[Python-ideas] Support parsing stream with `re`

[Ram Rachum]

Hi Ned! I'm happy to see you here.

I'm doing multi-color 3d-printing. The slicing software generates a GCode
file, which is a text file of instructions for the printer, each command
meaning something like "move the head to coordinates x,y,z while extruding
plastic at a rate of w" and lots of other administrative commands. (Turn
the print fan on, heat the extruder to a certain temperature, calibrate the
printer, etc.)

Here's an example of a simple GCode from a print I did last week:
https://www.dropbox.com/s/kzmm6v8ilcn0aik/JPL%20Go%20hook.gcode?dl=0

It's 1.8MB in size. They could get to 1GB for complex prints.

Multi-color prints means that at some points in the print, usually in a
layer change, I'm changing the color. This means I need to insert an M600
command, which tells the printer to pause the print, move the head around,
and give me a prompt to change the filament before continuing printing.

I'm sick of injecting the M600 manually after every print. I've been doing
that for the last year. I'm working on a script so I could say "Insert an
M600 command after layers 5, 88 and 234, and also before process Foo."

The slicer inserts comments saying "; layer 234" Or "; process Foo". I want
to identify the entire layer as one match. That's because I want to find
the layer and possibly process at the start, I want to find the last
retraction command, the first extrusion command in the new layer, etc. So
it's a regex that spans potentially thousands of lines.

Then I'll know just where to put my M600 and how much retraction to do
afterwards.


Thanks,
Ram.


On Sat, Oct 6, 2018 at 6:58 PM Ned Batchelder <ned at nedbatchelder.com> wrote:

> On 10/6/18 7:25 AM, Ram Rachum wrote:
>
> "This is a regular expression problem, rather than a Python problem."
>
> Do you have evidence for this assertion, except that other regex
> implementations have this limitation? Is there a regex specification
> somewhere that specifies that streams aren't supported? Is there a
> fundamental reason that streams aren't supported?
>
>
> "Can the lexing be done on a line-by-line basis?"
>
> For my use case, it unfortunately can't.
>
>
> You mentioned earlier that your use case doesn't have to worry about the
> "a.*b" problem.  Can you tell us more about your scenario?  How would the
> stream know it had read enough to match or not match?  Perhaps that same
> logic can be used to feed the data in chunks?
>
> --Ned.
>
>
> On Sat, Oct 6, 2018 at 1:53 PM Jonathan Fine <jfine2358 at gmail.com> wrote:
>
>> Hi Ram
>>
>> You wrote:
>>
>> > I'd like to use the re module to parse a long text file, 1GB in size. I
>> > wish that the re module could parse a stream, so I wouldn't have to load
>> > the whole thing into memory. I'd like to iterate over matches from the
>> > stream without keeping the old matches and input in RAM.
>>
>> This is a regular expression problem, rather than a Python problem. A
>> search for
>>     regular expression large file
>> brings up some URLs that might help you, starting with
>>
>> https://stackoverflow.com/questions/23773669/grep-pattern-match-between-very-large-files-is-way-too-slow
>>
>> This might also be helpful
>> https://svn.boost.org/trac10/ticket/11776
>>
>> What will work for your problem depends on the nature of the problem
>> you have. The simplest thing that might work is to iterate of the file
>> line-by-line, and use a regular expression to extract matches from
>> each line.
>>
>> In other words, something like (not tested)
>>
>>    def helper(lines):
>>        for line in lines:
>>            yield from re.finditer(pattern, line)
>>
>>     lines = open('my-big-file.txt')
>>     for match in helper(lines):
>>         # Do your stuff here
>>
>> Parsing is not the same as lexing, see
>> https://en.wikipedia.org/wiki/Lexical_analysis
>>
>> I suggest you use regular expressions ONLY for the lexing phase. If
>> you'd like further help, perhaps first ask yourself this. Can the
>> lexing be done on a line-by-line basis? And if not, why not?
>>
>> If line-by-line not possible, then you'll have to modify the helper.
>> At the end of each line, they'll be a residue / remainder, which
>> you'll have to bring into the next line. In other words, the helper
>> will have to record (and change) the state that exists at the end of
>> each line. A bit like the 'carry' that is used when doing long
>> addition.
>>
>> I hope this helps.
>>
>> --
>> Jonathan
>>
>>
>
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