About the changing id of an immutable string

Question

Something about the id of objects of type str (in python 2.7) puzzles me. The str type is immutable, so I would expect that once it is

Answer 1

This behavior is specific to the Python interactive shell. If I put the following in a .py file:

print id('so')
print id('so')
print id('so')

and execute it, I receive the following output:

2888960
2888960
2888960

In CPython, a string literal is treated as a constant, which we can see in the bytecode of the snippet above:

  2           0 LOAD_GLOBAL              0 (id)
              3 LOAD_CONST               1 ('so')
              6 CALL_FUNCTION            1
              9 PRINT_ITEM          
             10 PRINT_NEWLINE       

  3          11 LOAD_GLOBAL              0 (id)
             14 LOAD_CONST               1 ('so')
             17 CALL_FUNCTION            1
             20 PRINT_ITEM          
             21 PRINT_NEWLINE       

  4          22 LOAD_GLOBAL              0 (id)
             25 LOAD_CONST               1 ('so')
             28 CALL_FUNCTION            1
             31 PRINT_ITEM          
             32 PRINT_NEWLINE       
             33 LOAD_CONST               0 (None)
             36 RETURN_VALUE  

The same constant (i.e. the same string object) is loaded 3 times, so the IDs are the same.