Python's "only one way to do it" philosophy isn't good?

Steven D'Aprano steve at REMOVE.THIS.cybersource.com.au
Sun Jun 10 02:34:21 CEST 2007


On Sat, 09 Jun 2007 22:52:32 +0000, Josiah Carlson wrote:

> the only thing that optimization 
> currently does in Python at present is to discard docstrings

Python, or at least CPython, does more optimizations than that. Aside from
run-time optimizations like interned strings etc., there are a small
number of compiler-time optimizations done.

Running Python with the -O (optimize) flag tells Python to ignore
assert statements. Using -OO additionally removes docstrings.

Regardless of the flag, in function (and class?) definitions like the
following:

def function(args):
    "Doc string"
    x = 1 
    s = "this is a string constant"
    "and this string is treated as a comment"
    return s*x

The string-comment is ignored by the compiler just like "real" comments.
(The same doesn't necessarily hold for other data types.)


Some dead code is also optimized away:

>>> def function():
...     if 0:
...             print "dead code"
...     return 2
...
>>> dis.dis(function)
  4           0 LOAD_CONST               1 (2)
              3 RETURN_VALUE


Lastly, in recent versions (starting with 2.5 I believe) Python includes a
peephole optimizer that implements simple constant folding: 

# Python 2.4.3
>>> dis.dis(lambda: 1+2)
  1           0 LOAD_CONST               1 (1)
              3 LOAD_CONST               2 (2)
              6 BINARY_ADD
              7 RETURN_VALUE

# Python 2.5
>>> dis.dis(lambda: 1+2)
  1           0 LOAD_CONST               2 (3)
              3 RETURN_VALUE


The above all holds for CPython. Other Pythons may implement other
optimizations.



-- 
Steven.




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