如何动态地向类添加属性？

Question

I have many similar classes, they all contain their own class attributes:

class MyClass1(): 
    Context = ClassId1+"CONTEXTSTR"
    SubContext = ClassId1+"SUBCONTEXTSTR"
    UpVal = ClassID+"UPVAL"
    DoenVal = ClassID1+"DOWNVAL"
class MyClass2(): 
    Context = ClassId2+"CONTEXTSTR"
    SubContext = ClassId2+"SUBCONTEXTSTR"
    UpVal = ClassID2+"UPVAL"
    DoenVal = ClassID2+"DOWNVAL"
...

but all this soon becomes annoying and requires a lot of code repetition (error-prone).

I would like to be able to manipulate a sort of class_variable and to do something like:

class MyClass1():
    self_cls.MakeParameters(ClassId1)

even better if I could use inheritance and pass parameters to classes to do things like:

ClassID1 = "test01"    

class MyClass1(BaseClass,<ClassID1>):
    pass

print MyClass1.CONTEXT

obtaining as output "test01CONTEXTSTR" the code. How to do set the arguments of classes according to a given "template" that takes a parameter?

Answer 1

>>> class MyClass:
    def __init__(self, classid):
        self.context = "%s%s" % (classid, "CONTEXTSTR")


>>> somevar = MyClass("someid")
>>> print somevar.context
someidCONTEXTSTR
>>> 

Now if you want to inherit a class, it's a bit different. Continuning from above:

>>> class NewClass(MyClass):
    def __init__(self, classid, secondid):
        MyClass.__init__(self, classid)
        self.secondcontext = "%s_%s" % (secondid, "SECONDCONTEXT")


>>> secondvar = NewClass("someid", "someotherid")
>>> secondvar.context
'someidCONTEXTSTR'
>>> secondvar.secondcontext
'someotherid_SECONDCONTEXT'
>>> 

To auto-set the context (Which I think you're asking?), use keyword arguments:

>>> class NewClass(MyClass):
    def __init__(self, newclassid="newclassid", myclassid="myclassid"):
        MyClass.__init__(self, myclassid)
        self.newcontext = "%s%s" % (newclassid, " new context")


>>> NewClass().context
'myclassidCONTEXTSTR'
>>> NewClass().newcontext
'newclassid new context'
>>> NewClass(newclassid="only set this ones id").context
'myclassidCONTEXTSTR'
>>> NewClass(newclassid="only set this ones id").newcontext
'only set this ones id new context'
>>> 

Here I didn't assign the class, I just called it (Hence the brackets) and the keywords filled it in for me.

I think this is what you mean?

You can also set it as a keyword for MyClass , and it'll auto-assign if the NewClass doesn't set the variable. I don't think you'd need a code example for that though, but ask if you do.

I think this is what you're after, if not you'll need to clarify a bit more sorry.

Answer 2

Of course, you could add the attributes manually to the class like this:

def addToClass(cls, ClassId):
    cls.Context = ClassId1+"CONTEXTSTR"
    cls.SubContext = ClassId1+"SUBCONTEXTSTR"
    cls.UpVal = ClassID+"UPVAL"
    cls.DoenVal = ClassID1+"DOWNVAL"

class NewClass(MyClass):
    ...

Usage:

addToClass(NewClass, "someid")

But if you think that this is still too 'manual' and you would expect that a great language like Python should provide more and better, then you are right: metaclasses

Use metaclasses

You can achieve the wanted behavior using metaclasses (If you don't know what I am talking about I recommend reading the great answer posted in this issue: What is a metaclass in Python? )

Write a Metaclass factory method:

def getMetaClass(classId):
    class MyMetaClass(type):
        def __new__(cls, name, bases, dct):
            dct["Context"] =  "%sCONTEXTSTR" % classId
            dct["SubContext"] = "%sSUBCONTEXTSTR" % classId
            dct["UpVal"] = "%sUPVAL" % classId
            dct["DownVal"] = "%sDOWNVAL" % classId
            return super(MyMetaClass, cls).__new__(cls, name, bases, dct)
    return MyMetaClass

Define a class based on your Metaclass:

class MyClass1():
    __metaclass__ = getMetaClass("test01")

Use your class:

>>> A.Context
'test01CONTEXTSTR'

Update : If you don't like having this __metaclass__ in every of your classes, you could hide it in a superclass like this (proposed in comment by Anders):

Metaclass:

class MyMetaClass(type):
    def __new__(cls, name, bases, dct):
        classId = dct.get("CLASSID", "noClassId")
        dct["Context"] =  "%sCONTEXTSTR" % classId
        dct["SubContext"] = "%sSUBCONTEXTSTR" % classId
        dct["UpVal"] = "%sUPVAL" % classId
        dct["DownVal"] = "%sDOWNVAL" % classId
        return super(MyMetaClass, cls).__new__(cls, name, bases, dct)

Class hiding Meta logic:

class AutoContext:
    __metaclass__ = getMetaClass()

Usage:

class MyClass1(AutoContext):
    CLASSID = "test01"