python类关键字参数

Question

I'm writing a class for something and I keep stumbling across the same tiresome to type out construction. Is there some simple way I can set up class so that all the parameters in the constructor get initialized as their own name, ie fish = 0 -> self.fish = fish ?

class Example(object):
    def __init__(self, fish=0, birds=0, sheep=0):
        self.fish = fish
        self.birds = birds
        self.sheep = sheep

Answer 1

Short answer: no. You are not required to initialize everything in the constructor (you could do it lazily), unless you need it immediately or expose it (meaning that you don't control access). But, since in Python you don't declare data fields, it will become difficult, much difficult, to track them all if they appear in different parts of the code.

More comprehensive answer: you could do some magic with **kwargs (which holds a dictionary of argument name/value pairs), but that is highly discouraged, because it makes documenting the changes almost impossible and difficult for users to check if a certain argument is accepted or not. Use it only for optional, internal flags. It could be useful when having 20 or more parameters to pass, but in that case I would suggest to rethink the design and cluster data.

In case you need a simple key/value storage, consider using a builtin, such as dict .

Answer 2

You could use the inspect module:

import inspect

class Example(object):

    def __init__(self, fish=0, birds=0, sheep=0):
        frame = inspect.currentframe()
        args, _, _, values = inspect.getargvalues(frame)
        for i in args:
            setattr(self, i, values[i])

This works, but is more complicated that just setting them manually. It should be possible to hide this with a decorator:

@set_attributes
def __init__(self, fish=0, birds=0, sheep=0):
    pass

but defining set_attributes gets tricky because the decorator inserts another stack frame into the mix, and I can't quite get the details right.

Answer 3

For Python 3.7+, you can try using data classes in combination with type annotations.

https://docs.python.org/3/library/dataclasses.html

Import the module and use the decorator. Type-annotate your variables and there's no need to define an init method, because it will automatically be created for you.

from dataclasses import dataclass

@dataclass
class Example:
    fish: int = 0
    birds: int = 0
    sheep: int = 0