Using upstream - I'm getting the following results - is this what you expect (as it's not really clear to me from your description)?

[root@localhost tp-libvirt]# python
Python 2.7.5 (default, Feb 19 2014, 13:47:28) 
[GCC 4.8.2 20131212 (Red Hat 4.8.2-7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import libvirt
>>> con=libvirt.open(None)
>>> con.getLibVersion()
1002005
>>> con.baselineCPU([],0)
libvirt:  error : xmlCPUs in virConnectBaselineCPU must not be NULL
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3191, in baselineCPU
    if ret is None: raise libvirtError ('virConnectBaselineCPU() failed', conn=self)
libvirt.libvirtError: xmlCPUs in virConnectBaselineCPU must not be NULL
>>> ret = con.baselineCPU([], 0)
libvirt:  error : xmlCPUs in virConnectBaselineCPU must not be NULL
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3191, in baselineCPU
    if ret is None: raise libvirtError ('virConnectBaselineCPU() failed', conn=self)
libvirt.libvirtError: xmlCPUs in virConnectBaselineCPU must not be NULL
>>> print ret
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ret' is not defined
>>> 


Perhaps there's another example of a call that has an exception like you expect that will help me figure out exactly what you're looking for.

hmmm...

I reserved a RHEL7.0 beaker system, installed libvirt...

      Beaker Test information:
                         HOSTNAME=dell-pem600-01.rhts.eng.bos.redhat.com
                            JOBID=654101
                         RECIPEID=1357164
                    RESULT_SERVER=[::1]:7085
                           DISTRO=RHEL-7.0-20140507.0
                     ARCHITECTURE=x86_64


# yum install libvirt libvirt-python
...
---> Package libvirt-python.x86_64 0:1.1.1-29.el7 will be installed
...
# service libvirtd start
Redirecting to /bin/systemctl start  libvirtd.service
#  virsh list --all
 Id    Name                           State
----------------------------------------------------

#  python
Python 2.7.5 (default, Feb 11 2014, 07:46:25) 
[GCC 4.8.2 20140120 (Red Hat 4.8.2-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import libvirt
>>> con=libvirt.open(None)
>>> con.getLibVersion()
1001001
>>> con.baselineCPU([],0)
libvirt:  error : xmlCPUs in virConnectBaselineCPU must not be NULL
-1
>>> ret = con.baselineCPU([],0)
libvirt:  error : xmlCPUs in virConnectBaselineCPU must not be NULL
>>> 
#

But if I look at the git history for the 7.0 package - it's supposed to have the commit for bz 912170 which I believe is the fix for this issue. Additionally, if I look at the code in "/usr/lib64/python2.7/site-packages/libvirt.py" I see:

    def baselineCPU(self, xmlCPUs, flags=0):
        """Computes the most feature-rich CPU which is compatible with all given host CPUs. """
        ret = libvirtmod.virConnectBaselineCPU(self._o, xmlCPUs, flags)
        if ret is None: raise libvirtError ('virConnectBaselineCPU() failed', conn=self)
        return ret

...

Frustrating... but I think I found the commit I need now in the (now) split libvirt-python code - 8dd64f07ea2c4c53b51a5170f0a3db0c53a094da:

Author: Don Dugger <donald.d.dugger>
Date:   Tue Nov 26 16:53:20 2013 +0000

    Return right error code for baselineCPU
    
    This Python interface code is returning a -1 on errors for the
    `baselineCPU' API.  Since this API is supposed to return a pointer
    the error return value should really be VIR_PY_NONE.
    
    Signed-off-by: Don Dugger <donald.d.dugger>

diff --git a/libvirt-override.c b/libvirt-override.c
index 3ef81dd..4cc64b7 100644
--- a/libvirt-override.c
+++ b/libvirt-override.c
@@ -4496,13 +4496,13 @@ libvirt_virConnectBaselineCPU(PyObject *self ATTRIBUTE_U
 
         ncpus = PyList_Size(list);
         if (VIR_ALLOC_N(xmlcpus, ncpus) < 0)
-            return VIR_PY_INT_FAIL;
+            return VIR_PY_NONE;
 
         for (i = 0; i < ncpus; i++) {
             xmlcpus[i] = PyString_AsString(PyList_GetItem(list, i));
             if (xmlcpus[i] == NULL) {
                 VIR_FREE(xmlcpus);
-                return VIR_PY_INT_FAIL;
+                return VIR_PY_NONE;
             }
         }
     }
@@ -4514,13 +4514,13 @@ libvirt_virConnectBaselineCPU(PyObject *self ATTRIBUTE_U
     VIR_FREE(xmlcpus);
 
     if (base_cpu == NULL)
-        return VIR_PY_INT_FAIL;
+        return VIR_PY_NONE;
 
     pybase_cpu = PyString_FromString(base_cpu);
     VIR_FREE(base_cpu);
 
     if (pybase_cpu == NULL)
-        return VIR_PY_INT_FAIL;
+        return VIR_PY_NONE;
 
     return pybase_cpu;
 }

Since this is upstream and thus will be in 7.1, I assume you are not looking for a backport to 7.0, correct?  If not, then I'll figure what state to change this to...  If you are looking for a 7.0 backport, that'll probably mean this needs to be cloned to a 7.0.z stream (although I'm not quite sure of the process).

(In reply to John Ferlan from comment #3)
> Since this is upstream and thus will be in 7.1, I assume you are not looking
> for a backport to 7.0, correct?  If not, then I'll figure what state to
> change this to...  If you are looking for a 7.0 backport, that'll probably
> mean this needs to be cloned to a 7.0.z stream (although I'm not quite sure
> of the process).

Yes, I had set a flag to rhel-7.1.0, 
and there is no customer needs it be backport-ed to 7.0.

Thanks,
Jincheng Miao

Moving to POST since code is already upstream

Verify it as follows. The result is expected. Move its status to VERIFIED.

[root@localhost ~]# rpm -q libvirt libvirt-python
libvirt-1.2.8-8.el7.x86_64
libvirt-python-1.2.8-5.el7.x86_64


[root@localhost ~]# python
Python 2.7.5 (default, Feb 11 2014, 07:46:25) 
[GCC 4.8.2 20140120 (Red Hat 4.8.2-13)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import libvirt
>>> con=libvirt.open(None)
>>> con.baselineCPU([], 0)
libvirt:  error : xmlCPUs in virConnectBaselineCPU must not be NULL
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3326, in baselineCPU
    if ret is None: raise libvirtError ('virConnectBaselineCPU() failed', conn=self)
libvirt.libvirtError: xmlCPUs in virConnectBaselineCPU must not be NULL
>>> ret = con.baselineCPU([], 0)
libvirt:  error : xmlCPUs in virConnectBaselineCPU must not be NULL
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/site-packages/libvirt.py", line 3326, in baselineCPU
    if ret is None: raise libvirtError ('virConnectBaselineCPU() failed', conn=self)
libvirt.libvirtError: xmlCPUs in virConnectBaselineCPU must not be NULL
>>>

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-0323.html