THRIFT-4623 Fix python complex nested struct includes

[thomasbruggink]

Fix the bug where the following:

struct C {
    1: i32 value
}

struct A {
    1: C nested = {}
}

struct B {
    1: A itm = {}
}

would generate the following invalid python code:

class C:
    thrift_spec = None

    def __init__(self, value=None):
        self.value = value

class A:
    thrift_spec = None

    def __init__(self, nested=C()):
        if nested is self.thrift_spec[1][4]:
            nested = C()
        self.nested = nested

class B:
    thrift_spec = None

    def __init__(self, itm=A()):
        if itm is self.thrift_spec[1][4]:
            itm = A()
        self.itm = itm

The problem with the above code is that self.thrift_spec was initialized and set after the module was loaded. In python this would work when B is instantiated but since B immediately invokes A() it happens before the thrift_spec is set and this would result in TypeError: 'NoneType' object is not subscriptable. By first initializing a temporary placeholder with a unique address to compare to called _THRIFT_DEFAULT the generated code would look like:

_THRIFT_DEFAULT = object()

class C:
    def __init__(self, value=None):
        self.value = value

class A:
    def __init__(self, nested=_THRIFT_DEFAULT):
        if nested is _THRIFT_DEFAULT:
            nested = C()

class B:
    def __init__(self, itm=_THRIFT_DEFAULT):
        if itm is _THRIFT_DEFAULT:
            itm = A()

Here the instantiation of the default object is defered until after the module has been fully loaded resulting in the correct value being assigned.

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