functools.partial on class method
You are creating partials on the function, not the method. functools.partial()
objects are not descriptors, they will not themselves add the self
argument and cannot act as methods themselves. You can only wrap bound methods or functions, they don't work at all with unbound methods. This is documented:
partial
objects are likefunction
objects in that they are callable, weak referencable, and can have attributes. There are some important differences. For instance, the__name__
and__doc__
attributes are not created automatically. Also,partial
objects defined in classes behave like static methods and do not transform into bound methods during instance attribute look-up.
Use property
s instead; these are descriptors:
class RGB(object): def __init__(self, red, blue, green): super(RGB, self).__init__() self._red = red self._blue = blue self._green = green def _color(self, type): return getattr(self, type) @property def red(self): return self._color('_red') @property def blue(self): return self._color('_blue') @property def green(self): return self._color('_green')
As of Python 3.4, you can use the new functools.partialmethod()
object here; it'll do the right thing when bound to an instance:
class RGB(object): def __init__(self, red, blue, green): super(RGB, self).__init__() self._red = red self._blue = blue self._green = green def _color(self, type): return getattr(self, type) red = functools.partialmethod(_color, type='_red') blue = functools.partialmethod(_color, type='_blue') green = functools.partialmethod(_color, type='_green')
but these'd have to be called, whilst the property
objects can be used as simple attributes.
The issue with partialmethod
is that it is not compatible with inspect.signature
, functools.wraps
,...
Weirdly enough, if you re-implement functools.partial
yourself using the partial documentation implementation example, it will work:
# Implementation from:# https://docs.python.org/3/library/functools.html#functools.partialdef partial(func, /, *args, **keywords): def newfunc(*fargs, **fkeywords): newkeywords = {**keywords, **fkeywords} return func(*args, *fargs, **newkeywords) newfunc.func = func newfunc.args = args newfunc.keywords = keywords return newfunc
class RGB(object): def __init__(self, red, blue, green): super(RGB, self).__init__() self._red = red self._blue = blue self._green = green def _color(self, type): return getattr(self, type) red = partial(_color, type='_red') blue = partial(_color, type='_blue') green = partial(_color, type='_green')rgb = RGB(100, 192, 240)print(rgb.red()) # Print red
The reason is that newfunc
is a true function which implement the descriptor protocol with newfunc.__get__
. While type(functools.partial)
is a custom class with __call__
overwritten. Class won't add the self
parameter automatically.