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curriculum/challenges/english/blocks/learn-interfaces-by-building-an-equation-solver/663b83a28943e6aa6275a514.md

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--description--

Still within the Equation class, define a __str__ method to give a proper string representation to the equation objects you are going to create.

For now, within the __str__ method, declare a variable terms and assign it an empty list. You'll use this variable to store each term (coefficient times \( x^n \)) of your equation.

Then, declare a variable equation_string, assign it the result of joining the elements in the terms list with a space. Finally, return equation_string.

--hints--

You should define a __str__ method within the Equation class.

js
({ test: () => assert(runPython(`_Node(_code).find_class("Equation").has_function("__str__")`)) })

Your __str__ method should take one parameter, self.

js
({ test: () => assert(runPython(`_Node(_code).find_class("Equation").find_function("__str__").has_args("self")`)) })

You should declare a variable terms and assign it an empty list within the __str__ method.

js
({ test: () => assert(runPython(`_Node(_code).find_class("Equation").find_function("__str__").has_stmt("terms = []")`)) })

You should declare a variable equation_string and assign it the result of joining the elements in terms with a space within the __str__ method.

js
({ test: () => assert(runPython(`_Node(_code).find_class("Equation").find_function("__str__").has_stmt("equation_string = ' '.join(terms)")`)) })

You should return equation_string from your __str__ method.

js
({ test: () => assert(runPython(`_Node(_code).find_class("Equation").find_function("__str__").has_return("equation_string")`)) })

--seed--

--seed-contents--

py
from abc import ABC, abstractmethod

class Equation(ABC):
    degree: int
    
    def __init__(self, *args):
        if (self.degree + 1) != len(args):
            raise TypeError(
                f"'Equation' object takes {self.degree + 1} positional arguments but {len(args)} were given"
            )
        if any(not isinstance(arg, (int, float)) for arg in args):
            raise TypeError("Coefficients must be of type 'int' or 'float'")
        if args[0] == 0:
            raise ValueError("Highest degree coefficient must be different from zero")
        self.coefficients = {(len(args) - n - 1): arg for n, arg in enumerate(args)}

    def __init_subclass__(cls):
        if not hasattr(cls, "degree"):
            raise AttributeError(
                f"Cannot create '{cls.__name__}' class: missing required attribute 'degree'"
            )
--fcc-editable-region--
    
--fcc-editable-region--    
    @abstractmethod
    def solve(self):
        pass
        
    @abstractmethod
    def analyze(self):
        pass
        
class LinearEquation(Equation):
    degree = 1
    
    def solve(self):
        pass
    
    def analyze(self):
        pass


lin_eq = LinearEquation(2, 3)
print(lin_eq)