在类中设置默认值

Question

I am creating a class in Python, and I am unsure how to properly set default values. My goal is to set default values for all class instances, which can also be modified by a class method. However, I would like to have the initial default values restored after calling a method.

I have been able to make it work with the code shown below. It isn't very "pretty", so I suspect that are better approaches to this problem.

class plots:
    def __init__(self, **kwargs):
        self.default_attr = {'a': 1, 'b': 2, 'c': 3}
        self.default_attr.update(kwargs)
        self.__dict__.update((k, v) for k, v in self.default_attr.items())

    def method1(self, **kwargs):
        self.__dict__.update((k, v) for k, v in kwargs.items())

        #### Code for this method goes here

        # Then restore initial default values
        self.__dict__.update((k, v) for k, v in self.default_attr.items())

When I use this class, I would do something like my_instance = plots() and my_instance.method1() , my_instance.method1(b = 5) , and my_instance.method1() . When calling method1 the third time, b would be 5 if I don't reset the default values at the end of the method definition, but I would like it to be 2 again.

Note: the code above is just an example. The real class has dozens of default values, and using all of them as input arguments would be considered an antipattern.

Any suggestion on how to properly address this issue?

Answer 1

You can use class variables , and property to achieve your goal to set default values for all class instances. The instances values can be modified directly, and the initial default values restored after calling a method.

In view of the context that "the real class has dozens of default values", another approach that you may consider, is to set up a configuration file containing the default values, and using this file to initialize, or reset the defaults.

Here is a short example of the first approach using one class variable:

class Plots:

    _a = 1

    def __init__(self):
        self._a = None
        self.reset_default_values()

    def reset_default_values(self):
        self._a = Plots._a

    @property
    def a(self):
        return self._a

    @a.setter
    def a(self, value):
        self._a = value


plot = Plots()
print(plot.a)

plot.a = 42
print(plot.a)

plot.reset_default_values()
print(plot.a)

output:

1
42
1

Answer 2

You can use a context manager or a decorator to apply and reset the values without having to type the same code on each method.

Rather than having self.default_attr , I'd just return to the previous state.

Using a decorator you could get:

def with_kwargs(fn):
    def inner(self, **kwargs):
        prev = self.__dict__.copy()
        try:
            self.__dict__.update(kwargs)
            ret = fn(self)
        finally:
            self.__dict__ = prev
        return ret
    return inner


class plots:
    a = 1
    b = 2
    c = 3

    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

    @with_kwargs
    def method1(self):
        # Code goes here

---

IMHO this is a bad idea, and would at least suggest not mutating plots . You can do this by making a new object and passing that to method1 as self .

class Transparent:
    pass


def with_kwargs(fn):
    def inner(self, **kwargs):
        new_self = Transparent()
        new_self.__dict__ = {**self.__dict__, **kwargs}
        return fn(new_self)
    return inner

Answer 3

There is a whole bunch of ways to solve this problem, but if you have python 3.7 installed (or have 3.6 and install the backport ), dataclasses might be a good fit for a nice solution.

First of all, it lets you define the default values in a readable and compact manner, and also allows all the mutation operations you need:

>>> from dataclasses import dataclass
>>> @dataclass
... class Plots:
...     a: int = 1
...     b: int = 2
...     c: int = 3
...     
>>> p = Plots()        # create a Plot with only default values
>>> p
Plots(a=1, b=2, c=3)
>>> p.a = -1           # update something in this Plot instance
>>> p
Plots(a=-1, b=2, c=3)

You also get the option to define default factories instead of default values for free with the dataclass field definition . It might not be a problem yet, but it avoids the mutable default value gotcha , which every python programmer runs into sooner or later.

Last but not least, writing a reset function is quite easy given an existing dataclass, because it keeps track of all the default values already in its __dataclass_fields__ attribute:

>>> from dataclasses import dataclass, MISSING
>>> @dataclass
... class Plots:
...     a: int = 1
...     b: int = 2
...     c: int = 3
... 
...     def reset(self):
...         for name, field in self.__dataclass_fields__.items():
...             if field.default != MISSING:
...                 setattr(self, name, field.default)
...             else:
...                 setattr(self, name, field.default_factory())
...
>>> p = Plots(a=-1)     # create a Plot with some non-default values  
>>> p
Plots(a=-1, b=2, c=3)
>>> p.reset()           # calling reset on it restores the pre-defined defaults
>>> p
Plots(a=1, b=2, c=3)

So now you can write some function do_stuff(...) that updates the fields in a Plot instance, and as long as you execute reset() the changes won't persist.