Python virtual method without crapping names

Question

sample code what i am talking about

class A:
  def __init__(self):
    self.x = 5
  def virtual__getX(self):
    return self.x
  def getY(self):
    return self.virtual__getX()+2

class B(A):
  def __init__(self):
    A.__init__(self)
  def virtual__getX(self):
    return 20

a=A()
b=B()

print(a.getY())
print(b.getY())

is there a way in python 3 to have real virtual method without this really bad name "virtual__getX" i need clear names "__getX"

Python 3.5.2 (default, Dec 2015, 13:05:11)
[GCC 4.8.2] on linux

7
22

if i remove virtual from name the result is

Python 3.5.2 (default, Dec 2015, 13:05:11)
[GCC 4.8.2] on linux

7
7

is there better way to write real virtual methods like in "normal language" like delphi

Wait, so you want to inherit from another class and don't have inheritance? What are you actually trying to achieve? It seems like you need to use properties. https://docs.python.org/3/library/functions.html#property — Andrey Shipilov, Apr 24 '17 at 07:08
properties are usefull but not in this case, i need to have something like protected member, inheritance is important but from INSTANCE user should not call method, but question is answered - problem was in double underscore which is used by python internally — Livius, Apr 24 '17 at 08:38

score 1 · Accepted Answer · answered Apr 24 '17 at 07:11

1

You can't have a virtual function named __something. Attributes starting with a double underscore are considered private, and python internally applies name-mangling to them.

You can, however, rename virtual__getX to getX and it'll work as you expect.

answered Apr 24 '17 at 07:11

Aran-Fey

39,665
11
104
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really amaizing that double underscore have some "strange" behavior here. Is there a way to leve underscore and still have possibility to do this - something like @property decorator? – Livius Apr 24 '17 at 07:17
@Livius You can add another 2 backslashes at the end: `__getX__`. Names like that are usually used by python's magic methods though, so it isn't considered good practice. I don't know why you want to have getter functions, but in python we usually use properties instead. (Named `x`, not `getX` or anything else.) – Aran-Fey Apr 24 '17 at 07:21
i prefix all methods that do much more then only read property value with "get" but for internal class usage i prefix method by __ to "say" users do not call it from instance - but i see that if i need inheritance of that method then there is no good way to do this in python. In delphi i simple write "; virtual" after method and name dose not metter – Livius Apr 24 '17 at 07:25
1

@Livius why not just use a single underscore: `_getX` – juanpa.arrivillaga Apr 24 '17 at 07:29
good point but now i'm worry if in python are more magic with single underscore? – Livius Apr 24 '17 at 07:34
@Livius No, a single underscore doesn't do anything. – Aran-Fey Apr 24 '17 at 07:35
@Livius No, single underscore has no magic. Double-underscore specifically does *name-mangling*. – juanpa.arrivillaga Apr 24 '17 at 07:35
@Livius that being said, you shouldn't use getters and setters in Python. You should use properties *and that only when you need them*. That is the beauty of Python. – juanpa.arrivillaga Apr 24 '17 at 07:36

Python virtual method without crapping names

1 Answers1