How to access private variable of Python module from class

Question

In Python 3, prefixing a class variable makes it private my 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..

score 1 · Accepted Answer · edited Jan 07 '16 at 16:29

1

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 07 '16 at 16:29

Mad Physicist

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answered Jan 07 '16 at 15:37

Padraic Cunningham

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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. – Mad Physicist Jan 07 '16 at 15:45
1

@MadPhysicist, I added a couple of other ways to achieve the same, I cannot see any other way to avoid the name mangling – Padraic Cunningham Jan 07 '16 at 15:54
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. – Mad Physicist Jan 07 '16 at 16:00
@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. – Padraic Cunningham Jan 07 '16 at 16:14
I liked your other solutions (for completeness). Mind if I revert the deletion? – Mad Physicist Jan 07 '16 at 16:24
Signed up for python-list and posted there. Will post here if I get a better response there. – Mad Physicist Jan 07 '16 at 16:25
@MadPhysicist, work away, I meant to ask, I presume taking the arg is not possible? – Padraic Cunningham Jan 07 '16 at 16:26
I have already worked around the specific problem that this question represented (by trimming one of the underscores :)), but it bothers me that this specific thing can't be done directly in Python. – Mad Physicist Jan 07 '16 at 16:27
@MadPhysicist, it does seem to be bordering on a bug, I suppose if you create pass in into the class at initialisation `def __init__(self, somevar):` or to other methods it works fine but it is weird that you cannot even do something like `print(__a)` in the class – Padraic Cunningham Jan 07 '16 at 16:34
1

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 Jan 07 '16 at 16:36
Even doing `m = sys.modules[__name__]`; `self.a = m.__a` give `'module' object has no attribute '_B__a'` so the mangling seems to happen first – Padraic Cunningham Jan 07 '16 at 16:53
Well, I'm going to go ahead and submit a bug report. – Mad Physicist Jan 07 '16 at 19:13
I got a response to my bug report, which was closed almost immediately. I posted it as an answer here just for reference. I will keep your answer selected because it is better than the "real" answer. – Mad Physicist Jan 08 '16 at 11:08
@MadPhysicist. I had a feeling the might be the answer you would get, thanks for the letting me know. – Padraic Cunningham Jan 08 '16 at 11:11

score 1 · Answer 2 · answered Jan 08 '16 at 11:07

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.

score -2 · Answer 3 · edited Jan 06 '16 at 01:10

-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 06 '16 at 01:10

Mad Physicist

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answered Jan 05 '16 at 21:35

Wayne Morris

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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. – Mad Physicist Jan 06 '16 at 01:09

score -2 · Answer 4 · answered Jan 07 '16 at 15:24

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.

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. — Mad Physicist, Jan 07 '16 at 15:44

How to access private variable of Python module from class

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