How to handle python stray attribute assignments

Question

Sometimes I need to change the name of a Python class attribute that is being already extensively used in the rest of the code.

The problem is that for various reasons I may miss some places where the old name appears in an assignment statement such as class_instance.old_attribute_name = something. For example:

class theClass:
    def __init__(self):
        self.new_attribute_name = 0

o = theClass()
o.new_attribute_name += 3
if o.new_attribute_name > 3 or o.new_attribute_name < 4:
    o.old_attribute_name = 7
do_stuff_with(o.new_attribute_name)

The problem is that this can be difficult to spot, will rise no error and just create a useless attribute. I considered using __slot__, but feels like forcing my deep-seated C/C++ mindset on Python.

So I would like to ask, what am I doing wrong here? What will I lose if I try (somehow) to prevent attribute creation outside of instantiation? What is the pythonic way to handle stray attribute assignments?

John La Rooy · Accepted Answer · 2012-06-14 00:45:51Z

2

You can make the old attribute name a property

class theClass(object):
    def __init__(self):
        self.new_attribute_name = 0

    @property
    def old_attribute_name(self):
        print "Tried to access old_attribute_name"
        return self.new_attribute_name

    @old_attribute_name.setter
    def old_attribute_name(self, value):
        print "Tried to set old_attribute_name"
        return self.new_attribute_name

edited Jun 14, 2012 at 0:45

answered Jun 14, 2012 at 0:40

John La Rooy

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2 Comments

Gareth Latty Over a year ago

I would note that I would do this (personally, I'd throw an exception rather than just printing) to debug, to ensure the old code has been dealt with, then remove it once that is done.

user355252 Over a year ago

Ideally you'd use "warnings.warn" with "DeprecationWarning" to be able control whether access to the old name throws an exception or merely prints a warning to standard output.

Community · Accepted Answer · 2017-05-23 11:49:11Z

0

Instead of trying to find all the places you need to change with search-and-replace, you could instead try a "refactoring" tool.

What refactoring tools do you use for Python?

edited May 23, 2017 at 11:49

CommunityBot

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answered Jun 14, 2012 at 0:53

steveha

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3 Comments

user355252 Over a year ago

Note that due to Python's dynamic nature refactoring tools cannot catch all possible ways of attribute access, and hence likely miss names that are used in polymorphic code, or accessed dynamically (i.e. getattr()).

John La Rooy Over a year ago

@lunaryorn, Indeed. But it doesn't make such tools entirely worthless. A great deal of Python code exists that isn't very dynamic

user355252 Over a year ago

@gnibbler Didn't say any different. Rope is a good and important tool, but just keep in mind that it might miss occurrences when refactoring. Double check the sources afterwards, and have good test coverage before starting.

Wai Yip Tung · Accepted Answer · 2012-06-14 00:56:20Z

0

You can do this with property. It will block both getting and setting of the old attribute name.

class A(object):
    @property
    def old_name(self):
        raise NotImplementedError()

answered Jun 14, 2012 at 0:56

Wai Yip Tung

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