I'm trying to do the following:
class A:
#classmethod
def test_function(cls, message):
cls.__get_the_function()
class B(A):
#classmethod
def __get_the_function(cls):
return print("BBBB")
class C(A):
#classmethod
def __get_the_function(cls):
return print("CCCC")
however when I call:
B.test_function("Test")
I get the following:
AttributeError: type object 'B' has no attribute '_A__get_the_function'
I want class A to __get_the_function from the subclass (either class B or C depends on which one I use), but it looks like it is trying to look for it in itself.
NOTE: I'm using Python 3.8.2
__-prefixed names are handled specially during class creation. The name is replaced when the function is defined by a mangled name, as if you had defined the function as
#classmethod
def test_function(cls, message):
cls._A__get_the_function()
in the first place.
This is done to explicitly provide a way to hide a name from a subclass. Since you want to override the name, __get_the_function isn't an appropriate name; use an ordinary _-prefixed name if you want to mark it as private:
class A:
#classmethod
def test_function(cls, message):
cls._get_the_function()
# Define *something*, since test_function assumes it
# will exist. It doesn't have to *do* anything, though,
# until you override it.
#classmethod
def _get_the_function(cls):
pass
Related
I'm working on a project using abstract classes in Python (specifically, the abc module).
I have a few implementations of this abstract class, which have their own constructors and need to use self.
This is what my code looks like, but simplified:
from abc import ABC, abstractmethod
class BaseClass(ABC):
def __init__(self):
self.sublinks = [] # not meant to be passed in, that's why it isn't an argument in __init__
#classmethod
def display(cls):
print(cls.get_contents())
#abstractmethod
def get_contents():
pass
class ImplementationOne(Base):
def __init__(self, url):
self.url = url
def get_contents(self):
return "The url was: " + url
class ImplementationTwo(Base):
def get_contents():
return "This does not need a url"
test_one = ImplementationOne("https://google.com")
test_two = ImplementationTwo()
test_one.display()
When I run this, however, I get the error TypeError: get_contents() missing 1 required positional argument: 'self'.
I figured that this is because get_contents() in ImplementationOne takes self, but it's not specified in the abstract method.
So, if I changed:
#abstractmethod
def get_contents():
pass
to
#abstractmethod
def get_contents(self):
pass
But I get the same error.
I've tried many combinations, including putting self as an argument to every occurrence or get_contents, and passing in cls to get_contents in the abstract class - but no luck.
So, pretty much, how can I use the self keyword (aka access attributes) in only some implementations of an abstract method, that's called within a class method in the abstract class itself.
Also, on a side note, how can I access self.sublinks from within all implementations of BaseClass, while having its values different in each instance of an implementation?
There are a few things wrong here. One is that the #classmethod decorator should only be used when you need it to be called on a class.
Example:
class ImplementationOne:
#classmethod
def display(cls):
print(f'The class name is {cls.__name__}.')
ImplementationOne.display()
There is nothing special about the name self. It's just what is used by everyone to refer to the instance. In python the instance is implicitly handed to the first argument of the class unless you have a #classmethod decorator. In that case the class is handed as the first argument.
That is why you are getting the TypeError. Since you are calling the method on the instance test_one.display() you are essentially calling it as an instance method. Since you need to access the instance method get_contents from within it that is what you want. As a classmethod you wouldn't have access to get_contents.
That means you need both the ABC and ImplementationOne to have those methods implemented as instance methods.
Since it is now an instance method on the ABC it also should be an instance method in ImplementationTwo.
Your other question was how to get self.sublinks as an attribute in both subclasses.
