Attributes

util.attribute gives objects named, typed values – attributes – that are inherited down a type hierarchy, the way a class’s members are inherited by its subclasses and instances, but at run time and under rules you set per attribute.

Attributes and attributed objects

An Attribute is a value with:

• a type: a Python type, or the name of one of the known types – int, float, bool, str, text and mtext (multi-line text), color, list, dict, attrs, or choices[...] (one of a list of values). The names tell an editor how to edit the value (see Tk Widgets);

• a restriction on how it’s inherited (below);

• a note: a description, shown as a tooltip by the attribute editor.

An AttributedObject is a dict of attributes, by name, that also inherits attributes from its supertypes. A subclass says what those are, and whether the object is a type or an individual (an instance), by overriding getSupers() and isIndividual(). Then:

• obj.get(key) is the attribute’s effective value: the object’s own, or else the one it inherits (searching its supertypes in order);

• obj.getLocal(key) and obj.getInh(key) give just one or the other;

• obj[key] = value sets the object’s own value.

get() can also store a default – obj.get(key, storeDefault=v, storeType=t, note=...) – which is how an application declares its attributes (and preferences: see Preferences, Graphs and Utilities) as it first uses them.

An AttributedObject is a Subject: it announces "new attribute", "mod attribute" and "del attribute", so that, eg: the drawing of an object can follow its attributes. It’s Serializable and Dirtyable too.

Inheritance restrictions

A Restriction holds two RestrictionTypes flags: one for how individuals inherit the attribute, and one for how subtypes do:

NOINHERIT

The key is inherited, but not the value: it’s a slot that must be filled locally.

NOOVERRIDE

The inherited value can’t be overridden.

PRIVATE

The attribute isn’t inherited at all.

The useful combinations have names:

restriction	individuals	subtypes	eg
VAR_DEFAULTED	inherit	inherit	a default that anything may change
VAR	must fill in	inherit	an object’s name
CONST_TYPEOVERRIDE	can’t change	inherit	a constant that subtypes may change
CONST	can’t change	can’t change	a constant
UNINHERITED	must fill in	must fill in	a slot everything below must fill
TYPENAME	can’t change	must fill in	the name of a type
TYPEONLY	don’t see it	inherit	something only for types
LOCAL	don’t see it	don’t see it	only for the object itself

(Some combinations make no sense, and are refused: see legalityError().)

The attribute editor

util.attributeEditor edits an AttributedObject’s attributes in a dialog: their values, types and restrictions, with the values they inherit shown above them – see Tk Widgets.