What is a way to extract arguments from __init__ without creating new instance.
The code example:
class Super:
def __init__(self, name):
self.name = name
I am looking something like Super.__dict__.keys()type solution. Just to retrieve name argument information without adding any values. Is there such an option to do that?
Update for Python 3.3+ (as pointed out by beeb in the comments)
You can use inspect.signature introduced in Python 3.3:
class Super:
def __init__(self, name, kwarg='default'):
print('instantiated')
self.name = name
>>> import inspect
>>> inspect.signature(Super.__init__)
<Signature (self, name, kwarg='default')>
Original answer below
You can use inspect
>>> import inspect
>>> inspect.getargspec(Super.__init__)
ArgSpec(args=['self', 'name'], varargs=None, keywords=None, defaults=None)
>>>
Edit: inspect.getargspec doesn't actually create an instance of Super, see below:
import inspect
class Super:
def __init__(self, name):
print 'instantiated'
self.name = name
print inspect.getargspec(Super.__init__)
This outputs:
### Run test.a ###
ArgSpec(args=['self', 'name'], varargs=None, keywords=None, defaults=None)
>>>
Note that instantiated never got printed.
globals returns in a function call, which is not what you want. Incidentally, using inspect doesn't result in a function call, editing my answer for an example...
A metaclass that stores the concrete __init__ parameters in a class field:
class ParameterReader(ABCMeta):
def __init__(cls, *args, **kwargs):
parameters = inspect.signature(cls.__init__).parameters
parameters = {key: value for key, value in parameters.items() if key not in ['self', 'args', 'kwargs']}
try:
cls._init_parameters = cls.__bases__[0]._init_parameters.copy()
cls._init_parameters.update(parameters)
except AttributeError:
cls._init_parameters = parameters
super().__init__(*args, **kwargs)
_init_parameters can then be used within the class instance or outside of it:
class Fruit(metaclass=ParameterReader):
def __init__(self, color):
print(color)
class Corn(Fruit):
def __init__(self, size, *args, **kwargs):
super().__init__(*args, **kwargs)
print(size)
print(self._init_parameters)
print(Corn._init_parameters)
Producing:
{'color': <Parameter "color">, 'size': <Parameter "size">}
As well as in instantiation:
Corn(10, 'yellow')
Producing:
yellow
10
{'color': <Parameter "color">, 'size': <Parameter "size">}
Note how this handles using *args, **kwargs in Corn's __init__ parameters.
Also, note the naming difference between arguments and parameters.
__init__gets called. See this answer stackoverflow.com/questions/674304/pythons-use-of-new-and-init