapped, actions={}):
self.wrapped = wrapped
for name, func in actions.items():
setattr(self, name, types.MethodType(func, self))
def __getattr__(self, item):
return getattr(self.wrapped, item)

How to execute BaseClass method before it gets overridden by DerivedClass method in Python

I am almost sure that there is a proper term for what I want to do but since I'm not familiar with it, I will try to describe the whole idea explicitly. So what I have is a collection of classes that all inherit from one base class. All the classes consist almost entirely of different methods that are relevant within each class only. However, there are several methods that share similar name, general functionality and also some logic but their implementation is still mostly different. So what I want to know is whether it's possible to create a method in a base class that will execute some logic that is similar to all the methods but still continue the execution in the class specific method. Hopefully that makes sense but I will try to give a basic example of what I want.
So consider a base class that looks something like that:
class App(object):
def __init__(self, testName):
self.localLog = logging.getLogger(testName)
def access(self):
LOGIC_SHARED
And an example of a derived class:
class App1(App):
def __init__(self, testName):
. . .
super(App1, self).__init__(testName)
def access(self):
LOGIC_SPECIFIC
So what I'd like to achieve is that the LOGIC_SHARED part in base class access method to be executed when calling the access method of any App class before executing the LOGIC_SPECIFIC part which is(as it says) specific for each access method of all derived classes.
If that makes any difference, the LOGIC_SHARED mostly consists of logging and maintenance tasks.
Hope that is clear enough and the idea makes sense.
NOTE 1:
There are class specific parameters which are being used in the LOGIC_SHARED section.
NOTE 2:
It is important to implement that behavior using only Python built-in functions and modules.
NOTE 3:
The LOGIC_SHARED part looks something like that:
try:
self.localLog.info("Checking the actual link for %s", self.application)
self.link = self.checkLink(self.application)
self.localLog.info("Actual link found!: %s", self.link)
except:
self.localLog.info("No links found. Going to use the default link: %s", self.link)
So, there are plenty of specific class instance attributes that I use and I'm not sure how to use these attributes from the base class.

Sure, just put the specific logic in its own "private" function, which can overridden by the derived classes, and leave access in the Base.
class Base(object):
def access(self):
# Shared logic 1
self._specific_logic()
# Shared logic 2
def _specific_logic(self):
# Nothing special to do in the base class
pass
# Or you could even raise an exception
raise Exception('Called access on Base class instance')
class DerivedA(Base):
# overrides Base implementation
def _specific_logic(self):
# DerivedA specific logic
class DerivedB(Base):
# overrides Base implementation
def _specific_logic(self):
# DerivedB specific logic
def test():
x = Base()
x.access() # Shared logic 1
# Shared logic 2
a = DerivedA()
a.access() # Shared logic 1
# Derived A specific logic
# Shared logic 2
b = DerivedB()
b.access() # Shared logic 1
# Derived B specific logic
# Shared logic 2

The easiest method to do what you want is to simply call the parent's class access method inside the child's access method.
class App(object):
def __init__(self, testName):
self.localLog = logging.getLogger(testName)
def access(self):
LOGIC_SHARED
class App1(App):
def __init__(self, testName):
super(App1, self).__init__(testName)
def access(self):
App.access(self)
# or use super
super(App1, self).access()
However, your shared functionality is mostly logging and maintenance. Unless there is a pressing reason to put this inside the parent class, you may want to consider is to refactor the shared functionality into a decorator function. This is particularly useful if you want to reuse similar logging and maintenance functionality for a range of methods inside your class.
You can read more about function decorators here: http://www.artima.com/weblogs/viewpost.jsp?thread=240808, or here on Stack Overflow: How to make a chain of function decorators?.
def decorated(method):
def decorated_method(self, *args, **kwargs):
LOGIC_SHARED
method(self, *args, **kwargs)
return decorated_method
Remember than in python, functions are first class objects. That means that you can take a function and pass it as a parameter to another function. A decorator function make use of this. The decorator function takes another function as a parameter (here called method) and then creates a new function (here called decorated_method) that takes the place of the original function.
Your App1 class then would look like this:
class App1(App):
#logged
def access(self):
LOGIC_SPECIFIC
This really is shorthand for this:
class App1(App):
def access(self):
LOGIC_SPECIFIC
decorated_access = logged(App.access)
App.access = decorated_access
I would find this more elegant than adding methods to the superclass to capture shared functionality.

