Empty dictionaries evaluate to False in Python:

>>> dct = {}
>>> bool(dct)
False
>>> not dct
True
>>>

Thus, your isEmpty function is unnecessary. All you need to do is:

def onMessage(self, socket, message):
    if not self.users:
        socket.send("Nobody is online, please use REGISTER command" \
                    " in order to register into the server")
    else:
        socket.send("ONLINE " + ' ' .join(self.users.keys()))

if not a:
    print("List is empty")

Using the implicit booleanness of the empty list is quite Pythonic.

You can use all function for this task following way, consider following example

d = {"A": "abc", "B": "", "C": "ghi"}
any_empty = not all(d.values())
print(any_empty)

output:

True

I used all here to detect if all values are truth-y then negate is at will be False if any value is False. Note that this is actually sensitive to bool(value) so it will also detect None, False and so on, so please consider if this is acceptable in your case.

You need to implement the __nonzero__ method (or __bool__ for Python3)

https://docs.python.org/2/reference/datamodel.html#object.nonzero

class my_class(object):
    def __init__(self):
        self.lis1 = []
        self.dict1 = {}

    def __nonzero__(self):
        return bool(self.lis1 or self.dict1)

obj = my_class()
if obj:
    print "Available"
else:
    print "Not available"

Python also checks the __len__ method for truthiness, but that doesn't seem to make sense for your example.

If you have a lot of attributes to check you may prefer to

return any((self.lis1, self.dict1, ...))