On Jun 20, 8:03 am, eliben <[EMAIL PROTECTED]> wrote: > On Jun 20, 9:17 am, Bruno Desthuilliers <bruno. > > [EMAIL PROTECTED]> 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 -- http://mail.python.org/mailman/listinfo/python-list