how to use __str__ and __repr__?
Jim Newton
jimka at rdrop.com
Mon Jun 7 17:51:00 EDT 2004
thanks for responding,
i was expecting class().__str__()
to evaluate to the string "<__main__.another instance at 0x8132b64>"
because that what print does with class().
but alas it does not.
why does print class() not give me the same error as class().__str__()?
that's what i do not understand.
-jim
Larry Bates wrote:
> I don't understand what you are trying to do, but
> the problem is that when you define class 'another'
> the following line doesn't make sense:
>
> middle = " ".join( [ substr.__str__() for substr in self])
>
> The instance of another doesn't have a __str__ method
> defined (e.g. it's an empty class). All of your other
> tests have a class that does have a __str__ method
> because it was inherited from the baseclass list.
>
> You could try:
>
> class another(list):
> pass
>
> Larry Bates
> Syscon, Inc.
>
> "Jim Newton" <jimka at rdrop.com> wrote in message
> news:2ik7qrFo8httU1 at uni-berlin.de...
>
>>hi all, does anyone know what print does if there is no __str__ method?
>>i'm trying ot override the __repr__. If anyone can give me some advice
>>it would be great to have.
>>
>>I have defined a lisp-like linked list class as a subclass of list.
>>The __iter__ seems to work as i'd like, by traversing the links,
>>and the __repr__ seems to work properly for somethings but not others.
>>
>>The basic idea is that a list such as [ 1, 2, 3, 4] is converted to
>>[1, [2, [3, [4, nil]]], thus allowing me to push (cons) a new element
>>non-destructively onto the beginning of the list; e.g.
>>
>>x = seq2pair( [ 3,4,5]) --> [ 3, [4, [5, nil]]]
>>y = x.cons(2) --> [ 2, [3, [4, [5, nil]]]
>>z = y.cons(1) --> [ 1, [ 2, [3, [4, [5, nil]]]]
>>
>>for elt in z: # iterates elt=1, elt=2, elt=3 ...
>> pass
>>
>>I would love to know how to define the __repr__ or __str__
>>method so that it is able to print everything the way print
>>normally works, except that instances of my class gets printed
>>special. I want to print [ 1, [ 2, [3, [4, [5, nil]]]]
>>simple as a space seperated list. It works most of the time.
>>--> ( 1 2 3 4 5)
>>
>>Another question is whether there is a way to promote an
>>empty list [] to an instance of Pair?
>>
>>class Pair(list):
>>
>> def __iter__(self):
>> while self:
>> yield self.car()
>> self = self.cdr()
>>
>> def __repr__(self):
>> middle = " ".join( [ substr.__str__() for substr in self])
>> return "( " + middle + " )"
>>
>> # x = (cons x l_y)
>> # ==> x = l_y.cons(x)
>> def cons(self, car):
>> new = Pair()
>> new.append(car)
>> new.append(self)
>> return new
>>
>> def car(self):
>> if self:
>> return self[0]
>> return nil
>>
>> def cdr(self):
>> if len(self)<2:
>> return nil
>> return self[1]
>>
>>nil = Pair()
>>
>>
>># [ 1, 2, 3] --> [1, [2, [3, nil]]]
>>def seq2pair(seq):
>> new = Pair()
>> for index in xrange( len(seq), 0, -1):
>> new = new.cons(seq[index - 1])
>> return new
>>
>>mylist = seq2pair( [1,2,3,4,5])
>>print mylist # correctly prints --> ( 1 2 3 4 5)
>>
>>
>>mylist2 = seq2pair( [11.1, 21.1, 31.1, 41.1, mylist])
>>print mylist2
>># correctly prints --> ( 11.1 21.1 31.1 41.1 ( 1 2 3 4 5 ) )
>>
>>class another:
>> pass
>>
>>print another() # prints --> <__main__.another instance at 0x8132b64>
>>
>># ????????????????????????????????????????
>>print seq2pair( [ another(), another() ]) # FAILS
>># ????????????????????????????????????????
>>
>>Traceback (most recent call last):
>> File "xyz.py", line 52, in ?
>> print seq2pair( [ another(), another() ])
>> File "xyz.py", line 13, in __repr__
>> return "( " + " ".join( [ substr.__str__() for substr in self]) +
>
> " )"
>
>>AttributeError: another instance has no attribute '__str__'
>>
>
>
>
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