An idiom for code generation with exec

George Sakkis george.sakkis at gmail.com
Fri Jun 20 15:19:56 CEST 2008


On Jun 20, 8:03 am, eliben <eli... at gmail.com> wrote:
> On Jun 20, 9:17 am, Bruno Desthuilliers <bruno.
>
> 42.desthuilli... at websiteburo.invalid> wrote:
> > eliben a écrit :> Hello,
>
> > > In a Python program I'm writing I need to dynamically generate
> > > functions[*]
>
> > (snip)
>
> > > [*] I know that each time a code generation question comes up people
> > > suggest that there's a better way to achieve this, without using exec,
> > > eval, etc.
>
> > Just to make things clear: you do know that you can dynamically build
> > functions without exec, do you ?
>
> Yes, but the other options for doing so are significantly less
> flexible than exec.
>
> > > But in my case, for reasons too long to fully lay out, I
> > > really need to generate non-trivial functions with a lot of hard-coded
> > > actions for performance.
>
> > Just out of curiousity : could you tell a bit more about your use case
> > and what makes a simple closure not an option ?
>
> Okay.
>
> I work in the field of embedded programming, and one of the main uses
> I have for Python (and previously Perl) is writing GUIs for
> controlling embedded systems. The communication protocols are usually
> ad-hoc messages (headear, footer, data, crc) built on top of serial
> communication (RS232).
>
> The packets that arrive have a known format. For example (YAMLish
> syntax):
>
> packet_length: 10
> fields:
>   - name: header
>     offset: 0
>     length: 1
>   - name: time_tag
>     offset: 1
>     length: 1
>     transform: val * 2048
>     units: ms
>   - name: counter
>     offset: 2
>     length: 4
>     bytes-msb-first: true
>   - name: bitmask
>     offset: 6
>     length: 1
>     bit_from: 0
>     bit_to: 5
> ...
>
> This is a partial capability display. Fields have defined offsets and
> lengths, can be only several bits long, can have defined
> transformations and units for convenient display.
>
> I have a program that should receive such packets from the serial port
> and display their contents in tabular form. I want the user to be able
> to specify the format of his packets in a file similar to above.
>
> Now, in previous versions of this code, written in Perl, I found out
> that the procedure of extracting field values from packets is very
> inefficient. I've rewritten it using a dynamically generated procedure
> for each field, that does hard coded access to its data. For example:
>
> def get_counter(packet):
>   data = packet[2:6]
>   data.reverse()
>   return data
>
> This gave me a huge speedup, because each field now had its specific
> function sitting in a dict that quickly extracted the field's data
> from a given packet.

It's still not clear why the generic version is so slower, unless you
extract only a few selected fields, not all of them. Can you post a
sample of how you used to write it without exec to clarify where the
inefficiency comes from ?

George



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