how to use __str__ and __repr__?

Jim Newton jimka at rdrop.com
Mon Jun 7 23:21:48 CEST 2004


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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