python: nested classes: access outer class class member

Question

This does not work:

class A:
  a1 = 4

  class B:
    b1 = A.a1  # Fails
    b2 = 6

  class C:
    c1 = A.B.b2  # Fails

Any non cryptic way to solve it? I know I could take B and C out from A but I would like to keep them embedded. I also think it would be easier with no class members as they could easily passed to nested class as argument in constructor, but being all of them class members I do not know how to do some similar thing here. I have also read on some thread that this usage remembers using classes as namespaces and that should be solved using modules, not classes, but above classes are real classes for me (I construct instances) with class data in adition that I would like to share among them.

The "easy" way out is a parameter class that all of A, B, and C inherit from. — dhke
– dhke, Commented Sep 16, 2016 at 18:01
Not sure what you say. Adding another level of inheritance on top? — user2761220
– user2761220, Commented Sep 16, 2016 at 18:09
See below. Basically have A(Param) and either also B(Param) or pull in the class variables from Param instead of A if you need different names for the parameters. — dhke
– dhke, Commented Sep 16, 2016 at 18:27
B and C are in some cases namespaces for grouping static symbols used in A that I placed in classes inside A for hierarchical code organization in group of symbols (like enums, options). Some groups have symbols that references symbols in other group. — user2761220
– user2761220, Commented Sep 16, 2016 at 20:23

Robᵩ · Accepted Answer · 2016-09-16 18:08:19Z

3

You might defer the definition of B until A is fully defined, and defer C until both A and A.B are defined.

class A:
  a1 = 4

class B:
  b1 = A.a1
  b2 = 6
A.B = B
del B

class C:
  c1 = A.B.b2
A.C = C
del C

assert A.B.b1 == 4
assert A.C.c1 == 6

Alternatively, you could define B.b1 outside of B's definition:

class A:
  a1 = 4

  class B:
    pass
  B.b1 = a1
  B.b2 = 6

  class C:
    pass
  C.c1 = B.b2

assert A.B.b1 == 4
assert A.C.c1 == 6

answered Sep 16, 2016 at 18:08

Robᵩ

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Comments

Anthon · Accepted Answer · 2016-09-16 18:02:46Z

1

This fails for two different reasons. One is that A is not ready when you try to access A.a1 in B giving you NameError.

If you solve this using a subclass. The following will work:

class A:
  a1 = 4

class _A(A):

  class B:
    b1 = A.a1  # Fails
    b2 = 6

However accessing A.B.b2 in C will still not work as A has no attribute B. You will get an AttributeError on that.

answered Sep 16, 2016 at 18:02

Anthon

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Comments

dhke · Accepted Answer · 2016-09-16 18:43:36Z

1

One trick is to put the common parameters into a parameter class and inherit from that:

class Params:
   p = 4

class A(Params):
    # A has p
    class B(Params):
        # B has p
        pass

    class C(Params):
        # C has p
        pass

Or, if you need the params with different names in the inner classes:

class A(Params):
    # A has p
    class B:
        b = Params.p

    class C:
        c = Params.p

This avoids having to monkey-patch the class after creation.

edited Sep 16, 2016 at 18:43

answered Sep 16, 2016 at 18:24

dhke

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