does anyone have idea how to refer to class static method in the class attributes? for example:
class A(object):
var = A.staticMethod1 # this line doesn't work, as A has not be defined yet.
#staticmethod
def staticMethod1(*param):
pass
Class blocks are just temporary scopes, so, it's actually pretty simple. Just move the alias below the method definition, and take off the class name (since you're in scope directly)
class A(object):
#staticmethod
def staticMethod1(*param):
pass
var = staticMethod1
Python 3 doesn't allow you to reference a class inside its body (except in methods):
class A:
static_attribute = A()
def __init__(self):
...
This raises a NameError in the second line because 'A' is not defined.
Alternatives
I have quickly found one workaround:
class A:
#property
#classmethod
def static_property(cls):
return A()
def __init__(self):
...
Although this isn't exactly the same since it returns a different instance every time (you could prevent this by saving the instance to a static variable the first time).
Are there simpler and/or more elegant alternatives?
EDIT:
I have moved the question about the reasons for this restriction to a separate question
The expression A() can't be run until the class A has been defined. In your first block of code, the definition of A is not complete at the point you are trying to execute A().
Here is a simpler alternative:
class A:
def __init__(self):
...
A.static_attribute = A()
When you define a class, Python immediately executes the code within the definition. Note that's different than defining a function where Python compiles the code, but doesn't execute it.
That's why this will create an error:
class MyClass(object):
a = 1 / 0
But this won't:
def my_func():
a = 1 / 0
In the body of A's class definition, A is not yet defined, so you can't reference it until after it's been defined.
There are several ways you can accomplish what you're asking, but it's not clear to me why this would be useful in the first place, so if you can provide more details about your use case, it'll be easier to recommend which path to go down.
The simplest would be what khelwood posted:
class A(object):
pass
A.static_attribute = A()
Because this is modifying class creation, using a metaclass could be appropriate:
class MetaA(type):
def __new__(mcs, name, bases, attrs):
cls = super(MetaA, mcs).__new__(mcs, name, bases, attrs)
cls.static_attribute = cls()
return cls
class A(object):
__metaclass__ = MetaA
Or you could use descriptors to have the instance lazily created or if you wanted to customize access to it further:
class MyDescriptor(object):
def __get__(self, instance, owner):
owner.static_attribute = owner()
return owner.static_attribute
class A(object):
static_attribute = MyDescriptor()
Using the property decorator is a viable approach, but it would need to be done something like this:
class A:
_static_attribute = None
#property
def static_attribute(self):
if A._static_attribute is None:
A._static_attribute = A()
return A._static_attribute
def __init__(self):
pass
a = A()
print(a.static_attribute) # -> <__main__.A object at 0x004859D0>
b = A()
print(b.static_attribute) # -> <__main__.A object at 0x004859D0>
You can use a class decorator:
def set_static_attribute(cls):
cls.static_attribute = cls()
return cls
#set_static_attribute
class A:
pass
Now:
>>>> A.static_attribute
<__main__.A at 0x10713a0f0>
Applying the decorator on top of the class makes it more explicit than setting static_attribute after a potentially long class definition. The applied decorator "belongs" to the class definition. So if you move the class around in your source code you will more likely move it along than an extra setting of the attribute outside the class.
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
class Something(object):
our_random = Something.random_thing
#staticmethod
def random_thing():
return 4
Of course, this doesn't work, becauese Something doesn't exist when I attempt to call its method. Nor does this:
class Something(object):
our_random = random_thing
#staticmethod
def random_thing():
return 4
I've "solved" this by Just placing random_thing()'s definition above the class, but I find this messy.
Call it in the .__init__() initializer then:
class Something(object):
def __init__(self):
self.our_random = Something.random_thing()
or call the static method after you defined it, but are still defining the class; because it is a static method, you'd have to access it through the __func__ attribute:
class Something(object):
#staticmethod
def random_thing():
return 4
our_random = random_thing.__func__()
If you didn't mean to call it, just create a copy of the method with a different name, just do so after you defined it:
class Something(object):
#staticmethod
def random_thing():
return 4
our_random = random_thing # our_random as an alias for random_thing
The class body is executed as a function, with the local namespace of the function then forming the class attributes. So, like a function, if you want to refer to other objects you need to make sure they are defined first.
class Something(object):
#staticmethod
def random_thing():
return 4
our_random = random_thing
Class definitions create a namespace, so you can refer to other names (class attributes) within the class body without needing to access them through the class.
Let's say I have this code:
class class1(object):
def __init__(self):
#don't worry about this
def parse(self, array):
# do something with array
class class2(object):
def __init__(self):
#don't worry about this
def parse(self, array):
# do something else with array
I want to be able to call class1's parse from class2 and vice-versa. I know with c++ this can be done quite easily by doing
class1::parse(array)
How would I do the equivalent in python?
It sounds like you want a static method:
class class1(object):
#staticmethod
def parse(array):
...
Note that in such cases you leave off the usually-required self parameter, because parse is not a function called on a particular instance of class1.
On the other hand, if you want a method which is still tied to its owner class, you can write a class method, where the first argument is actually the class object:
class class1(object):
#classmethod
def parse(cls, array):
...