How to return string representation of self in a class in Python?

To 'pretty print' the object itself, define __str__ like so:

class Test(object):
    def __init__(self, thing):
        self.thing = thing

    def __str__(self):
        return self.thing

 >>> a=Test('apple')
 >>> print a
 apple

If you want the representation to be custom, add __repr__:

class Test(object):
    def __init__(self, thing):
        self.thing = thing
    def __repr__(self):
        return self.thing 

>>> Test('pear')
pear

If you want to create a string as stated in your edit, you can do this:

class Test(object):
    def __init__(self, thing):
        self.thing = thing

    def andthis(self, other):
        return '{} and {}'.format(self.thing, other)

>>> apple=Test('apple')
>>> apple.andthis('cinnamon')
'apple and cinnamon'
>>> Test('apple').andthis('carrots')
'apple and carrots' 

you should add

def __str__(self):
    return self.thing

so it looks like this

class Test:
    def __init__(self, thing):
        self.thing = thing

    def f1(self, thing):
        return str(self) + " and " + thing

    def __str__(self):
        return self.thing

a = Test("apples")
print a
>> "apples"
print a.f1("orange")
>> "apples and orange"

If you wanted f1() to return a string, then do so:

def f1(self, otherthing):
    return '{} and {}'.format(self.thing, otherthing)

Here we use str.format() to put self.thing and otherthing together into a new string, which is returned. Note that you need to explicitly refer to self.thing here.

You could also use string concatenation, like in your own code:

def f1(self, otherthing):
    return self.thing + ' and ' + otherthing

but again, you need to refer to self.thing explicitly.

Demo:

>>> class Test:
...     def __init__(self, thing):
...         self.thing = thing
...     def f1(self, otherthing):
...         return '{} and {}'.format(self.thing, otherthing)
... 
>>> a = Test("apples")
>>> a.f1("cinnamon")
'apples and cinnamon'