If I understand well this commment (How to execute BaseClass method before it gets overridden by DerivedClass method in Python) you want that additional arguments passed to the parent class used in derived class
based on Jonathon Reinhart's answer
it's how you could do
class Base(object):
def access(self,
param1 ,param2, #first common parameters
*args, #second positional parameters
**kwargs #third keyword arguments
):
# Shared logic 1
self._specific_logic(param1, param2, *args, **kwargs)
# Shared logic 2
def _specific_logic(self, param1, param2, *args, **kwargs):
# Nothing special to do in the base class
pass
# Or you could even raise an exception
raise Exception('Called access on Base class instance')
class DerivedA(Base):
# overrides Base implementation
def _specific_logic(self, param1, param2, param3):
# DerivedA specific logic
class DerivedB(Base):
# overrides Base implementation
def _specific_logic(self, param1, param2, param4):
# DerivedB specific logic
def test():
x = Base()
a = DerivedA()
a.access("param1", "param2", "param3") # Shared logic 1
# Derived A specific logic
# Shared logic 2
b = DerivedB()
b.access("param1", "param2", param4="param4") # Shared logic 1
# Derived B specific logic
# Shared logic 2

I personally prefer Jonathon Reinhart's answer, but seeing as you seem to want more options, here's two more. I would probably never use the metaclass one, as cool as it is, but I might consider the second one with decorators.
With Metaclasses
This method uses a metaclass for the base class that will force the base class's access method to be called first, without having a separate private function, and without having to explicitly call super or anything like that. End result: no extra work/code goes into inheriting classes.
Plus, it works like maaaagiiiiic </spongebob>
Below is the code that will do this. Here http://dbgr.cc/W you can step through the code live and see how it works :
#!/usr/bin/env python
class ForceBaseClassFirst(type):
def __new__(cls, name, bases, attrs):
"""
"""
print("Creating class '%s'" % name)
def wrap_function(fn_name, base_fn, other_fn):
def new_fn(*args, **kwargs):
print("calling base '%s' function" % fn_name)
base_fn(*args, **kwargs)
print("calling other '%s' function" % fn_name)
other_fn(*args, **kwargs)
new_fn.__name__ = "wrapped_%s" % fn_name
return new_fn
if name != "BaseClass":
print("setting attrs['access'] to wrapped function")
attrs["access"] = wrap_function(
"access",
getattr(bases[0], "access", lambda: None),
attrs.setdefault("access", lambda: None)
)
return type.__new__(cls, name, bases, attrs)
class BaseClass(object):
__metaclass__ = ForceBaseClassFirst
def access(self):
print("in BaseClass access function")
class OtherClass(BaseClass):
def access(self):
print("in OtherClass access function")
print("OtherClass attributes:")
for k,v in OtherClass.__dict__.iteritems():
print("%15s: %r" % (k, v))
o = OtherClass()
print("Calling access on OtherClass instance")
print("-------------------------------------")
o.access()
This uses a metaclass to replace OtherClass's access function with a function that wraps a call to BaseClass's access function and a call to OtherClass's access function. See the best explanation of metaclasses here https://stackoverflow.com/a/6581949.
Stepping through the code should really help you understand the order of things.
With Decorators
This functionality could also easily be put into a decorator, as shown below. Again, a steppable/debuggable/runnable version of the code below can be found here http://dbgr.cc/0
#!/usr/bin/env python
def superfy(some_func):
def wrapped(self, *args, **kwargs):
# NOTE might need to be changed when dealing with
# multiple inheritance
base_fn = getattr(self.__class__.__bases__[0], some_func.__name__, lambda *args, **kwargs: None)
# bind the parent class' function and call it
base_fn.__get__(self, self.__class__)(*args, **kwargs)
# call the child class' function
some_func(self, *args, **kwargs)
wrapped.__name__ = "superfy(%s)" % some_func.__name__
return wrapped
class BaseClass(object):
def access(self):
print("in BaseClass access function")
class OtherClass(BaseClass):
#superfy
def access(self):
print("in OtherClass access function")
print("OtherClass attributes")
print("----------------------")
for k,v in OtherClass.__dict__.iteritems():
print("%15s: %r" % (k, v))
print("")
o = OtherClass()
print("Calling access on OtherClass instance")
print("-------------------------------------")
o.access()
The decorator above retrieves the BaseClass' function of the same name, and calls that first before calling the OtherClass' function.

May this simple approach can help.
class App:
def __init__(self, testName):
self.localLog = logging.getLogger(testName)
self.application = None
self.link = None
def access(self):
print('There is something BaseClass must do')
print('The application is ', self.application)
print('The link is ', self.link)
class App1(App):
def __init__(self, testName):
# ...
super(App1, self).__init__(testName)
def access(self):
self.application = 'Application created by App1'
self.link = 'Link created by App1'
super(App1, self).access()
print('There is something App1 must do')
class App2(App):
def __init__(self, testName):
# ...
super(App2, self).__init__(testName)
def access(self):
self.application = 'Application created by App2'
self.link = 'Link created by App2'
super(App2, self).access()
print('There is something App2 must do')
and the test result:
>>>
>>> app = App('Baseclass')
>>> app.access()
There is something BaseClass must do
The application is None
The link is None
>>> app1 = App1('App1 test')
>>> app1.access()
There is something BaseClass must do
The application is Application created by App1
The link is Link created by App1
There is something App1 must do
>>> app2 = App2('App2 text')
>>> app2.access()
There is something BaseClass must do
The application is Application created by App2
The link is Link created by App2
There is something App2 must do
>>>

