When the Python interpreter deals with a .py file, is it different from dealing with a single statement?

Question

Running:

a = 257
b = 257
print id(a) == id(b)

Results in:

Same statement but opposite results. Why?

Answer 1

The code in test.py is parsed together which is more optimizable than the code parsed as separate statements in the interpreter

When you put it in a test.py and run it as a whole, the byte code compiler has a better chance of analyzing the usage of literals and optimizing them. (Hence you get a and b pointing to the same place)

As opposed to when you run the separate statements (parsed separately) in the interpreter (where I think it only optimizes up to 256 but not 257 via preallocation)

Play with this in the interpreter to see the effect of separate statements:

>>> a, b = 257, 257      # or if you prefer: a = 257; b = 257
>>> print a is b
True

>>> a = 257
>>> b = 257
>>> print a is b
False

Defining a function in the interperter also gives it a change to analyze and optimize the used literals

>>> def test():
...     a = 257
...     b = 257
...     print a is b
... 
>>> test()
True

This optimization is not limited to integers only, works e.g. for floats too (floats are not subject to a cache like integers in the [-5, 256] range are)

>>> def test():
...     pi = 3.14
...     x = 3.14
...     return x is pi
... 
>>> test()
True

# As opposed to separate statements:
>>> pi = 3.14
>>> x = 3.14
>>> x is pi
False

Looking at the byte code to see that it indeed reuses the same constant

>>> dis.dis(test)
  2           0 LOAD_CONST               1 (3.14)
              3 STORE_FAST               0 (pi)

  3           6 LOAD_CONST               1 (3.14) <-- Same constant 1 reused
              9 STORE_FAST               1 (x)

  4          12 LOAD_FAST                1 (x)
             15 LOAD_FAST                0 (pi)
             18 COMPARE_OP               8 (is)
             21 RETURN_VALUE