CODEWITHSUNDEEP
pythonoptimization
Justin Standard
Justin Standard

Reputation: 21545

Does Assignment before return have a cost in python?

Given 2 similar python functions:

def get_text(filename):
    with open(filename, 'rb') as f:
       text = pickle.load(f)
    return text

and 

def get_text2(filename):
     with open(filename, 'rb') as f:
         return pickle.load(f)

Would there be any performance difference betwen get_text and get_text2 ? Or will the python interpreter still have to do an assignment under the covers before returning?

This question is mostly about the assignment to a local variable before return -- the unpickling a file part is just a convenient example I had handy.

Upvotes: 3

Views: 174

Answers (1)

Justin Standard
Justin Standard

Reputation: 21545

Thanks to @jonrsharpe for mentioning that I could dis this which I think provides the answer:

This done with python 3.7.1

>>> def get_text(filename):
...    with open(filename, 'rb') as f:
...       text = pickle.load(f)
...    return text
... 
>>> def get_text2(filename):
...    with open(filename, 'rb') as f:
...       return pickle.load(f)
... 
>>> import dis
>>> dis.dis(get_text)
  2           0 LOAD_GLOBAL              0 (open)
              2 LOAD_FAST                0 (filename)
              4 LOAD_CONST               1 ('rb')
              6 CALL_FUNCTION            2
              8 SETUP_WITH              16 (to 26)
             10 STORE_FAST               1 (f)

  3          12 LOAD_GLOBAL              1 (pickle)
             14 LOAD_METHOD              2 (load)
             16 LOAD_FAST                1 (f)
             18 CALL_METHOD              1
             20 STORE_FAST               2 (text)
             22 POP_BLOCK
             24 LOAD_CONST               0 (None)
        >>   26 WITH_CLEANUP_START
             28 WITH_CLEANUP_FINISH
             30 END_FINALLY

  4          32 LOAD_FAST                2 (text)
             34 RETURN_VALUE
>>> dis.dis(get_text2)
  2           0 LOAD_GLOBAL              0 (open)
              2 LOAD_FAST                0 (filename)
              4 LOAD_CONST               1 ('rb')
              6 CALL_FUNCTION            2
              8 SETUP_WITH              12 (to 22)
             10 STORE_FAST               1 (f)

  3          12 LOAD_GLOBAL              1 (pickle)
             14 LOAD_METHOD              2 (load)
             16 LOAD_FAST                1 (f)
             18 CALL_METHOD              1
             20 RETURN_VALUE
        >>   22 WITH_CLEANUP_START
             24 WITH_CLEANUP_FINISH
             26 END_FINALLY
             28 LOAD_CONST               0 (None)
             30 RETURN_VALUE

The get_text2 version which returns right away avoids the STORE_FAST, POP_BLOCK and LOAD_FAST instructions -- but I agree the cost is probably negligible.

I think in many cases returning right away results in shorter cleaner code as well.

Upvotes: 3

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