How to import object from builtins affecting just one class?

Question

I am converting code from python2 to python3 for newstyle classes using future. My project is in Django 1.11

I have a class in forms.py as:

class Address:
    ...rest of code...

class AddressForm(Address, forms.ModelForm):
    ...rest of code...

in Python 2

which is converted to :

from buitlins import object
class Address(object):
        ...rest of code...

class AddressForm(Address, forms.ModelForm):
    ...rest of code...

in Python 3

I have a selenium test that fails when this Form is invoked after it is converted to Python3 with the following error:

File "<path_to_venv>/local/lib/python2.7/site-packages/django/utils/six.py", line 842, in <lambda>
klass.__str__ = lambda self: self.__unicode__().encode('utf-8')
File "<path_to_venv>/local/lib/python2.7/site-packages/future/types/newobject.py", line 78, in __unicode__
s = type(self).__str__(self)
RuntimeError: maximum recursion depth exceeded

However, when I remove the import from buitlins import object the test passes.

But as I have added a future check, I get a future difference error & thus every class has to be converted to newstyle. I want it to work in both Python2 and Python3.

Is there a way this module builtins module import can affect just one class and not others in the forms.py file. Or is there some other method to handle this?

I feel you get it reversed. `class Address:` should be used in Python 3 and `class Address(object)` should be used in Python2 — Mia, Aug 29 '18 at 13:37
`class Address:` and `class Address(object)` do *exactly* the same thing in Python 3, whether or not you import `object` from `builtins` explicitly. — chepner, Aug 29 '18 at 13:38
Your error comes from a Python 2.7 library, which maybe does not work properly if used from Python 3. — chepner, Aug 29 '18 at 13:39
Does one of these classes define a `next` or `__next__` method? Can we see them? — Patrick Haugh, Aug 29 '18 at 13:39
I believe You can find the answer of your question in this link : https://stackoverflow.com/questions/14222416/recursion-in-python-runtimeerror-maximum-recursion-depth-exceeded-while-callin/14222843 — amirhosein majidi, Aug 29 '18 at 13:48
@PatrickHaugh there is none of `__str__` or `__unicode__` method. But there is a unicode(variable) method in one of the form classes in the forms.py file. — Deesha, Aug 29 '18 at 13:50

Patrick Haugh · Accepted Answer · 2018-08-29T15:03:13.950

The problem you're running up against seems to be from two different Python 2 modernization tools fighting. You seem to be using the python_2_unicode_compatible decorator from django.utils.six

def python_2_unicode_compatible(klass):
    """
    A decorator that defines __unicode__ and __str__ methods under Python 2.
    Under Python 3 it does nothing.
    To support Python 2 and 3 with a single code base, define a __str__ method
    returning text and apply this decorator to the class.
    """
    if PY2:
        if '__str__' not in klass.__dict__:
            raise ValueError("@python_2_unicode_compatible cannot be applied "
                             "to %s because it doesn't define __str__()." %
                             klass.__name__)
        klass.__unicode__ = klass.__str__
        klass.__str__ = lambda self: self.__unicode__().encode('utf-8')
    return klass

and inheriting from newobject, which has this __unicode__ method

def __unicode__(self):
    # All subclasses of the builtin object should have __str__ defined.
    # Note that old-style classes do not have __str__ defined.
    if hasattr(self, '__str__'):
        s = type(self).__str__(self)
    else:
        s = str(self)
    if isinstance(s, unicode):
        return s
    else:
        return s.decode('utf-8')

And because the two have slightly different strategies for providing both __unicode__ and __str__ methods, they ed up calling each other infinitely, which leads to your recursion error.

The module that provides builtins.object provides its own python_2_unicode_compatible decorator. Have you tried using that over the one from django.utils.six?

I tried `class Address(object)`: without `from builtins import object` and the test passes. But since `newstyle` is in the list of checks in the CI CD, it needs `from builtins import object` else there is difference in futurize error. — Deesha, Aug 29 '18 at 14:40
The module that provides `builtins.object` provides its own [`python_2_unicode_compatible`](http://python-future.org/reference.html?highlight=python%20unicode%20compatible#future.utils.python_2_unicode_compatible) decorator. Have you tried using that over the one from `django.utils.six`? — Patrick Haugh, Aug 29 '18 at 14:51
Just tried this [python_2_unicode_compatible](http://python-future.org/reference.html?highlight=python%20unicode%20compatible#future.utils.python_2_unicode_compatible), the decorator worked. Thanks alot. This was really helpful :) — Deesha, Aug 29 '18 at 14:57

score 0 · Answer 2 · answered Aug 29 '18 at 13:40

0

This is the python2 way.

class Address(object):

In python3 classes inherit the object implicitly, so it should be like this;

class Address:

answered Aug 29 '18 at 13:40

ruohola

21,987
6
62
97

[old-style and new-style classes in Python 2.7?](https://stackoverflow.com/questions/13816849/old-style-and-new-style-classes-in-python-2-7) isn't it the the other way round? Moving from py2 to py3 – Deesha Aug 29 '18 at 13:47
1

In Python 2 `class Address:` is an old-style class and `class Address(object):` is a new-style class. In Python 3, all classes are new-style. For the transition, you should change all of your classes to `class Adddress(object):`. The `from builtins import object` is importing a shim that knows about Python 3 interfaces like `__str__` and `__next__` that Python 2 doesn't have. – Patrick Haugh Aug 29 '18 at 13:50
@PatrickHaugh that makes sense. But I still wonder the class that is being converted to newstyle doesn't use `__str__` and `__next__` anywhere. – Deesha Aug 29 '18 at 13:56
`object` defines a `__str__` method, so your `Address(object)` class will inherit that method. – Patrick Haugh Aug 29 '18 at 14:00

score 0 · Answer 3 · answered Aug 15 '20 at 01:29

Ran into this today, and Patrick Haugh mostly describes the problem, except that six and python_2_unicode_compatible is not referenced in django 1.11, the version in the question and the one I am using. In our case, the problem was that a django model was inheriting from a mixin, which inherited from future.builtins.newobject.

newobject (from builtins import object) adds an attribute called unicode: https://github.com/PythonCharmers/python-future/blob/master/src/future/types/newobject.py#L41
django admin has a logging feature creates a LogEntry containing a text representation of the object. https://github.com/django/django/blob/stable/1.11.x/django/contrib/admin/options.py#L741
The text representation used is object.__unicode__: https://github.com/django/django/blob/stable/1.11.x/django/utils/encoding.py#L77
__unicode__ is a wrapper around __str__ in the future package
__str__ for django db models is implemented as a wrapper around unicode in django https://github.com/django/django/blob/stable/1.11.x/django/db/models/base.py#L595

We don't have a great solution, apart from explicitly importing future as from builtins import object as future_object if we need access to both, and disabling the entire fix by running futurize --stage2 -x object instead of futurize --stage2

How to import object from builtins affecting just one class?

3 Answers3