How to sort list of objects properly in Python

Question

There is a Location class

class Location(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y
    def move(self, deltaX, deltaY):
        return Location(self.x + deltaX, self.y + deltaY)
    def getX(self):
        return self.x
    def getY(self):
        return self.y
    def dist_from(self, other):
        xDist = self.x - other.x
        yDist = self.y - other.y
        return (xDist**2 + yDist**2)**0.5
    def __eq__(self, other):
        return (self.x == other.x and self.y == other.y)
    def __str__(self):
        return '<' + str(self.x) + ',' + str(self.y) + '>'

and a get_cars method which I wrote to retrieve data from the list (doesn't belong to LOcation class) according to the spec below:

def get_cars(self):
    """ Returns a list of all cars on the parking. The list should contain 
    the string representation of the Location of a car. The list should 
    be sorted by the x coordinate of the location. """


    result = ''
    for i in sorted(self.car_loc_list, key=lambda Location: Location.x):
        if self.car_loc_list[0] == i:
            result += '\'' + str(i) + '\''
        else:    
            result += ', ' + '\'' + str(i) + '\''

    return '[' + result + ']'

self.car_loc_list is just a list which holds objects of Location class and they contain some coordinates(x,y) (unsorted):

for i in c.car_loc_list:
    print(i)

<0,0>
<1,5>
<7,2>
<2,1>
<1,7>
<4,3>

The online grader examine my code in 2 ways:

print(c.get_cars()) - OK
print(sorted(c.get_cars())) - NOT OK

When I follow the first way:

print(c.get_cars())

It prints out the next result sorted by X coordinate(1-st digit):

print(c.get_cars())
Out[539]: "['<0,0>', '<1,5>', '<1,7>', '<2,1>', '<4,3>', '<7,2>']"

It is also the result which I (and grader) expected to receive.

When I do print(sorted(c.get_cars)) I get:

print(sorted(c.get_cars()))
[' ', ' ', ' ', ' ', ' ', "'", "'", "'", "'", "'", "'", "'", "'", "'", "'", "'", "'", ',', ',', ',', ',', ',', ',', ',', ',', ',', ',', ',', '0', '0', '1', '1', '1', '2', '2', '3', '4', '5', '7', '7', '<', '<', '<', '<', '<', '<', '>', '>', '>', '>', '>', '>', '[', ']']

I stuck at this place. Also I understand that it somehow transforms my output to the string again and that is why I get such result. Is there any idea how to implement so that both solutions give the same result according to the spec above e.g ['<0,0>', '<1,5>', '<1,7>', '<2,1>', '<4,3>', '<7,2>'] ?

UPD Seems I didin't understand the next sentence:

The list should contain the string representation of the Location of a car.

Why are you returning a string then? Your grader expects a list. That you managed to make your string look like a list shows the grader is too simplistic to detect the difference, but sorted() on a list returns individual characters in sorted order. — Martijn Pieters
– Martijn Pieters, Commented Aug 1, 2017 at 19:10
In other words, I'm pretty sure you should not be returning a single string object. You should be returning a list of strings, each string of the form '<x,y>', so ['<0,0>', '<1,5>', '<1,7>', '<2,1>', '<4,3>', '<7,2>'] (note that there are no double quotes around that value). — Martijn Pieters
– Martijn Pieters, Commented Aug 1, 2017 at 19:11
Seems I don't understand this sentence properly: The list should contain the string representation of the Location of a car. — Robert
– Robert, Commented Aug 1, 2017 at 19:13

Martijn Pieters · Accepted Answer · 2017-08-01 19:13:38Z

1

The description of your method is pretty clear where you are going wrong:

""" Returns a list of all cars on the parking. The list should contain 
the string representation of the Location of a car. The list should 
be sorted by the x coordinate of the location. """

You are not returning a list. You are returning a string whose contents look like a list. That's not the same thing.

Return a list instead:

def get_cars(self):
    """ Returns a list of all cars on the parking. The list should contain 
    the string representation of the Location of a car. The list should 
    be sorted by the x coordinate of the location. """

    result = []
    for loc in sorted(self.car_loc_list, key=lambda loc: loc.x):
        result.append(str(loc))
    return result

or more simply with a list comprehension:

def get_cars(self):
    """ Returns a list of all cars on the parking. The list should contain 
    the string representation of the Location of a car. The list should 
    be sorted by the x coordinate of the location. """

    return [str(loc) for loc in sorted(self.car_loc_list, key=lambda loc: loc.x)]

answered Aug 1, 2017 at 19:13

Martijn Pieters

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1 Comment

Robert Over a year ago

OMG! Thank you so much. I've spent half of day trying to figure it out because I have no prior coding experience.

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