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classification
Title: getattr function w/ default
Type: Stage:
Components: Interpreter Core Versions:
process
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: jhylton Nosy List: chriscog, doerwalter, glchapman, jhylton, lemburg, mwh, tim.peters
Priority: high Keywords:

Created on 2001-05-01 00:14 by glchapman, last changed 2022-04-10 16:04 by admin. This issue is now closed.

Messages (9)
msg4566 - (view) Author: Greg Chapman (glchapman) Date: 2001-05-01 00:14
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'
>>> 
msg4567 - (view) Author: Chris Cogdon (chriscog) * Date: 2001-05-04 00:55
Logged In: YES 
user_id=67116

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
msg4568 - (view) Author: Michael Hudson (mwh) (Python committer) Date: 2001-05-04 11:11
Logged In: YES 
user_id=6656

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.
msg4569 - (view) Author: Jeremy Hylton (jhylton) (Python triager) Date: 2001-05-08 04:26
Logged In: YES 
user_id=31392

This looks like a bug to me, too.  I think a non-string attr
argument
to getattr()  should raise TypeError.
msg4570 - (view) Author: Walter Dörwald (doerwalter) * (Python committer) Date: 2001-05-22 18:29
Logged In: YES 
user_id=89016

But unicode attr arguments should be allowed, because 
someday in the far future Python might be unicode based.
msg4571 - (view) Author: Michael Hudson (mwh) (Python committer) Date: 2001-05-22 18:42
Logged In: YES 
user_id=6656

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.
msg4572 - (view) Author: Marc-Andre Lemburg (lemburg) * (Python committer) Date: 2001-05-22 20:13
Logged In: YES 
user_id=38388

I guess the right thing to do here, would be to only mask
AttributeErrors in the getattr() builtin.
msg4573 - (view) Author: Tim Peters (tim.peters) * (Python committer) Date: 2001-05-23 01:18
Logged In: YES 
user_id=31435

Oh no:  getattr hooks can raise any exception whatsoever, 
and existing code relies on getattr() treating any 
exception whatsoever as meaning "no attribute of that 
name".  For example, IndexError and KeyError are often 
raised naturally by getattr hooks that fail to resolve a 
name via tree or dict structures.

The specific complaint here is a different story:  the docs 
say the attribute argument must be a string (Unicode, 
whatever -- "a string" <wink>), and I agree that calling 
getattr() with something other than a string argument 
should raise a TypeError instead of pretending it's a 
*possible* attribute name and that there simply isn't any 
attribute with that name (it's not a name at all, so the 
call doesn't make sense).

Jeremy, I see this is assigned to you now.  There are 
enough conflicting opinions here (at least Michael's and 
mine seem to coincide) that perhaps it's time to dump it on 
Guido for Pronouncement.
msg4574 - (view) Author: Jeremy Hylton (jhylton) (Python triager) Date: 2001-07-30 22:41
Logged In: YES 
user_id=31392

Don't think there's any actual dissent here.  You, Michael,
and I all seem to agree.   I'll commit Michael's suggestion
and live with the duplicated code.

Fixed in rev 2.220 of bltinmodule.c
History
Date User Action Args
2022-04-10 16:04:01adminsetgithub: 34437
2001-05-01 00:14:28glchapmancreate