Since your are overriding __init__ in ImplementationOne you need to call the parent class's __init__ as well. You can do this by using super() to call the Super or Base class's methods.
class ImplementationOne(BaseClass):
def __init__(self, url):
self.url = url
super().__init__()
Full working code:
from abc import ABC, abstractmethod
class BaseClass(ABC):
def __init__(self):
self.sublinks = []
def display(self):
print(self.get_contents())
#abstractmethod
def get_contents(self):
pass
class ImplementationOne(BaseClass):
def __init__(self, url):
self.url = url
super().__init__()
def get_contents(self):
return "The url was: " + self.url
class ImplementationTwo(BaseClass):
def get_contents(self):
return "This does not need a url"
test_one = ImplementationOne("https://google.com")
test_two = ImplementationTwo()
test_one.display()
test_two.display()
print(test_one.sublinks)
Let me give a brief explanation of the issue:
I have a server object with several functionalities.
all functionalities have some common code, so this warrants a functionalities base class
Each functionality has its own set of constants, defined in a constants class within the functionality.
The functionality base class has a set of common constants as well.
here is a sample code:
class server:
class base_func:
class common_consts:
name = "name"
def validate(self):
pass
def execute(self):
pass
class func1(base_func):
class consts:
new_name = base_func.common_consts.name #this is where the problem occurs
def get_result(self):
self.validate()
self.execute()
so when i try to use the common_consts from base_func, in func1.consts, I get the following error:
NameError: global name 'base_func' is not defined
I do not know why this happens. Can someone help?
Is there a limitation to the scope of nesting in python, especially 2.7
Also if i remove the top level server class, and have the functionality classes as independent classes, everything seems to work fine. The example of the working code is here:
class base_func:
class common_consts:
name = "name"
def validate(self):
pass
def execute(self):
pass
class func1(base_func):
class consts:
new_name = base_func.common_consts.name #this is where the problem occurs
def get_result(self):
self.validate()
self.execute()
This leads me to believe that there definitely exists some limitation to the nesting depth and namespace scopes in python. I just want to be sure before i make changes to my design.
class server:
class base_func:
class common_consts:
name = "name"
def validate(self):
pass
def execute(self):
pass
# base_func and func1 are at same, level. So, here you can directly use base_func and func1 anywhere
# at top level of the server class
class func1(base_func):
class consts:
new_name = base_func.common_consts.name # this is where the problem occurs
def get_result(self):
self.validate()
self.execute
For a class(classes have their own namespace), variable lookup works like this:
While parsing the class body any variable defined inside the class body can be access directly, but once
it is parsed it becomes a class attribute.
As, the class base_func is inside server class which is still being parsed the func1(base_func) will work fine.
But, for class consts base_func is not at the same level. So, after looking the variable in its body it will directly jump
to its enclosing scope, i.e global namespace in this case.
A fix will be to do the assignement like this:
class server:
class base_func:
class common_consts:
name = "name"
def validate(self):
pass
def execute(self):
pass
class func1(base_func):
class consts:
pass
def get_result(self):
self.validate()
self.execute
func1.consts.new_name = base_func.common_consts.name
You've hit a problem with class scope. The class scope is never made available except to operations that directly occur in the class scope. This is why you can't call method within another method without referencing self.
eg.
class A(object):
def f(self):
pass
def g(self):
f() # error, class scope isn't available
The same is true when you create a nested class. The initial class statement class ClassName(Base): has access to the class scope, but once in the nested class scope you lose access to the enclosing class scope.
Generally, there is no good reason to nest classes in python. You should try to create all classes at module level. You can hide internal classes by either placing them in a sub module or all prefixing them with an underscore.
class _BaseFunctionality(object):
# common constants
name = "name"
value = "value"
def execute(self):
return (self.name, self.value)
class _SpecificFunctionality(_BaseFunctionality):
# specific constants
# override value of value attribute
value = "another_value"
def get_result(self):
assert self.name == "name"
return self.execute()
class Server(object):
functionality = _SpecificFunctionality()
assert _BaseFunctionality.value == "value"
assert _SpecificFunctionality.value == "another_value"
assert Server().functionality.get_result() == ("name", "another_value")
class Email():
def __init__(self, store_number):
self.store_number = store_number
def amethod(self):
pass
What is the correct way to pass variables from a sub-class to a parent-class?
should I do:
class MoreSpecificEmail():
def __init__(self, store_number):
Email.__init__(self, store_number=store_number)
def another_method(self):
pass
or:
class MoreSpecificEmail():
def __init__(self, store_number):
self.store_number = store_number
Email.__init__(self, store_number=self.store_number)
I have just been using different abbreviations of store_number in each sub-class to help clarify what's going on in my head. I am sure that is the wrong way, though.
