Python: Checking if a 'Dictionary' is empty doesn't seem to work

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()))

Here are three ways you can check if dict is empty. I prefer using the first way only though. The other two ways are way too wordy.

test_dict = {}

if not test_dict:
    print "Dict is Empty"


if not bool(test_dict):
    print "Dict is Empty"


if len(test_dict) == 0:
    print "Dict is Empty"

d = {}
print(len(d.keys()))

If the length is zero, it means that the dict is empty.

Simple ways to check an empty dict are below:

a = {}

1. if a == {}:
     print ('empty dict')
   

2. if not a:
     print ('empty dict')
   

Method 1 is more strict, because when a = None, method 1 will provide the correct result, but method 2 will give an incorrect result.