[Python-bugs-list] [ python-Bugs-420304 ] getattr function w/ default

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Bugs item #420304, was updated on 2001-04-30 17:14
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Category: Python Interpreter Core
Group: None
Status: Open
Resolution: None
Priority: 6
Submitted By: Greg Chapman (glchapman)
Assigned to: Jeremy Hylton (jhylton)
Summary: getattr function w/ default

Initial Comment:
In Python 2.1, when calling the builtin getattr 
function and supplying the (optional) default 
argument, this default is returned on any exception 
raised during PyObject_GetAttr, even if the exception 
is not an AttributeError.  I did not expect this 
behavior after reading the getattr documentation.  Is 
this the  intended behavior?  (If so, I think the 
documentation should be made clearer on this point.)

Example (providing a non-string as the attribute name):

>>> class Test:
... 	pass
... 
>>> t = Test()
>>> getattr(t, 1, "hello")
'hello'
>>> 


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Comment By: Michael Hudson (mwh)
Date: 2001-05-22 11:42

Message:
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Well, the patch I posted below allows unicode (though
admittedly only for strings that can be encoded using the
default encoding).

Have you read the code to object.c:PyObject_GetAttr?  All my
code does is hoist some of the error checking out of there
into getattr to better decide whether to return the provided
default value.

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Comment By: Walter Dörwald (doerwalter)
Date: 2001-05-22 11:29

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But unicode attr arguments should be allowed, because 
someday in the far future Python might be unicode based.

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Comment By: Jeremy Hylton (jhylton)
Date: 2001-05-07 21:26

Message:
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This looks like a bug to me, too.  I think a non-string attr
argument
to getattr()  should raise TypeError.


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Comment By: Michael Hudson (mwh)
Date: 2001-05-04 04:11

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But consider this:

>>> getattr({},1)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: attribute name must be string
>>> getattr({},1,"e")
'e'

or

>>> getattr(None,u"\343","e")
'e'
>>> getattr(None,u"\343")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
UnicodeError: ASCII encoding error: ordinal not in range(128)

Here's a possible patch:

Index: bltinmodule.c
===================================================================
RCS file: /cvsroot/python/python/dist/src/Python/bltinmodule.c,v
retrieving revision 2.200
diff -c -r2.200 bltinmodule.c
*** bltinmodule.c       2001/05/02 07:39:38     2.200
--- bltinmodule.c       2001/05/04 11:09:35
***************
*** 845,850 ****
--- 845,854 ----
  The globals and locals are dictionaries, defaulting to the
current\n\
  globals and locals.  If only globals is given, locals
defaults to it.";
  
+ /* Internal API needed by builtin_getattr(): */
+ extern 
+ PyObject *_PyUnicode_AsDefaultEncodedString(PyObject *unicode,
+                                 const char *errors);
  
  static PyObject *
  builtin_getattr(PyObject *self, PyObject *args)
***************
*** 854,859 ****
--- 858,874 ----
  
        if (!PyArg_ParseTuple(args, "OO|O:getattr", &v,
&name, &dflt))
                return NULL;
+       if (PyUnicode_Check(name)) {
+               name =
_PyUnicode_AsDefaultEncodedString(name, NULL);
+               if (name == NULL)
+                       return NULL;
+       } 
+       else if (!PyString_Check(name)) {
+               PyErr_Format(PyExc_TypeError,
+            "getattr: attribute name must be string, not
\%.500s\",
+                            name->ob_type->tp_name);
+               return NULL;
+       }
        result = PyObject_GetAttr(v, name);
        if (result == NULL && dflt != NULL) {
                PyErr_Clear();

... though by the time you've done all that, there's not
much point in calling PyObject_GetAttr at all...

OTOH, this changes behaviour, so maybe one should just
clarify this in the docs.

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Comment By: Chris Cogdon (chriscog)
Date: 2001-05-03 17:55

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Looks like an attribute error to me. consider this:

>>> getattr (t,fred,"thingy")
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: There is no variable named 'fred'

Ie, in your own example, it couldn't find an attribute '1' 
in the class, even if its an integer attribute (which is 
only obtainable using getattr/setattr). That's exactly what 
AttributeError is

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