Class variable assignment inside a class without init

Question

Consider the following:

from module1 import foo_ext()

    class A(object):
        x = foo_ext()
        @classMethod
        def foo1():
          . . .
        @classMethod
        def foo2():
          . . .
        @classMethod
        def foo3():
          . . .

Is it true that everytime we call A.foo1() or A.foo2() x class variable is not being assigned again by a call to foo_ext(), only on the first time there is some call to "static class" A with some of its class methods :foo1, foo2, etc?

bruno desthuilliers · Accepted Answer · 2018-10-22 10:39:29Z

Is it true that everytime we call A.foo1() or A.foo2() x class variable is not being assigned again by a call to foo_ext()

Well yes of course why ???

only on the first time there is some call to "static class" A with some of its class methods :foo1, foo2, etc?

Not even. This statement - like all statements at the top-level of class block - are eval'd as part of the class statement execution, which is - assuming the most common case of a class statement at the module's top-level (=> not within a function) - when the module is imported for the first time for a given process.

This:

class Foo(object):
   x = somefunc()

   def bar(self):
       print("foo")

   @staticmethod
   def baaz():
       print("quux")

is actually just syntactic sugar for:

x = somefunc()

def bar(self):
  print("foo")

def baaz():
   print("quux")

attribs = {
    "x" : x,
    "bar": bar,
    "baaz": staticmethod(baaz)
    }

del x
del bar
del baaz

Foo = type("Foo", (object,), attribs)

so why would you expect the x = somefunc() statement to be executed each time you instanciate Foo or call bar or baaz ?

Julian Camilleri · Accepted Answer · 2018-10-22 10:05:07Z

1

The issue is that you're creating a class attribute and not an instance attribute.

def in_here():
    print('Called')

    return True

class A:
    x = in_here()

    @classmethod
    def test(cls):
        print('In test')
        return cls.x


A.test()

This will only print Called once. - Because the class attributes are not rebuilt every time you call the class, but rather once when executed.

If you'd like to have it run it more than once, then why not use __init__? or change the class attribute to a property? or better yet, just call the function when required?

edited Oct 22, 2018 at 10:05

answered Oct 22, 2018 at 10:00

Julian Camilleri

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