How to access private variable of Python module from class

Question

In Python 3, prefixing a class variable makes it private by mangling the name within the class. How do I access a module variable within a class?

For example, the following two ways do not work:

__a = 3
class B:
    def __init__(self):
        self.a = __a
b = B()

results in:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in __init__
NameError: name '_B__a' is not defined

Using global does not help either:

__a = 3
class B:
    def __init__(self):
        global __a
        self.a = __a
b = B()

results in:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 4, in __init__
NameError: name '_B__a' is not defined

Running locals() shows that the variable __a exists unmangled:

>>> locals()
{'__package__': None, '__name__': '__main__',
 '__loader__': <class '_frozen_importlib.BuiltinImporter'>,
 '__doc__': None, '__a': 3, 'B': <class '__main__.B'>,
 '__builtins__': <module 'builtins' (built-in)>, '__spec__': None}

[Newlines added for legibility]

Running same code in a module (as opposed to interpreter) results in the exact same behavior. Using Anaconda's Python 3.5.1 :: Continuum Analytics, Inc..

Mad Physicist · Accepted Answer · 2016-01-08 11:07:05Z

3

Apparently the "official" answer is not to use double underscores outside of a class. This is implied in the documentation here: https://docs.python.org/2/tutorial/classes.html#private-variables-and-class-local-references. Furthermore, the following (failed) bug report (and this response) make it explicit.

answered Jan 8, 2016 at 11:07

Mad Physicist

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Comments

Mad Physicist · Accepted Answer · 2016-01-07 16:29:01Z

2

It's ugly but You could access globals:

__a = 3
class B:
    def __init__(self):
        self.a = globals()["__a"]
b = B()

You can also put it in a dict:

__a = 3

d = {"__a": __a}

class B:
    def __init__(self):
        self.a = d["__a"]
b = B()

Or a list, tuple etc.. and index:

__a = 3

l = [__a]

class B:
    def __init__(self):
        self.a = l[0]
b = B()

edited Jan 7, 2016 at 16:29

Mad Physicist

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answered Jan 7, 2016 at 15:37

Padraic Cunningham

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14 Comments

Mad Physicist Over a year ago

I agree that this is ugly, but unless someone comes up with a better solution, it is the only one that seems to work so far. Thanks. I think that I'll ask this on the Python mailing list to see if it gets any different response.

Padraic Cunningham Over a year ago

@MadPhysicist, I added a couple of other ways to achieve the same, I cannot see any other way to avoid the name mangling

Mad Physicist Over a year ago

Looks like conceptually the same thing. You are storing a reference to __a in a non-mangled container outside the class first, whether globals() is doing it for you or you are doing it manually.

Padraic Cunningham Over a year ago

@MadPhysicist, it is, I just added it as accessing it gives you the option to name the list or dict with a name that may be more explicative of what is happening but you would probably want to use a weakref or dek d after so probably not worth the effort so I removed it.

Mad Physicist Over a year ago

My thoughts exactly. Sure there are workarounds, but the way I see it, either mangling is getting too greedy or global is not doing its job right.

|

Mad Physicist · Accepted Answer · 2016-01-06 01:10:20Z

-2

You are instantiating a class by passing a variable which is not defined. putting __a outside the class will not not work as the class will not see this variable. What you should do instead is:

__a = 3
class B:
def __init__(self, __a):
   self.a = __a
b = B(__a)

This way you would have passed an argument in the constructor for initializing.

edited Jan 6, 2016 at 1:10

Mad Physicist

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answered Jan 5, 2016 at 21:35

Wayne Morris

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1 Comment

Mad Physicist Over a year ago

A) You can use normal global variables the way I am doing. This one is causing a problem because of Python 3's name mangling, therefore B) I am not looking for a workaround but an actual solution that would let me use my global variable if at all possible.

Wayne Morris · Accepted Answer · 2016-01-07 15:24:03Z

-2

If you are going to mangle the names as you are trying to do then I would refer you to this article: http://shahriar.svbtle.com/underscores-in-python

As such, my solution to what you are trying to do is as follows:`

class R:
    global _R__a
    _R__a = 3
    def __init__(self):
        pass

class B:    
    global _R__a    
    def __init__(self):     
        self.a = _R__a
b = B()
print b.a
#3`

This way, you are also more specific about the variable you are calling without much room for modifying it later. Hope this works.

answered Jan 7, 2016 at 15:24

Wayne Morris

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1 Comment

Mad Physicist Over a year ago

This does not answer my question at all. I am trying to access a module-level attribute with two underscores in the name inside a class method.

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