oop - Python - typechecking OK when error wouldn't be thrown otherwise? -

i have python classes that, if simplified, like:

class base:     def __init__(self, v):         self.value = v      def dothings(self):         print "doing things."      def domorethings(self):         print "doing more things."      def combine(self, b):         self.value += b.value  class foo(base):     def showvalue(self):         print "foo value %d." % self.value  class bar(base):     def showvalue(self):         print "bar value %d." % self.value 

the base class contains methods (represented above dothings , domorethings) implement functionality common both foo , bar subclasses. foo , bar subclasses differ in how interpret value field. (above, differ show when printing it, in actual application, several other things more complicated.) base can thought of "abstract": users ever work foos , bars. base exists home code common subclasses.

the method want ask combine, lets take 2 of these objects , make third. because foo , bar interpret value differently, doesn't make sense combine 2 subclasses of different types: can combine 2 foos foo or 2 bars bar not foo , bar. so, procedure combine same subclasses, makes sense have factored out , defined in 1 place.

i signal error if user tries combine 2 incompatible objects, don't see way without introducing ugly typechecks. practice this? or should usual thing , not check, document issue, , assume user won't try use combine in way wasn't intended, though such use appear "succeed" , return garbage object instead of raising error?

thank help.

i see several approaches here:

1. don't check , trust user right thing. might appropriate or dangerous, depending on situation.

2. check type. right looks ugly, easiest thing.

3. don't have value, name them way intended have (pressure, temperature) , let them combine themselves.

4. same 3, additionally have subclasses have property maps accesses .value respective "real" value variable. way, can keep .__init__(), .combine() have done every subclass.

5. same 4, don't use property, self-crafted descriptor. in this answer, show how done.