Parsing a serial stream too slowly
cs at zip.com.au
Tue Jan 24 03:23:43 EST 2012
On 24Jan2012 05:08, Steven D'Aprano <steve+comp.lang.python at pearwood.info> wrote:
| On Tue, 24 Jan 2012 10:49:41 +1100, Cameron Simpson wrote:
| > | def OnSerialRead(self, event):
| > | text = event.data
| > | self.sensorabuffer = self.sensorabuffer + text
| > | self.sensorbbuffer = self.sensorbbuffer + text
| > | self.sensorcbuffer = self.sensorcbuffer + text
| > Slow and memory wasteful. Supposing a sensor never reports? You will
| > accumulate an ever growing buffer string. And extending a string gets
| > expensive as it grows.
| I admit I haven't read this entire thread, but one thing jumps out at me.
| It looks like the code is accumulating strings by repeated +
| concatenation. This is risky.
| In general, you should accumulate strings into a list buffer, then join
| them into a single string in one call:
| buffer = 
| while something:
| return ''.join(buffer)
Yeah, but the OP needs to examine the string after every packet arrival,
so he doesn't get to defer the joining of the strings.
| Use of repeated string addition risks slow quadratic behaviour. The OP is
| reporting slow behaviour... alarms bells ring.
He's _inferring_ slow behaviour from the wrong results he was getting.
In fact he doesn't have slow behaviour (in that python isn't slow enough
in his case to cause trouble).
[...snip references to more depth on string concatenation...]
| > The slow: You're compiling the regular expression _every_ time you come
| > here (unless the re module caches things, which I seem to recall it may.
| It does.
| > But that efficiency is only luck.
| More deliberate design than good luck :)
The luck is in his program benefiting from it, not the decision of the
re authors to cache the source->compiled mapping.
| > Regex _is_ slow. It is good for flexible lexing, but generally Not Fast.
| I hope I will never be mistaken for a re fanboy, but credit where credit
| is due: if you need the full power of a regex, you almost certainly can't
| write anything in Python that will beat the re module.
Indeed not. But many many uses of the re module are for trivial lexing
stuff that is way simpler than the complexities involved in regexps
themselves (parse regexp, compiler from parse, parse string from regexp
And perhaps equally important, regexps are both cryptic and powerful, a
combination that makes it very easy to write either highly expensive
regexps or bighly buggy regexps - in his case a small regexp error was
his bug, and programs speed had nothing to do with it.
His program did have inefficiencies though, which were worth discussing.
Cameron Simpson <cs at zip.com.au> DoD#743
There are two ways of dealing with this problem: one is complicated and
messy, and the other is simple and very elegant. We don't have much time
left, so I'll just show you the complicated and messy way.
- Richard Feynman, 1981
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