How to validate the __init__ parameters

[Jean-Michel Pichavant]

Denis Doria wrote:
> Hi;
>
> I'm checking the best way to validate attributes inside a class. Of
> course I can use property to check it, but I really want to do it
> inside the __init__:
>
> class A:
>     def __init__(self, foo, bar):
>         self.foo = foo #check if foo is correct
>         self.bar = bar
>
> All examples that I saw with property didn't show a way to do it in
> the __init__. Just to clarify, I don't want to check if the parameter
> is an int, or something like that, I want to know if the parameter do
> not use more than X chars; and want to do it when I'm 'creating' the
> instance; not after the creation:
>
> a = A('foo', 'bar')
>
> not
>
> class A:
>     def __init__(self, foo = None, bar = None):
>         self._foo = foo
>         self._bar = bar
>     def  set_foo(self, foo):
>         if len(foo) > 5:
>              raise <something>
>         _foo = foo
>     foo = property(setter = set_foo)
>
> a = A()
> a.foo = 'foo'
>
>
> I thought in something like:
>
> class A:
>     def __init__(self, foo = None, bar = None):
>         set_foo(foo)
>         self._bar = bar
>     def  set_foo(self, foo):
>         if len(foo) > 5:
>              raise <something>
>         _foo = foo
>     foo = property(setter = set_foo)
>
> But looks too much like java
>   
One possible way, straight and simple

class A:
    def __init__(self, foo = None, bar = None):
        if len(foo) > 5:
		raise ValueError('foo cannot exceed 5 characters')
	self._foo = foo
        self._bar = bar


JM