How to print 2d array of objects in python?

Question

I have a 2d array of objects, where I've assigned each index an object:

for row in range(0, 9):
        for column in range(0, 9):
            self.Matrix[row][column] = Square(row, column)

where Square() is an object that takes in a specific index. Each Square object has a method constructor (def __str__) that will print a specific text(ex. "KN") for its specific coordinates. I've tried just printing the matrix:

print self.Matrix()

but I end up getting a really long that's something like

[[<__main__.Square object at 0x101936d90>, <__main__.Square object at 0x101936dd0>, <__main__.Square object at 0x101936e10>, .....

How can I print the actual objects instead?

Are you sure you had the parentheses in the `print`, after `Matrix`? Because I think you should have gotten an error if you did. — Tom Karzes, Nov 23 '15 at 07:55
You are printing as repr instead of str. How self.Matrix() is implemented? — Roman Susi, Nov 23 '15 at 07:57

TigerhawkT3 · Answer 1 · 2015-11-23T08:06:48.657

This is happening because you're printing the Matrix that contains the Squares. This calls the __str__() for the Matrix class. If you haven't defined a __str__() for that class that returns a string comprising the str() of each of its contained objects, it'll give you the repr() of each of those objects, as defined by their __repr__(). I don't suppose you've defined one. The default is a mere memory location, as you see.

Here's a demo with a stub class:

>>> class A:
...     def __str__(self):
...             return 'a'
...     def __repr__(self):
...             return 'b'
...
>>> print(A())
a
>>> A()
b
>>> [A(), A()]
[b, b]
>>> print([A(), A()])
[b, b]
>>> print(*[A(), A()])
a a

The solution is to either define a specific __str__() for Matrix that returns the str() for each of its contained objects, or define a suitable __repr__() for the Square objects (which should be something that could be passed to eval() to recreate the object, not just something like "KN").

score 1 · Answer 2 · edited May 23 '17 at 12:30

You should use __repr__

difference between __repr__ and __str__

class A():
    def __str__(self):
        return "this is __str__"

class B():
    def __repr__(self):
        return "this is __repr__"

a = [A() for one in range(10)]
b = [B() for one in range(10)]

print a
[<__main__.A instance at 0x103e314d0>, <__main__.A instance at 0x103e31488>]
print b
[this is __repr__, this is __repr__]

How to print 2d array of objects in python?

2 Answers2