访问超类中的属性

Question

I am building up a tree and a dictionary mapping nodes in the tree to unique ids. When trying to access objects in the dictionary I am experiencing some unexpected behaviour. I am able to access some inherited attributes of the object, but not other. I've extracted part of the project and modified it such that it is hopefully understandable:

#!/usr/bin/env python

IMPORTS = vars()

class Node(object):
    SYMTAB = {}
    def __init__(self, kwargs={}):
        self.ati = kwargs.get(u'@i')
        self._add_symbol(self.ati, self)
        self.atr = kwargs.get(u'@r')

    def _add_symbol(self, k, v):
        self.SYMTAB[k] = v

class CompilationUnit(Node):
    def __init__(self, kwargs={}):
        super(CompilationUnit, self).__init__(kwargs)
        self.types = map(lambda x: IMPORTS[x['@t']](x),
                      kwargs.get('types').get('@e', []))

class BodyDeclaration(Node):
    def __init__(self, kwargs={}):
        super(BodyDeclaration, self).__init__(kwargs)

class TypeDeclaration(BodyDeclaration):
    def __init__(self, kwargs={}):
        super(TypeDeclaration, self).__init__(kwargs)
        self.members = map(lambda x: IMPORTS[x[u'@t']](x),
                        kwargs.get(u'members').get(u'@e', []))

class ClassOrInterfaceDeclaration(TypeDeclaration):
    def __init__(self, kwargs={}):
        super(ClassOrInterfaceDeclaration, self).__init__(kwargs)

class FieldDeclaration(BodyDeclaration):
    def __init__(self, kwargs={}):
        super(FieldDeclaration, self).__init__(kwargs)
        print '*'*10, 'SYMTAB:'
        for k,v in self.SYMTAB.items():
            print k,v
        print '*'*10
        print 'SYMTAB[self.atr]:',self.SYMTAB[self.atr]
        print self.SYMTAB[self.atr].atr
        print self.SYMTAB[self.atr].members

d = {u'@i': 0, u'@r': None, u'@t': u'CompilationUnit', 'types':
     {u'@e':
      [{u'@t': u'ClassOrInterfaceDeclaration', u'@i': 1, u'@r': 0,
        u'members':
        {u'@e':
         [{u'@t': 'FieldDeclaration', u'@i': 2, u'@r': 1}]}}]}}

c = CompilationUnit(d)
print c

This will produce the following output:

********** SYMTAB:
0 <__main__.CompilationUnit object at 0x105466f10>
1 <__main__.ClassOrInterfaceDeclaration object at 0x10547c050>
2 <__main__.FieldDeclaration object at 0x10547c150>
**********
SYMTAB[self.atr]: <__main__.ClassOrInterfaceDeclaration object at 0x10547c050>
0
Traceback (most recent call last):
  File "class_error.py", line 74, in <module>
    c = CompilationUnit(d)
  File "class_error.py", line 30, in __init__
    kwargs.get('types').get('@e', []))
  File "class_error.py", line 29, in <lambda>
    self._types = map(lambda x: IMPORTS[x['@t']](x),
  File "class_error.py", line 54, in __init__
    super(ClassOrInterfaceDeclaration, self).__init__(kwargs)
  File "class_error.py", line 45, in __init__
    kwargs.get(u'members').get(u'@e', []))
  File "class_error.py", line 44, in <lambda>
    self._members = map(lambda x: IMPORTS[x[u'@t']](x),
  File "class_error.py", line 65, in __init__
    print self.SYMTAB[self.atr].members
AttributeError: 'ClassOrInterfaceDeclaration' object has no attribute 'members'

I'm not even sure where to begin trying to fix this. Recently I added the print self.SYMTAB[self.atr].atr line and saw that this actually worked. The only thing I can think of is that FieldDeclaration doesn't inherit from TypeDeclaration , which is where the members attribute is actually defined. But why should this matter? I am accessing the ClassOrInterfaceDeclaration node, which does inherit from TypeDeclaration ? This object should have a members attribute. What am I missing about how to access this attribute?

Answer 1

Problem

There seem to multiple other problems in your program but here:

class TypeDeclaration(BodyDeclaration):
    def __init__(self, kwargs={}):
        super(TypeDeclaration, self).__init__(kwargs)
        self.members = map(lambda x: IMPORTS[x[u'@t']](x),
                        kwargs.get(u'members').get(u'@e', []))

the part:

kwargs.get(u'members')

tries to access self.members which you are just about to create.

You put u'@t': u'ClassOrInterfaceDeclaration' into d . Then in here:

        self.members = map(lambda x: IMPORTS[x[u'@t']](x),
                        kwargs.get(u'members').get(u'@e', []))

You try access ClassOrInterfaceDeclaration().members , which you are just about to create. Instanziation of ClassOrInterfaceDeclaration calls the __init__() of TypeDeclaration that contains the cod above.

Solution

You need to let all __init__() functions to finish before trying to access members. For example, you can move all your printing into a own method:

class FieldDeclaration(BodyDeclaration):
    def __init__(self, kwargs={}):
        super(FieldDeclaration, self).__init__(kwargs)

    def meth(self):
        print '*'*10, 'SYMTAB:'
        for k,v in self.SYMTAB.items():
            print k,v
        print '*'*10
        print 'SYMTAB[self.atr]:',self.SYMTAB[self.atr]
        print self.SYMTAB[self.atr].atr
        print self.SYMTAB[self.atr].members


d = {u'@i': 0, u'@r': None, u'@t': u'CompilationUnit', 'types':
     {u'@e':
      [{u'@t': u'ClassOrInterfaceDeclaration', u'@i': 1, u'@r': 0,
        u'members':
        {u'@e':
         [{u'@t': 'FieldDeclaration', u'@i': 2, u'@r': 1}]}}]}}

c = CompilationUnit(d)
print c
c.SYMTAB[2].meth()

Output:

<__main__.CompilationUnit object at 0x104ef9510>
********** SYMTAB:
0 <__main__.CompilationUnit object at 0x104ef9510>
1 <__main__.ClassOrInterfaceDeclaration object at 0x104ef9610>
2 <__main__.FieldDeclaration object at 0x104ef9710>
**********
SYMTAB[self.atr]: <__main__.ClassOrInterfaceDeclaration object at 0x104ef9610>
0
[<__main__.FieldDeclaration object at 0x104ef9710>]