Question

I created the following example to understand the instances in python

import time;
class test:
    mytime = time.time();   
    def __init__(self):
        #self.mytime = time.time();
        time.sleep(1);
        pass


from test import test

test1 = test()
test2 = test()

print test1.mytime
print test2.mytime

test1.mytime = 12

print test1.mytime
print test2.mytime

in this case the output id the following:

1347876794.72
1347876794.72
12
1347876794.72

I expected that the test2.mytime is bigger with 1 second than the test1.mytime. Why doesn’t created a copy about the mytime in each instance?

Answer 1

Let’s look at those lines:

class test:
   mytime = time.time();   

Here you set class member value, which is calculated once when class definition is executed, so time.time() is calculated once when module that contains class test is loaded.

Every instance of the class test will receive that precalculated value and you must override that value (e.g. in __init__ method), accessing it via self (which is a special argument that stores reference to an instance), thus setting instance member value.