Someone accidentally discovered a way to set attributes of built-in
types, even though the implementation tries to prevent this. For
example, you cannot modify the str type to add a new method. Let's
define the method first:
>>> def reverse(self):
... return self[::-1]
...
>>>
Using direct attribute assignment doesn't work:
>>> str.reverse = reverse
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: can't set attributes of built-in/extension type 'str'
>>>
Using the dictionary doesn't work either:
>>> str.__dict__['reverse'] = reverse
Traceback (most recent call last):
File "<stdin>", line 1, in ?
TypeError: object does not support item assignment
>>>
But here's a trick that *does* work:
>>> object.__setattr__(str, 'reverse', reverse)
>>>
Proof that it worked:
>>> "hello".reverse()
'olleh'
>>>
What to do about this? I *really* don't want changes to built-in
types to become a standard "hack", because there are all sorts of
things that could go wrong. (For one, built-in type objects are
static C variables, which are shared between multiple interpreter
contexts in the same process.)
--Guido van Rossum (home page: http://www.python.org/~guido/)
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