Changing the field of one class from another method

Question

Here, slowly, I am glad to be a python. Background 0, if that.

I want to understand if I did the right thing. There are two classes: rope and knife. There is one object of each class. The rope has a length, the knife is able to cut the rope. Did I understand correctly the essence of using the method of one class that changes the attribute of another? Or is it a children's code curve? thanks for the advice

class Rope: length = 30 def get_info(self): print("\n", "---"*8, "\n", "class Rope, get_info(): ", "\n", "Rope's length is %s meters" % self.length, "\n", "---"*8, "\n" ) class Knife: def cut_rope(self, length, a): print("Now we will cut %s meters off" % length) a.length -= length rope = Rope() knife = Knife() rope.get_info() knife.cut_rope(12, rope) rope.get_info() C:\Users\олег\Python34\classes>ex2.py ------------------------ class Rope, get_info(): Rope's length is 30 meters ------------------------ Now we will cut 12 meters off ------------------------ class Rope, get_info(): Rope's length is 18 meters

it has methods, say, to tie into a knot (with the help of the object "hand" or "person").
And then in one thread the rope will be cut, in the other they will increase ...
As for the rope.get_info() - the rope introspection method should not be either)

Answer 1 · 2016-08-24T02:40:19

As already wrote in the commentary, I propose to make the rope an immutable object. And maximize the use of built-in methods.

 # coding=utf-8 class Rope: # зачем хардкодить длину верёвки? Лучше будем задавать её в конструкторе. def __init__(self, length): if length <= 0: raise ValueError('length should be > 0') self._length = length # сработает с методом len() def __len__(self): return self._length # а это сработает с str(), соответственно и с print def __str__(self): return "This rope's length is {0!s} meters".format(self._length) class Knife: # Полиморфизм! Ножу всё равно, что резать. На выходе должно быть два того же. # Если нарезаемое нельзя инициировать только по длине, задаваемой числом, будет ошибка. def cut(self, object, length): _class = object.__class__ # по-хорошему, тут ещё должны быть проверки, что length не None и что число # и, конечно, что object не None if len(object) < length: raise ValueError('cut length exceeds available length') elif len(object) == length: # даже для одного объекта мы возвращаем список, чтобы всегда был перечислимый (iterable) результат return [_class(len(object))] else: return [_class(len(object) - length), _class(length)] rope = Rope(30) knife = Knife() print rope for r in knife.cut(rope, 20): print r

Conclusion:

 This rope's length is 30 meters This rope's length is 10 meters This rope's length is 20 meters

One wants to pull the cut method out of the class, and leave it just a "free" function.
Although, of course, the example was originally a toy, and the condition of the knife is indispensable)

Answer 2 · 2016-08-24T12:33:21

 # -*- coding: UTF-8 -*- class Rope(str): def __init__(self, s: str): self.len = len(self) print("%s rope's length is %s meters" % (self, self.len)) def __getitem__(self, item: slice): return self.__class__(super().__getitem__(item)) class Knife: @staticmethod def cut(string: str, length: int): rope = Rope(string) yield from (rope, rope[:length], rope[length:]) print('{s} cut {}[{}] pos'.format(rope.len, length, s=rope)) if __name__ == '__main__': rope = '-qwertyuiop-' print([(r, r.len) for r in Knife.cut(rope, 5)])

out:

 -qwertyuiop- rope's length is 12 meters -qwer rope's length is 5 meters tyuiop- rope's length is 7 meters -qwertyuiop- cut 12[5] pos [('-qwertyuiop-', 12), ('-qwer', 5), ('tyuiop-', 7)]

It is a little strange that the rope is somehow tied to the knife.
Moreover, all the ropes, which were cut with a knife, are tied to it. Oo
the method of planting a tree is the method of a forest or a forester)))

Changing the field of one class from another method

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