Why is local variable access faster than class member access in Python?

Question

While trying to tackle a more complex problem, I came to compare access speed to local variable vs member variables.

Here a test program:

#!/usr/bin/         


        

Answer 1

Local names is faster because Python does some optimization that local names don't need dict access, on the other hand, instance attributes need to access the __dict__ of the object.

This is also the reason why local names are faster than global names.

Answer 2

self.m += 1 means you have to look up a local variable called self and then find the attribute called m

Of course if you just have to look up a local variable, it will be faster without the extra step.

It can be useful to look at what is happening under the hood:

>>> import dis
>>> dis.dis(StressTestLocal.do_work)
 18           0 LOAD_CONST               1 (0)
              3 STORE_FAST               1 (m)

 19           6 LOAD_FAST                1 (m)
              9 LOAD_CONST               2 (1)
             12 INPLACE_ADD         
             13 STORE_FAST               1 (m)

 20          16 LOAD_FAST                1 (m)
             19 LOAD_CONST               3 (2)
             22 INPLACE_MULTIPLY    
             23 STORE_FAST               1 (m)
             26 LOAD_CONST               0 (None)
             29 RETURN_VALUE        
>>> dis.dis(StressTestMember.do_work)
 10           0 LOAD_FAST                0 (self)
              3 DUP_TOP             
              4 LOAD_ATTR                0 (m)
              7 LOAD_CONST               1 (1)
             10 INPLACE_ADD         
             11 ROT_TWO             
             12 STORE_ATTR               0 (m)

 11          15 LOAD_FAST                0 (self)
             18 DUP_TOP             
             19 LOAD_ATTR                0 (m)
             22 LOAD_CONST               2 (2)
             25 INPLACE_MULTIPLY    
             26 ROT_TWO             
             27 STORE_ATTR               0 (m)
             30 LOAD_CONST               0 (None)
             33 RETURN_VALUE