What you currently have isn't inheritance; neither of your classes actually inherits from anything! Firstly, Email should be a "new-style class", inheriting from object:
class Email(object):
# ^ note inheritance from object
def __init__(self, store_number):
self.store_number = store_number
def amethod(self):
pass
Then MoreSpecificEmail should inherit from Email - as it doesn't have any additional instantiation parameters, it can just use the inherited __init__ and doesn't need to define its own:
class MoreSpecificEmail(Email):
# ^ note inheritance from Email
# note no need to define __init__
def another_method(self):
pass
For an example where there are additional __init__ parameters, note that you should use super and rely on the superclass's __init__ to assign the parameters it takes - you only need to assign the attributes that don't get handled by the superclass:
class MoreSpecificEmail(Email):
def __init__(self, store_number, something_else):
super(MoreSpecificEmail, self).__init__(store_number)
# ^ pass it straight on
self.something_else = something_else
def another_method(self):
pass
For more information, see the Python class tutorial.
I'd like to automatically run some code upon class creation that can call other class methods. I have not found a way of doing so from within the class declaration itself and end up creating a #classmethod called __clsinit__ and call it from the defining scope immediately after the class declaration. Is there a method I can define such that it will get automatically called after the class object is created?
You can do this with a metaclass or a class decorator.
A class decorator (since 2.6) is probably easier to understand:
def call_clsinit(cls):
cls._clsinit()
return cls
#call_clsinit
class MyClass:
#classmethod
def _clsinit(cls):
print "MyClass._clsinit()"
Metaclasses are more powerful; they can call code and modify the ingredients of the class before it is created as well as afterwards (also, they can be inherited):
def call_clsinit(*args, **kwargs):
cls = type(*args, **kwargs)
cls._clsinit()
return cls;
class MyClass(object):
__metaclass__ = call_clsinit
#classmethod
def _clsinit(cls):
print "MyClass._clsinit()"
I'm trying to provide framework which allows people to write their own plugins. These plugins are basically derived classes. My base class needs some variables to initialize, how can I initialize my base class without having to let my derived class feed the variable in the base class initialization?
#!/bin/python
class BaseClass():
def __init__(self,config):
self.config=config
def showConfig(self):
print "I am using %s" % self.config
class UserPlugin(BaseClass):
def __init__(self,config):
BaseClass.__init__(self,config)
def doSomething(self):
print "Something"
fubar = UserPlugin('/tmp/config.cfg')
fubar.showConfig()
My goal is to avoid the need to define the config parameter in the UserPlugin class, since this is something I don't want the user who writes a plugin to be bothered with.
You can use argument lists to pass any remaining arguments to the base class:
class UserPlugin(BaseClass):
def __init__(self, *args, **kwargs):
BaseClass.__init__(self, *args, **kwargs)
Based on your Pastebin code, how about this? This avoids using a separate global, instead using a class attribute, which is accessible as a member to all derived classes and their instances.
#!/bin/python
class BaseClass():
config = '/tmp/config.cfg'
def __init__(self):
pass
def showConfig(self):
print "I am using %s" % self.config
class UserPlugin(BaseClass):
def __init__(self):
BaseClass.__init__(self)
def doSomething(self):
print "Something"
fubar = UserPlugin()
fubar.showConfig()
This was the other way to do it that I mentioned before. Keep in mind that if you want to change the value of BaseClass.config itself, you should access it directly (i.e. BaseClass.config = '/foo/path'; otherwise, you wind up creating a custom UPinstance.config value, leaving BaseClass.config unchanged.