Read/Write stream tags in Python

aris · July 13, 2012, 5:52pm

Hello all,
I’ve been trying to work with stream tags in python, mainly using the
grextras blocks coding guide
here:Home · guruofquality/grextras Wiki · GitHub…
https://github.com/guruofquality/grextras/wiki/Blocks-Coding-Guide#wiki-the-work-function.

I’ve set up a very simple flow graph, just a vector source, throttle and
file sink, and I would like to place a tag on the first element of the
vector that contains time metadata. I’m using this code:

def work(self, input_items, output_items):
item_index = ? #which output item gets the tag?
offset = self.nitems_written(0) + item_index
key = pmt.pmt_string_to_symbol(“example_key”)
value = pmt.pmt_string_to_symbol(“example_value”)

#write at tag to output port 0 with given absolute item offset
self.add_item_tag(0, offset, key, value)

I think the problem has something to do with not implementing the
correct modules. My question is, what are the appropriate modules to
import to read and write stream tags in python?

Thank You,

-Anisha

anisha · July 13, 2012, 6:54pm

On 07/13/2012 08:51 AM, Anisha G. wrote:

def work(self, input_items, output_items):
item_index = ? #which output item gets the tag?

set item_index to x so the tag gets associated with outputs_items[0][x]

You might want to look more that that coding guide, at lot of the
snippets are just work functions, but many contain full class
definitions with import statements. For example:

from gnuradio import gr
import gnuradio.extras

class my_sync_block(gr.block):

def __init__(self, args):
    gr.block.__init__(self, name="my block", in_sig=[numpy.int32],

out_sig=[numpy.int32])

def work(self, input_items, output_items):
    #work stuff...
    return len(output_items[0])

-josh

anisha · July 13, 2012, 7:10pm

Thanks!
Do you by any chance have links to specific code examples that could
help
out? I’ve been trying to look at old examples you have posted to others
on
this forum, however, some of the links from github seem to be broken.

anisha · July 13, 2012, 8:41pm

On 07/13/2012 10:08 AM, Anisha G. wrote:

Thanks!
Do you by any chance have links to specific code examples that could help
out? I’ve been trying to look at old examples you have posted to others on
this forum, however, some of the links from github seem to be broken.

Well, I dont have an example from python, but its just the same thing
without semicolons. So this should be very helpful:
http://gnuradio.org/cgit/gnuradio.git/tree/gr-uhd/examples/c++/

Let me know if the links are bad on one of my wiki pages.
At least I can fix those

-josh

anisha · July 13, 2012, 9:26pm

On 07/13/2012 12:08 PM, Anisha G. wrote:

Thanks for the examples, I will definitely be looking into them!
As for links, This one: http://gnuradio.org/cgit/jblum.git/log/?h=next is
broken, it comes from one of the blocks coding guide here:
http://gnuradio.org/redmine/projects/gnuradio/wiki/BlocksCodingGuide/diff/2which
i found really helpful.

There was a lot of work in various branches off of gnuradio which I had
merged into next. That branch is gone, and that work has been combined
into the GrExtras project. The link and relevant coding guide was
moved/updated: Home · guruofquality/grextras Wiki · GitHub

I also had one more quick question, I’m getting this error:
‘gr_top_block_sptr’ object has no attribute ‘add_item_tag’, though I
thought add_item_tag was inherited from gr_block.
Thanks again!

Not sure, you might send a code snippet. It looks like you are calling a
method of the top block, and not from the block’s work function?

-josh

anisha · July 13, 2012, 9:58pm

I don’t think I’m writing the work function…

def work(self, input_items, output_items):
item_index = 1 #which output item gets the tag?
offset = self.nitems_written(0) + item_index
key = pmt.pmt_string_to_symbol(“example_key”)
value = pmt.pmt_string_to_symbol(“example_value”)

    self.add_item_tag(0, offset, key, value)

#call work function
self.work(self.gr_vector_source_x_0, self.gr_file_sink_0)

Error: AttributeError: ‘gr_top_block_sptr’ object has no attribute
‘nitems_written’

anisha · July 13, 2012, 10:21pm

Also, I’ve been assuming that nitems_written/read is from
gr_block_detail,
and add_item_tag is from gr_block and that they have both been swigged,
since you used them in your example. Is it possible that they haven’t
been?
Thanks
-Anisha

anisha · July 13, 2012, 10:28pm

Aha, I knew I had unit tests.

Study these carefully:

github.com

guruofquality/grextras/blob/master/python/qa_block_gateway.py#L104


      
                      in_sig = [numpy.float32],
                      out_sig = [numpy.float32],
                  )
                  self.set_relative_rate(2.0)
          
          
    def work(self, input_items, output_items):
                  output_items[0][1::2] = input_items[0]
                  output_items[0][::2] = input_items[0]
                  return len(output_items[0])
          
          
class tag_source(gr.block):
              def __init__(self):
                  gr.block.__init__(
                      self,
                      name = "tag source",
                      in_sig = None,
                      out_sig = [numpy.float32],
                  )
          
          
    def work(self, input_items, output_items):
                  num_output_items = len(output_items[0])

And just for sanity, make sure that “make test” passed for you

-josh

anisha · July 13, 2012, 9:10pm

Thanks for the examples, I will definitely be looking into them!
As for links, This one: http://gnuradio.org/cgit/jblum.git/log/?h=next
is
broken, it comes from one of the blocks coding guide here:
http://gnuradio.org/redmine/projects/gnuradio/wiki/BlocksCodingGuide/diff/2which
i found really helpful.

I also had one more quick question, I’m getting this error:
‘gr_top_block_sptr’ object has no attribute ‘add_item_tag’, though I
thought add_item_tag was inherited from gr_block.
Thanks again!

anisha · July 16, 2012, 5:12pm

Thanks, this was a huge help!
-Anisha