Where is this python method defined?

Question

I'm currently reading the code of virt-manager/virt-clone, but did not do much python before.

Here is the important parts of the code:

https://github.com/virt-manager/virt-manager/blob/master/virt-clone#L169

conn = cli.getConnection(options.connect)
design = Cloner(conn)

https://github.com/virt-manager/virt-manager/blob/master/virtinst/cli.py#L263

from .connection import VirtualConnection
conn = VirtualConnection(uri)

https://github.com/virt-manager/virt-manager/blob/master/virtinst/connection.py#L35

class VirtualConnection(object):

https://github.com/virt-manager/virt-manager/blob/master/virtinst/cloner.py#L591

return self.conn.lookupByName(name)

The last line looks like there is a method lookupByName called for an object of class VirtualConnection. However I cannot find such a method defined in that class and it does not have a parent class to get such a method from, either.

Can you help me spot the place where lookupByName is defined?

What might be a good way to find such an answer myself? (debugger, etc?)

Dan D. · Accepted Answer · 2016-03-23 11:44:18Z

2

The lookupByName method of the virConnect object is defined in C as part of libvirt's Python bindings. The following code forwards the attribute accesses from it to that object which was returned by libvirt.openAuth and assigned to self._libvirtconn:

https://github.com/virt-manager/virt-manager/blob/master/virtinst/connection.py#L94

def __getattr__(self, attr):
    if attr in self.__dict__:
        return self.__dict__[attr]

    # Proxy virConnect API calls
    libvirtconn = self.__dict__.get("_libvirtconn")
    return getattr(libvirtconn, attr)

This causes conn.lookupByName to be conn._libvirtconn.lookupByName.

answered Mar 23, 2016 at 11:44

Dan D.

75k15 gold badges111 silver badges129 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Marcin Cuprjak · Accepted Answer · 2016-03-23 11:43:13Z

0

I haven't investigated your case, but in Python methods can be dynamically added to a class or to an instance

class A:
    def a(self):
        print 'a'

class C(A):
    def c(self):
        print 'c'

def foo(arg):
    print 'foo',arg

a = C()
print dir(a)

C.f = foo

a.a()
a.c()
a.f()
print dir(a)

the result is:

['__doc__', '__module__', 'a', 'c']
a
c
foo <__main__.C instance at 0x024CB9B8>
['__doc__', '__module__', 'a', 'c', 'f']

As you can see the second call to dir shows new method 'f'. So in some cases it might be tricky to find the origin of that.

answered Mar 23, 2016 at 11:43

Marcin Cuprjak

6945 silver badges6 bronze badges

Comments

Ronald Oussoren · Accepted Answer · 2016-03-23 11:44:38Z

0

The VirtualConnection class you refer to has an implementation of __getattr__, which is a method that will be called when an attribute cannot be resolved.

This __getattr__ method looks up attributes on a "_libvirtconn" attribute of the VirtualConnection instance.

See also: https://docs.python.org/3/reference/datamodel.html#object.getattr

answered Mar 23, 2016 at 11:44

Ronald Oussoren

2,82021 silver badges31 bronze badges

Collectives™ on Stack Overflow

Where is this python method defined?

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related