Traits are Mixins

Traits are Mixins, behaviours. All these terms recovers loosely the same idea.

In this case refering to even older conventions traits are concrete classes for abstract classes/interfaces.

collections.MutableMapping defines an interface and some concrete methods. Since isinstance relies on interfaces (ducktyping) I can safely use it to implement methods that don’t exists and will normally work for most Mappings.

General Rules:

Generic behaviour

The Operation is propagated for each keys of both Mappings and will be propagated to the ending values. If the values are MutableMapping with the operation they will propagate. If the values are not MutableMapping with the trait, the operation will apply in place.


If your MutableMapping with Addition is made of MutableMapping without it, you’ll have a problem. To solve the problem use :ref:`bowyer`_

Scalar Operations

A scalar is everything that is not a MutableMapping. Trait support things such as integer, array operation by applying the operation on each values of the MutableMapping. Order is respected.

Inclusive Trait

If a key is absent on one of the Mapping, it will be considered the neutral element. An empty list, for list, 0 for int, 0.0 for float...

The behaviour of addition and substraction is consistently deriving from the boolean algebrae meaning of + in a set context where + means union.

Thus Addition and substraction are inclusive.

Exclusive Trait

Multiplication operates as an intersection, because on one hand it is consistentwith the set/boolean meaning of multiplication, and also that neutral element of addition, is normaly the null element of multiplication. Since multiplication implies division, instead of multiplying by 0 and keeping present in at least one dict, I prefer to avoid the raging division by zero. In short, I try to avoid my dict to explode when dividing by 0. I am weak I know.

Summary of the behaviours and dependancies

Operation Short Behaviour Requires Safe Name
Copier copy None      
Addition add Inclusive copy Yes InclusiveAdder
Multiplication mul Exclusive add,copy Yes ExclusiveMuler
Substraction sub Inclusive add,mul,copy Yes InclusiveSubber
Division div Exclusive add,mul,sub,copy No TaintedExclusiveDiver