Why the unexpected naming of nested classes in Python? How to "fix"?

Question

Please consider the following short Python 2.x script:

#!/usr/bin/env python
class A(object):
    class B(object):
        class C(object):
            pass
        def __init__(self):
            self.c = A.B.C()
    def __init__(self):
        self.b = A.B()

def main():
    a = A()
    print "%s: %r" % (type(a).__name__, type(a))
    print "%s: %r" % (type(a.b).__name__, type(a.b))
    print "%s: %r" % (type(a.b.c).__name__, type(a.b.c))

if __name__ == "__main__":
    main()

The output of which, when run in Python 2.7.6, is:

A: <class '__main__.A'>
B: <class '__main__.B'>
C: <class '__main__.C'>

I was expecting a different output here. Something more along the lines of:

A: <class '__main__.A'>
A.B: <class '__main__.A.B'>
A.B.C: <class '__main__.A.B.C'>

In particular I expected to see the same qualified name that I have to give to instantiate A.B and A.B.C classes respectively.

Could anyone shed any light on why those new type classes identify themselves as rooted in __main__ instead of how they were nested in the code?

Also: is there a way to fix this by naming the nested classes explicitly, such that they will identify themselves as A.B and A.B.C respectively (or possibly in the type representation as __main__.A.B and __main__.A.B.C respectively)?

@vaultah: thanks. Much appreciated. So there is no remedy in Python 2.x? — 0xC0000022L
– 0xC0000022L, Commented Nov 30, 2014 at 15:38

vaultah · Accepted Answer · 2014-11-30 19:27:56Z

2

Qualified names for classes were introduced in Python 3.3 (see PEP 3155) and AFAIK there's no non-hacky way to do this on Python 2.

edited Nov 30, 2014 at 19:27

answered Nov 30, 2014 at 15:46

vaultah

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Comments

0xC0000022L · Accepted Answer · 2014-11-30 17:28:45Z

In order to allow me to get the name of a class I introduced the following functions to introduce the PEP 3155 attribute __qualname__:

def fixup_module_class_names(modname=__name__):
    """
    Fixes the class types of the given module to have an attribute __qualname__
    as stipulated by PEP 3155.
    """
    def fixup_classtype_members(obj, prefix=None):
        """
        Fixes the __qualname__ attribute of the object as well as its contained
        member classes recursively.
        """
        from inspect import getmembers, isclass
        for n, t in getmembers(obj, isclass):
            if (t.__module__ != modname) or n == "__class__":
                continue
            setattr(t, "__qualname__", n if prefix is None else "%s.%s" % (prefix, n))
            fixup_classtype_members(t, getattr(t, "__qualname__"))
    fixup_classtype_members(sys.modules[modname])

Just call it as fixup_module_class_names() or by passing the module name whose class type members you want to have with __qualname___ attributes.

In order to account for the missing __qualname__ attributes on other class types I am then using typename(x) instead of type(x).__name__ where typename is defined as:

def typename(clsobj):
    "Retrieve the qualified name from an object or the name from anything else."
    return getattr(type(clsobj), "__qualname__", None) or getattr(type(clsobj), "__name__")

Note: If you, distinguished reader, find anything wrong with this approach for Python 2.x, please leave a comment or simply edit and fix the code as appropriate.

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