Default fied_heads with instances will cause shared parameters

[hyperzy]

Parameters default to an instantiated object would remain the same according to Python doc

Default parameter values are evaluated from left to right when the function definition is executed. This means that the expression is evaluated once, when the function is defined, and that the same “pre-computed” value is used for each call. This is especially important to understand when a default parameter is a mutable object, such as a list or a dictionary: if the function modifies the object (e.g. by appending an item to a list), the default value is in effect modified....

Therefore, the coarse_field and fine_field would share the same RGBFieldHead, which is not the expected behavior.