Return value from 2D array in Python class

Question

I'll frame this in the fact that I'm kind of a newbie to Python and I'm taking my first stab at writing a class. What I'd like to achieve is for the class to open a text file and return a value from the file based on user input. The text file contains information in a 2D array like this:

 20,21,22
 23,24,25
 26,27,28

The value is retrieved from this text file and assigned to a new variable, so that the variable can be printed and also used later in a calculation. I've been able to do this without difficulty outside of a class, but getting this to work in a class has been frustrating. Here's the code I have so far in Python 2.7:

im_new = []

class Read(object):
    def __init__(self, row, col, newvar):
        self.row = row
        self.col = col
        self.newvar = newvar

        display_list = []
        self.newvar = []

        with open("Array.txt", "r") as data_file:
            for line in data_file:
                display_list.append(line.strip().split(','))

    def __str__(self):
        self.newvar = (display_list[self.row][self.col])
        return self.newvar

immem = Read( 1, 1, im_new)


print "OUTPUT: ", im_new

Ideally, I'd get "OUTPUT: 24", but instead I get "OUTPUT: []". I'm not sure what I'm doing wrong. Any help would be appreciated.

You're printing im_new, which is initialized as an empty list and never modified. — TigerhawkT3
– TigerhawkT3, Commented May 5, 2015 at 19:34
The first big problem is just that you forgot the self. before display_list. (You've got it on all of your other attributes, so I assume you understand what it means, and this is effectively just a typo?) — abarnert
– abarnert, Commented May 5, 2015 at 19:35
The second big problem is that you're printing im_new instead of immem. It's the latter that holds your Read instance, which has a __str__ method, which does what you want. — abarnert
– abarnert, Commented May 5, 2015 at 19:37

abarnert · Accepted Answer · 2015-05-05 19:38:49Z

2

First, you're mixing up two of your variables. im_new is an empty list; immem is a Read instance. Change the last line to print 'OUTPUT:', immem.

When you fix that, you'll get another problem: a NameError on display_list. This is because you forgot the self. on display_list, but in your __init__ method and in your __str__ method. (I assume you know what self. means, because you use it correctly for all of your other attributes.)

As a side note, it's a little weird to override self.newvar every time you get the __str__ of your object. There's really no reason for this attribute to exist, at least in the code you've shown. So you could just do this:

class Read(object):
    def __init__(self, row, col):
        self.row = row
        self.col = col

        self.display_list = []

        with open("Array.txt", "r") as data_file:
            for line in data_file:
                self.display_list.append(line.strip().split(','))

    def __str__(self):
        return self.display_list[self.row][self.col]

immem = Read(1, 1)

answered May 5, 2015 at 19:38

abarnert

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1 Comment

credenzamatic Over a year ago

Thanks for walking me through this. I'd been through a number of iterations of this, had gotten the errors you mention, which left me scratching my head. Your explanation makes this clear. Thank you.

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