Adding a combine function we can combine two functions and execute them one after other as bellow
def combine(*fun):
def new(*s):
for i in fun:
i(*s)
return new
class base():
def x(self,i):
print 'i',i
class derived(base):
def x(self,i):
print 'i*i',i*i
x=combine(base.x,x)
new_obj=derived():
new_obj.x(3)
Output Bellow
i 3
i*i 9
it need not be single level hierarchy it can have any number of levels or nested

Trying to program a "to_class" general decorator

Suppose I have defined:
def to_class(cls):
""" returns a decorator
aimed to force the result to be of class cls. """
def decorating_func(func):
def wrapper(*args, **kwargs):
return cls(func(*args, **kwargs))
return wrapper
return decorator(decorating_func)
I wish to use it to create decorators which turn function results to objects of a given class. However, this will not work:
class TestClass(object):
def __init__(self, value):
self._value = (value, value)
def __str__(self):
return str(self._value)
#staticmethod
#to_test_class
def test_func(value):
return value
to_test_class = to_class(TestClass)
as test_func will look for to_test_class and will not find it. On the other hand, putting the assignment to to_test_class before the class definition will fail as well, as TestClass will not be defined yet.
Trying to put #to_class(TestClass) above the definition of test_func will also fail, as the method is constructed before the class (if I am not wrong).
The only workaround I have found is to define to_test_class manually as a decorator, and not as one returned from the general "to_class" def.
It might be important to mention that this is only a basic example, but I wish to use to_class for many applications, such as modifying the returned value before 'plugging' it into the class' constructor; and I wish to use it as a decorator for other class' methods as well.
I am sure some think a "to_class" decorator is pointless; manipulations can be done within the decorated method, instead. Though, I find it convenient, and it helps me with readability.
Finally I wish to add that this interests me 20% for practical reasons and 80% for studying reasons, as I find this is something I do not fully understand about decorators in Python in general.

Indeed, at class construction time, the class object itself has not yet been constructed, thus you cannot use it as the basis of a decorator.
One work-around I can think of, is to not use the staticmethod decorator. Instead, internally in your own decorator, re-use the classmethod decorator. That way you ensure that Python at the very least passes in the associated class for you:
def to_class(func):
""" returns a decorator
aimed to force the result to be of class cls. """
def wrapper(cls, *args, **kwargs):
return cls(func(*args, **kwargs))
return classmethod(wrapper)
Then use it like this:
class TestClass(object):
def __init__(self, value):
self._value = (value, value)
def __str__(self):
return str(self._value)
#to_class
def test_func(value):
return value
Demonstration:
>>> def to_class(func):
... """ returns a decorator
... aimed to force the result to be of class cls. """
... def wrapper(cls, *args, **kwargs):
... return cls(func(*args, **kwargs))
... return classmethod(wrapper)
...
>>> class TestClass(object):
... def __init__(self, value):
... self._value = (value, value)
... def __str__(self):
... return str(self._value)
... #to_class
... def test_func(value):
... return value
...
>>> TestClass.test_func('foo')
<__main__.TestClass object at 0x102a77210>
>>> print TestClass.test_func('foo')
('foo', 'foo')
A generic version of your decorator is not easy; the only other workaround to your conundrum is to use a metaclass hack; see another answer of mine where I describe the method in more detail.
You basically need to reach into the class-under-construction namespace, set a temporary metaclass, and then rely on there being at least one instance of the class before your decorator will work; the temporary metaclass approach hooks into the class creation mechanisms to retrieve the constructed class at a later time.
Seeing as you are using this decorator as an alternative class factory however, that is probably not going to be ideal; if someone used your decorated functions to create class instances exclusively the metaclass would be called too late.

Well, you forgot that class is the first parameter passed to method decorated with classmethod, so you can write it like this:
def to_this_class(func):
def wrapped(cls, value):
res = func(cls, value)
return cls(res)
return wrapped
class TestClass(object):
def __init__(self, value):
self._value = (value, value)
def __str__(self):
return str(self._value)
#classmethod
#to_this_class
def test_func(cls, value):
return value
x = TestClass('a')
print x.test_func('b')

The problem is that a decorator gets evaluated upon defining the thing it decorates, so when defining the method test_func(), the decorator to_test_class gets called, and even if it already exists, the thing it shall work on (the class TestClass) does not exist yet (as this is created after all methods are created).
Maybe you can use a placeholder at the point where the class is used and later (after the class is created) fill in that value (the class) at the point of the placeholder.
Example:
lazyClasses = {}
def to_lazy_class(className):
""" returns a decorator
aimed to force the result to be of class cls. """
def decorating_func(func):
def wrapper(*args, **kwargs):
return lazyClasses[className](func(*args, **kwargs))
return wrapper
return decorating_func
class TestClass(object):
def __init__(self, value):
self._value = (value, value)
def __str__(self):
return str(self._value)
#staticmethod
#to_lazy_class('TestClass')
def test_func(value):
return value
lazyClasses['TestClass'] = TestClass
>>> TestClass.test_func('hallo')
<__main__.TestClass object at 0x7f76d8cba190>