Question

我正在为CS-600开设麻省理工学院开放课件，我无法弄清楚为什么最后一个印刷语句没有打印任何内容。这是我写的代码：

#!/usr/bin/env python
# encoding: utf-8
# 6.00 Problem Set 9
#
# Name:
# Collaborators:
# Time:

from string import *

class Shape(object):
    def area(self):
        raise AttributeException("Subclasses should override this method.")

class Square(Shape):
    def __init__(self, h):
        """
        h: length of side of the square
        """
        self.side = float(h)
    def area(self):
        """
        Returns area of the square
        """
        return self.side**2
    def __str__(self):
        return 'Square with side ' + str(self.side)
    def __eq__(self, other):
        """
        Two squares are equal if they have the same dimension.
        other: object to check for equality
        """
        return type(other) == Square and self.side == other.side

class Circle(Shape):
    def __init__(self, radius):
        """
        radius: radius of the circle
        """
        self.radius = float(radius)
    def area(self):
        """
        Returns approximate area of the circle
        """
        return 3.14159*(self.radius**2)
    def __str__(self):
        return 'Circle with radius ' + str(self.radius)
    def __eq__(self, other):
        """
        Two circles are equal if they have the same radius.
        other: object to check for equality
        """
        return type(other) == Circle and self.radius == other.radius

#
# Problem 1: Create the Triangle class
#
## TO DO: Implement the `Triangle` class, which also extends `Shape`.

class Triangle(Shape):
    def __init__(self, base, height):
        self.base = float(base)
        self.height = float(height)
    def area(self):
        return self.base*self.height/2
    def __str__(self):
        return 'Triangle with base ' + str(self.base) + 'and height ' + str(self.height)
    def __eq__(self, other):
        return type(other) == Triangle and self.base == other.base and self.height == other.height
#
# Problem 2: Create the ShapeSet class
#
## TO DO: Fill in the following code skeleton according to the
##    specifications.

class ShapeSet(object):
    def __init__(self):
        """
        Initialize any needed variables
        """
        self.allCircles = []
        self.allSquares = []
        self.allTriangles = []
        self.allShapes = self.allCircles + self.allSquares + self.allTriangles
        self.place = None 
    def addShape(self, sh):
        """
        Add shape sh to the set; no two shapes in the set may be
        identical
        sh: shape to be added
        """
        if not isinstance(sh, Shape): raise TypeError('not a shape')
        if isinstance(sh, Square):
            for sq in self.allSquares:
                if sh == sq:
                    raise ValueError('shape already in the set')
            self.allSquares.append(sh)
        if isinstance(sh, Triangle):
            for tri in self.allTriangles:
                if sh == tri:
                    raise ValueError('shape already in the set')
            self.allTriangles.append(sh)
        if isinstance(sh, Circle):
            for circ in self.allCircles:
                if sh == circ:
                    raise ValueError('shape already in the set')
            self.allCircles.append(sh)
    def __iter__(self):
        """
        Return an iterator that allows you to iterate over the set of
        shapes, one shape at a time
        """
        self.place = 0
        return self
    def next(self):
        if self.place >= len(self.allShapes):
            raise StopIteration
        self.place += 1
        return self.allShapes[self.place - 1]

    def __str__(self):
        """
        Return the string representation for a set, which consists of
        the string representation of each shape, categorized by type
        (circles, then squares, then triangles)
        """
        shapeList = ""
        for item in self.allShapes:
            shapeList += item.get__str__ + "br/"
        return shapeList

#
# Problem 3: Find the largest shapes in a ShapeSet
#
def findLargest(shapes):
    """
    Returns a tuple containing the elements of ShapeSet with the
       largest area.
    shapes: ShapeSet
    """
    ## TO DO

#
# Problem 4: Read shapes from a file into a ShapeSet
#
def readShapesFromFile(filename):
    """
    Retrieves shape information from the given file.
    Creates and returns a ShapeSet with the shapes found.
    filename: string
    """
    ## TO DO

def main():
    sq1 = Square(4.0)
    sq2 = Square(5.0)
    sq3 = Square(3.0)
    circ1 = Circle(3.0)
    circ2 = Circle(3.2)
    tri1 = Triangle(3.0, 4.0)
    tri2 = Triangle(4.0, 3.0)
    tri3 = Triangle(1.0, 1.0)
    thisSet = ShapeSet()
    thisSet.addShape(sq1)
    thisSet.addShape(sq2)
    thisSet.addShape(sq3)
    thisSet.addShape(circ1)
    thisSet.addShape(circ2)
    thisSet.addShape(tri1)
    thisSet.addShape(tri2)
    thisSet.addShape(tri3)
    print thisSet



if __name__ == '__main__':
    main()

Answer 1

这一行：

self.allShapes = self.allCircles + self.allSquares + self.allTriangles

不符合您的想法。它将allShapes设置为空列表，然后在您稍后添加形状时，不会更新allShapes。

然后，您的__str__函数只会循环allShapes，它仍为空，因此您的__str__会返回一个空字符串。

Answer 2

这一行使allShapes成为一个空列表：

self.allShapes = self.allCircles + self.allSquares + self.allTriangles

如果修改allCircles，则不会影响allShapes。我个人会消除allShapes，并在 str 方法中，在最后一秒添加它们：

for item in self.allCircles + self.allSquares + self.allTriangles:

Answer 3

问题在于：

self.allShapes = self.allCircles + self.allSquares + self.allTriangles

当您连接这样的列表时，结果是组件列表的副本。因此，当稍后更改这些列表时，不会更改连接列表。在这种情况下，self.allCircles等都是空的。所以self.allShapes也是一个空列表; ShapeSet.__str__中的for循环不会向ShapeList附加任何内容，因此结果为空字符串。

解决此问题的一种简单方法是使allShapes成为您调用的方法，并且每次返回self.allCircles ...等的新连接它被称为。这样，allShapes始终是最新的。

Answer 4

如果这是您的实际代码，则必须是因为

item.get__str__

应该引发异常。

编辑：正如其他人所说，这不是实际问题，但我将此留在这里作为进一步进展的提示。请注意，直接调用x.__str__()被视为坏风格（“unpythonic”），正如您可能想要的那样。即使在str(x)的实施中，也请致电__str__。

Answer 5

在init方法的 start 中将allShapes指定为self.allCircles + self.allSquares + self.allTriangles的值（当其他列表为空时））。

它的值永远不会改变，所以它仍然是空的。

你需要在addShape中使用它：

self.allShapes.append(sh)

为什么main方法底部的print语句不能打印任何内容？

5 个答案: