Making functions callable

[Anthony Hawkes]

Hi Guys,

I'm running into a problem with django(I guess this would also affect Python in general) where if I create a view eg

def view(year=today.year())

The year is never re-evaluated until the server is reloaded/restarted

I'm trying to figure out how to make a callable method accessible as a property if this is even possible to try and rectify this(not sure if this is even the correct approach). I've had a look at some magic methods but can't figure it out.

Basically I want to make eg

def year(something):

    return 'blah'

Accessible using object.year as well as object.year()

Any pointers/ideas etc?

[John DeRosa]

For the default value to work as you expect, do this:

def view(request, year=None):

    if year is None:

        year = today.year()

Kwarg defaults are evaluated when the module is interpreted for the first time.

John

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[Anthony]

Thanks John,

I've yet to test it but I read a few people stating to use lambdas variables/properties. Is it definitely the case that it will only evaluate once?

Either way I've embarked on the mission to make methods return as properties and have found @property decorator to fit my needs, the issue was I was calling Object.property not ObjectI().property, However I want to apply this functionality to all my methods and am using:

__getattribute__

    def __getattribute__(self, item):

        super(DateCalc, self).__getattribute__(item)

But I've run into the problem that my __init__ properties don't seem to be initialised eg I get

self._date_actual = datetime.strptime("{Y}-{m}-{d}".format(Y=year, m=month, d=day), self._format_global).date() TypeError: must be string, not None

Anyone have an idea about what's going on?

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Anthony Hawkes
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Ph: 0400 372 260

[Mike Dewhirst]

On 27/03/2014 7:57am, Anthony Hawkes wrote:
> Hi Guys,
>
> I'm running into a problem with django(I guess this would also affect
> Python in general) where if I create a view eg
> def view(year=today.year())
> The year is never re-evaluated until the server is reloaded/restarted
>
> I'm trying to figure out how to make a callable method accessible as a
> property if this is even possible to try and rectify this(not sure if
> this is even the correct approach). I've had a look at some magic
> methods but can't figure it out.
>
> Basically I want to make eg
>
> def year(something):
> return 'blah'

You should be able to pass the callable as an argument. It then gets
evaluated each time it is called ...

Here is one I use ... from time to time :)

def when():
return datetime.now(tz=pytz.utc)

>
> Accessible using object.year as well as object.year()
>
> Any pointers/ideas etc?
>

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[John DeRosa]

Not sure what is going on without seeing the code. But something is None when it should be a string, as the exception is telling you.

It’s often easiest to try out problem code interactively in Python, and zero in on the problem.

John

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[Javier Guerra Giraldez]

On Wed, Mar 26, 2014 at 5:53 PM, Anthony <lifesi...@gmail.com> wrote:
> I've yet to test it but I read a few people stating to use lambdas
> variables/properties. Is it definitely the case that it will only evaluate
> once?

from the python docs (2.7):

Default parameter values are evaluated when the function definition is
executed. This means that the expression is evaluated once, when the
function is defined, and that the same “pre-computed” value is used
for each call. This is especially important to understand when a
default parameter is a mutable object, such as a list or a dictionary:
if the function modifies the object (e.g. by appending an item to a
list), the default value is in effect modified. This is generally not
what was intended. A way around this is to use None as the default,
and explicitly test for it in the body of the function.

--
Javier

[Anthony Hawkes]

Thanks Guys, especially Javier! I've spent so much time trawling Google for this info!