On 13.02.13 08:42, Lennart Regebro wrote:
Something is needed - a patch for PyPy or for the documentation I guess.
Not arguing that it wouldn't be good, but I disagree that it is needed.
This is only an issue when you, as in your proof, have a loop that
does concatenation. This is usually when looping over a list of
strings that should be concatenated together. Doing so in a loop with
concatenation may be the natural way for people new to Python, but the
"natural" way to do it in Python is with a ''.join() call.
This:
s = ''.join(('X' for x in xrange(x)))
Is more than twice as fast in Python 2.7 than your example. It is in
fact also slower in PyPy 1.9 than Python 2.7, but only with a factor
of two:
Python 2.7:
time for 10000000 concats = 0.887
Pypy 1.9:
time for 10000000 concats = 1.600
(And of course s = 'X'* x takes only a bout a hundredth of the time,
but that's cheating. ;-)
This is not about how to write efficient concatenation and not
for me. It is also not about a constant factor, which I don't really
care about but in situations where speed matters.
This is about a possible algorithmic trap, where code written for
CPython may behave well with some roughly O(n) behavior,
and by switching to PyPy you get a surprise when the same
code now has O(n**2) behavior. Such runtime explosions can damage
the trust in PyPy, with code sitting in some module which you even
did not write but "pip install"-ed it.
So this is important to know, especially for newcomers, and for people
who are giving advice to them.
For algorithmic compatibility, there should no longer
be a feature with this drastic side effect, if that cannot be supported by
all other dialects.
To avoid such hidden traps in larger code bases, documentation is
needed that clearly gives a warning saying "don't do that", like CS
students learn for most other languages.
cheers - chris
--
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