Python class object in array

Question

I'm trying to make an array of class objects. When I create the object, it works great:

class Complex(object):

    def __init__(self, realpart, imagpart):
        #creates complex number
        self.r = realpart
        self.i = imagpart

    def __str__(self):
        '''Returns complex number as a string'''
        return '(%s + %s j)' % (self.r, self.i) 

a = Complex(1,0)
print a

(1 + 0 j)

But when I try to put a in an array, I get an error:

arr1 = [a]

[<__ main __.Complex object at 0x5afab0>]

Why could this be happening? Thanks in advance.

That's not really an error, and, depending on how you want to use this list of objects, may not cause problems for you at all. It's just a display issue. — BrenBarn, Jan 22 '14 at 07:49
Your title is slightly misleading: you don't pout a class object into the array, but an instance of it. — glglgl, Jan 22 '14 at 09:02

score 3 · Answer 1 · answered Jan 22 '14 at 07:47

3

Because __repr__ was used instead of __str__. Override __repr__ as Ashwini Chaudhary commented.

>>> class Complex(object):
...     def __init__(self, realpart, imagpart):
...         self.r = realpart
...         self.i = imagpart
...     def __str__(self):
...         '''Returns complex number as a string'''
...         return '(%s + %s j)' % (self.r, self.i)
...     __repr__ = __str__ # <-----
...
>>> a = Complex(1,0)
>>> [a]
[(1 + 0 j)]

answered Jan 22 '14 at 07:47

falsetru

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1

Note that it may be better to provide a less ambiguous `repr` representation, such as `Complex(1, 0)`. With this implementation, it's difficult to tell exactly what kind of object you're dealing with. It looks like the built-in type, but slightly mangled. – user2357112 Jan 22 '14 at 08:10

score 1 · Answer 2 · edited May 23 '17 at 11:56

As per the docs, print normally calls __str__, so when you printed the object, __str__ is invoked. But when you print the builtin collections, the __str__ method of them invoke __iter__ of the individual elements in it. So, your must implement __iter__ on your own, or let the default __repr__ as it is.

If you decide to implement __repr__ yourself, the docs say,

If at all possible, this should look like a valid Python expression that could be used to recreate an object with the same value (given an appropriate environment). If this is not possible, a string of the form <...some useful description...> should be returned. The return value must be a string object.

So, if you are implementing it yourself, give as much information as possible, so that the __repr__ output is unambiguous and useful for debugging.

def __str__(self):
    '''Returns complex number as a string'''
    return '< Complex({} + {}j) >'.format(self.r, self.i)

You can read more about this, in this answer.

Python class object in array

2 Answers2