Re: Django strange behaviour

[monoBOT]

Lists are inmutable!!!! You are modifiying the same list everytime you invoque that view

El 07/11/2014 10:08, "termopro" <term...@gmail.com> escribió:

I have a view in Django which calls external library/class. The problem is that for some reason Django keeps caching results coming from previous calls of that class.

Please consider the following simple example:

Django view:

from some_path import Demo
def test_view(request):
    demo = Demo()
    result = demo.do_something()
    return render(request, 'test.html',
                            { 'result':result }
                )

Demo class:

class Demo():
    result = []

    def do_something(self):
        self.result.append(1)
        self.result.append(2)
        self.result.append(3)
        return self.result

You expect result to be [1, 2, 3], right ? WRONG!

The first time a page is loaded you'll get the correct result. But on all following requests it will keep incrementing: [1, 2, 3, 1, 2, 3]... [1, 2, 3, 1, 2, 3, 1, 2, 3] ...

So my question is obvious - what is going on here ? How do i receive [1, 2, 3] every time i call a class inside Django view ?

p.s. - Django 1.7 / MacOSx

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[Aliane Abdelouahab]

you are calling the function twice, appending every element, and then appending the whole list!

[Aliane Abdelouahab]

here is what is gives in the console:

class Demo():

    result = []

    def do_something(self):

        self.result.append(1)

        self.result.append(2)

        self.result.append(3)

        return self.result

    

demo = Demo()

result = demo.do_something()

result

[1, 2, 3]

demo = Demo()

result = demo.do_something()

demo = Demo()

result

[Rafał Pitoń]

This happens because "result" is class level attribute, not instance one, and its expected behaviour in Python.

To made result instance attribute, define it in your __init__:

class Demo():

    def __init__(self):
        self.result = []

    def do_something(self):
        self.result.append(1)
        self.result.append(2)
        self.result.append(3)
        return self.result

[Scot Hacker]

On Saturday, November 8, 2014 9:07:04 AM UTC-8, monoBOT monoBOT wrote:

Lists are inmutable!!!!

Lists are absolutely mutable in Python - hence the availability of `result.append()`. 

Perhaps you're thinking of tuples, which are not mutable and have no methods for alteration. 

./s