为什么这两个功能不同？

Question

Take a look at this:

>>> def f():
...     return (2+3)*4
... 
>>> dis(f)
  2           0 LOAD_CONST               5 (20)
              3 RETURN_VALUE  

Evidently, the compiler has pre-evaluated (2+3)*4 , which makes sense.

Now, if I simply change the order of the operands of * :

>>> def f():
...     return 4*(2+3)
... 
>>> dis(f)
  2           0 LOAD_CONST               1 (4)
              3 LOAD_CONST               4 (5)
              6 BINARY_MULTIPLY     
              7 RETURN_VALUE  

The expression is no longer fully pre-evaluated! What is the reason for this? I am using CPython 2.7.3.

Answer 1

In the first case the unoptimized code is LOAD 2 LOAD 3 ADD LOAD 4 MULTIPLY and in the second case it's LOAD 4 LOAD 2 LOAD 3 ADD MULTIPLY . The pattern matcher in fold_binops_on_constants() must handle the first ADD ok (replacing LOAD LOAD ADD with LOAD ) and then follows on to do the same thing to MULTIPLY . In the second case by the time the ADD (now the second argument to MULTIPLY instead of the first) is turned into a constant the scanner is too far ahead to see LLM (when the "cursor" was on LOAD 4 it didn't look like a LLM yet).

Answer 2

Looks like this issue was patched in Python 3.3, as can be seen here .

>>> def f():
...     return (2+3)*4
... 
>>> dis(f)
  2           0 LOAD_CONST               5 (20)
              3 RETURN_VALUE  
>>> def f():
...     return 4*(2+3)
... 
>>> dis(f)
  2           0 LOAD_CONST               5 (20)
              3 RETURN_VALUE