Getting the definition order of class attributes in python¶
Same question was in StackOverflow, but the answer was not so soft on me.
It gave me a some hints, but it actually says ‘Watch a code’. Ok, so I read the code and understood that behavior, I will write that description here.
Why is definition order necessary¶
The best example is on Form libraries. The definition order will affect to rendering order directory.
Consider on a suprious form library, like this:
class MyForm(Form): name = StringField() text = StringField()
and it should be rendered that the first place is name and the second text like this oreder:
<input value='name' /> <input value='text' />
Of cause, general form libraries consider the definition order, such as Djnago Form, deform (actually, colander’s behavior).
On my case, I should handle it on creating Uiro framework, definition views in controller.
To handle the order, you should place a counter. The counter will be increased on each constraction of orderd attributes. And then, each attributes stores that counter value to it’s own.
>>> import itertools >>> class Field(object): ... _counter = itertools.count() ... def __init__(self): ... self._order = next(Field._counter) ... >>> class Form(object): ... name = Field() ... text = Field() ... >>> Form.name <__main__.Field object at 0x7f0abfb26250> >>> Form.name._order 0 >>> Form.text._order 1
Ok, seems good. Then, apply this practice on writing metaclass:
>>> class FormMetaClass(type): ... def __new__(cls, name, base, attrs): ... new_class = super(FormMetaClass, cls).__new__(cls, name, base, attrs) ... ... fields = [(name, value) for name, value in attrs.items() ... if isinstance(value, Field)] ... ... # sorting manually corresponds to the definision order of Fields. ... fields.sort(key=lambda e: e._order) ... ... new_class.fields = fields ... return new_class ... >>> class Form(metaclass=FormMetaClass): ... name = Field() ... text = Field() ... >>> Form.fields ('name', <__main__.Field object at 0x7f0abfb26410>) >>> Form.fields ('text', <__main__.Field object at 0x7f0abfb26490>)
sweet, the definition order didn’t lost. In __new__ method, the fields before sorted, the order of Fields is not considered in fields, because the attrs is just a dictionary. so we should apply _oreder attribute to Field and re-consider the order manually using that _order.
I used this tips on this change, check it out.