The built-in function map() currently swallows any exception that might have occured while trying to get an iterator from any parameter. This produces unexpected behaviour for applications that require a certain type of exception to be raised when __iter__() is called on their objects.

From 24179f82b7de, inside map_new():

973 /* Get iterator. */
974 curseq = PyTuple_GetItem(args, i+1);
975 sqp->it = PyObject_GetIter(curseq);
976 if (sqp->it == NULL) {
977 static char errmsg[] =
978 "argument %d to map() must support iteration";
979 char errbuf[sizeof(errmsg) + 25];
980 PyOS_snprintf(errbuf, sizeof(errbuf), errmsg, i+2);
981 PyErr_SetString(PyExc_TypeError, errbuf);
982 goto Fail_2;
983 }

We *must* check if there has been any other kind of exception already being set when returning from PyObject_GetIter before setting PyExc_TypeError in line 981. If there is none, it is ok to raise a TypeError; any other exception must be passed on.

For example: raising TooManyCacheMissesException in __iter__() causes map(foobar, myobject) to raise TypeError instead of TooManyCacheMissesException.

Workaround: use map(foobar, iter(myobject)). The explicit call to iter will either produce an iterator object (which returns self to map()) or raises the correct exception.

Python 3 is not affected as map_new() does not throw it's own TypeError in case PyObject_GetIter() fails.

There maybe compatibility issues which prevent such behavior change.

I agree with Ray. This is essentially a feature request which you say has already been implemented in Py 3 but which cannot go into Py2.7. Only fixes for bugs (discrepancies between doc and behavior) can go into 2.7. I suspect 2.6 and before acted the same way.