what is the different between function and method in python

Recently, I’m concentrating on nose plugin development.
Some people would be confused with some descriptions in http://nose.readthedocs.io/en/latest/plugins/interface.html

wantFunction(function)
Return true to collect this function as a test, false to prevent it from being collected, and None if you don’t care.

Parameters: function – The function object being examined by the selector


wantMethod(method)
Return true to collect this method as a test, false to prevent it from being collected, and None if you don’t care.

Parameters: method (unbound method) – The method object being examined by the selector

what is the different between function and method?

After some experiments, we get the conclusion below:
- function is defined out of class in python
- method is defined in a class

>>> def a(): pass
>>> a
0x107063aa0>
>>> type(a)
'function'>

>>> class A:
...   def a(self): pass
>>> A.a

>>> type(A.a)
'instancemethod'>

import unittest

def test_function():
      print "test_function will be invoked by wantFunction"

class TestStringMethods(unittest.TestCase)：
     def test_method(self):
           print "test_method will be invoked by wantMethod"