[Python-bugs-list] [ python-Bugs-468061 ] __str__ ignored in str subclass

[noreply@sourceforge.net]

Bugs item #468061, was opened at 2001-10-04 13:52
You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=468061&group_id=5470

Category: Type/class unification
Group: Python 2.2
>Status: Closed
>Resolution: Fixed
Priority: 5
Submitted By: Tim Peters (tim_one)
Assigned to: Tim Peters (tim_one)
Summary: __str__ ignored in str subclass

Initial Comment:
>From c.l.py; looks messy (e.g., if PyObject_String 
didn't ignore octetstring.__str__, the call to str() 
in the latter would just recurse forever; and he's 
right that b2a_hex requires a genuine string; and e.g.

    return b2a_hex(self + "")

would be mighty obscure).

 ----Original Message-----
From: Andreas.Trawoeger
Sent: Thursday, October 04, 2001 7:10 AM
To: python-list
Subject: Python 2.2a4: Problems with overloading 
__str__ in subclasses of str

I would like to write a class that behaves like a 
string only with the difference that it displays the 
string in hex.  In Python 2.2a4 this should be 
possible by subclassing str and overloading __repr__ 
and __str__ :

from binascii import b2a_hex

class octetstring(str):
    def __repr__(self):
        return b2a_hex(str(self))
    def __str__(self):
        return b2a_hex(str(self))

o = octetstring('A')

print "o.__repr__():",o.__repr__()
print "o.__str__() :",o.__str__()
print "str(o)      :",str(o)
print "o           :",o

The result is pretty strange:

o.__repr__(): 41
o.__str__() : 41
str(o)      : A
o           : A

I'm not shure what's the exact reason for this 
problem. Either __str__ isn't used for str() and 
print.  Or I'm trapped in an chicken egg problem:
I have to convert self to an string (otherwise b2a_hex 
won't work) to do this Python should use __str__ 
(which is the method I'm changing).

cu andreas


----------------------------------------------------------------------

>Comment By: Tim Peters (tim_one)
Date: 2001-10-16 13:21

Message:
Logged In: YES 
user_id=31435

Closed.  str() and print applied to subclasses of str now 
respect a __str__ method override (if any).  Ditto for the 
corresponding C API functions (PyObject_Str() in 
particular).

Lib/test/test_descr.py; new revision: 1.90
Objects/object.c; new revision: 2.157
Objects/stringobject.c; new revision: 2.139


----------------------------------------------------------------------

Comment By: Tim Peters (tim_one)
Date: 2001-10-11 19:44

Message:
Logged In: YES 
user_id=31435

Bug 470040 has been fixed, so you'll be able to pass 
instances of str subclasses directly to b2a_hex (which uses 
the buffer interface to obtain the value; buffer methods 
simply weren't getting inherited; OTOH, I'm not sure this 
is such a great thing, since I'm not sure a str subclass in 
Python can override the buffer methods now inherited from 
str!).

----------------------------------------------------------------------

Comment By: Tim Peters (tim_one)
Date: 2001-10-10 15:52

Message:
Logged In: YES 
user_id=31435

The business about b2a_hex not accepting an object of a 
subclass of str is now the subject of bug 470040.

----------------------------------------------------------------------

Comment By: Tim Peters (tim_one)
Date: 2001-10-04 13:57

Message:
Logged In: YES 
user_id=31435

Looks like I assigned this to Guido by mistake; back to me.

----------------------------------------------------------------------

You can respond by visiting: 
http://sourceforge.net/tracker/?func=detail&atid=105470&aid=468061&group_id=5470