List props returns invalid empty list

[rowanc1]

When not initialized the list properties return an empty list rather than None. This is confusing as in the case below, that is not even a valid thing:

import properties
class Array(properties.HasProperties):
    shape = properties.List(
        'shape of the 2d array',
        prop=properties.Integer('', min=1),
        min_length=2,
        max_length=2,
    )
a = Array()
print(a.shape)
#> []

[rowanc1]

What is worse is that if the property is also required=False, it will fail validation.

Work around is to use a default=properties.undefined.

[fwkoch]

I disagree that the first comment is a bug, it's just the way things are designed (for now...). A container property's _class_default value is an empty container, regardless of anything else. Looking through the documentation, though, looks like that's never stated... You can always set default=properties.undefined; then a.shape is None. In this case, though, a.validate() will still fail until shape is set correctly. Whether or not this is the correct behavior is a different story...

...which brings up your second comment. I agree that is a very bad bug-like consequence that I hadn't thought of (if there is no min_length requirement and the list property is unset, None vs [] shouldn't be a big difference). Anyway, I think this is probably enough reason to change the default behavior.

That has possible implications for existing code that may assume the list is instantiated by default. Needs some investigation before acting.

Another possibly safer but definitely more complicated solution is making the default undefined if required=False but [] otherwise... Something to consider, but I really don't like this. Yeah. Really don't like it.

Anyway, as a temporary fix, just set default=properties.undefined. Then things should behave as you expect.

[fwkoch]

I guess really it's just a question of taste. Do we require users who do not want an initialized list to set default=properties.undefined? or do we require users who do want an initialized list to set default=list?

Edit: The latter fits a bit better with other property types, but isn't backwards compatible.

[fwkoch]

Another related bug-like thing here with defaults that are not undefined. Given something like this:

    values = properties.Union(
        'Lists of similar type values',
        props=[
            properties.List('', properties.Integer(''), min_length=2),
            properties.List('', properties.Float(''), min_length=2),
            properties.List('', properties.DateTime(''), min_length=2),
        ],
        default=properties.undefined,
    )

I'd expect the undefined default to override the list [] default. But it does not, the default instead is set to the list default.

This leads a bit into another question - do we want union properties to grab sub-props's defaults? Makes more sense if you look at something like this:

    value = properties.Union(
        'Float, Int, or DateTime',
        props=[
            properties.Integer('', default=5),
            properties.Float(''),
            properties.DateTime(''),
        ],
    )

The the default for the union property will be 5 as well. But maybe that isn't what we want either...?

Anyway, definitely need to rethink the empty list default.

[fwkoch]

Empty list defaults have been removed with #219 - the backwards incompatibility means the next release is 0.4.0, but this is a necessary change to fix the above behaviour and add some consistency to property defaults