How to automate accessor definition?

Steven D'Aprano steve at
Sun Mar 21 19:18:57 CET 2010

On Sun, 21 Mar 2010 16:57:40 +0000, kj wrote:

> Just accessing attributes looks a bit dangerous to me, due to bugs like
> typing
>   i.typo = 'foo'
> when what you meant is
>   i.type = 'foo'

That's the price you pay for using a dynamic language like Python with no 
declarations. But honestly, the price isn't very high, particularly if 
you use an editor or IDE with auto-completion. I can't think of the last 
time I had an error due to the above sort of mistake.

Besides, is that error really so much more likely than this?

    i.type = 'fpo'

when you meant 'foo'? The compiler can't protect you from that error, not 
in any language.

> I tried fixing this by mucking with __setattr__, but I didn't hit on a
> satisfactory solution (basically, I couldn't find a good,
> self-maintaining, way to specify the attributes that were OK to set from
> those that weren't).  Is there anything built-in?


You could abuse __slots__, but it really is abuse: __slots__ are a memory 
optimization, not a typo-checker.

In Python 3.x, you can (untested) replace the class __dict__ with a 
custom type that has more smarts. At the cost of performance. This 
doesn't work in 2.x though, as the class __dict__ is always a regular 

Something like this might work, at some minor cost of performance:

# Untested
def __setattr__(self, name, value):
    if hasattr(self, name):
        super(MyClassName, self).__setattr__(name, value)
        raise TypeError('cannot create new attributes')

Then, in your __init__ method, to initialise an attribute use:

    self.__dict__['attr'] = value

to bypass the setattr.

Or you can use something like PyChecker or PyLint to analyse your code 
and warm about likely typos.

But really, it's not a common form of error. YMMV.

> Regarding properties, is there a built-in way to memoize them? For
> example, suppose that the value of a property is obtained by parsing the
> contents of a file (specified in another instance attribute). It would
> make no sense to do this parsing more than once.  Is there a standard
> idiom for memoizing the value once it is determined for the first time?

Google for "Python memoization cookbook". This will get you started:

Then just apply the memoize decorator to the property getter.


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