# Fwd: Write a source block in python **URL:** \ **Category:** GNU Radio\ **Created:** [January 22, 2015, 10:13pm UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928 "2015-01-22T22:13:35Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [January 22, 2015, 10:13pm UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928/1 "2015-01-22T22:13:35Z")
Hi I wrote a simple source block that reads a string (as a parameter) and outputs the corresponding array of bytes. Here is the code: class myblock(gr.sync\_block): ``` def __init__ (self, helloMsg): gr.sync_block. __init__ (self, name="myblock", in_sig=None, out_sig=[numpy.byte]) self.helloMsg = helloMsg def work(self, input_items, output_items): out = output_items[0] test = array.array('B',self.helloMsg) for j in range(len(test)): out[j] = test[j] return len(output_items[0][:len(test)]) ``` In the output I repeatedly get the helloMsg. (it seems that the work() function is always running) Is it possible to get in the output my helloMsg only one time (or a specific amount of times) and then stop the execution of the flowgraph? Thank you in advance! Thanasis ---
**Author:** ![Thanasis\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/7ea924/32.png) [@Thanasis\_B](https://www.ruby-forum.com/u/Thanasis_B)\ **Post date:** [January 22, 2015, 10:33pm UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928/2 "2015-01-22T22:33:09Z")
I think having the work fn return “-1” will cause the flowgraph to exit, so you could potentially have a “self.count = 1” in **init** and then when you’ve output \>= self.count, have work return -1 -Doug Douglas Anderson | Intern DOC/NTIA/ITS-T | 325 Broadway St., Boulder, CO 80305 | P: 303 497 3582 * * * From: discuss-gnuradio-bounces+danderson=removed\_email\_address@domain.invalid [discuss-gnuradio-bounces+danderson=removed\_email\_address@domain.invalid] on behalf of Thanasis B. [removed\_email\_address@domain.invalid] Sent: Thursday, January 22, 2015 2:12 PM To: removed\_email\_address@domain.invalid Subject: [Discuss-gnuradio] Fwd: Write a source block in python Hi I wrote a simple source block that reads a string (as a parameter) and outputs the corresponding array of bytes. Here is the code: class myblock(gr.sync\_block): ``` def __init__ (self, helloMsg): gr.sync_block. __init__ (self, name="myblock", in_sig=None, out_sig=[numpy.byte]) self.helloMsg = helloMsg def work(self, input_items, output_items): out = output_items[0] test = array.array('B',self.helloMsg) for j in range(len(test)): out[j] = test[j] return len(output_items[0][:len(test)]) ``` In the output I repeatedly get the helloMsg. (it seems that the work() function is always running) Is it possible to get in the output my helloMsg only one time (or a specific amount of times) and then stop the execution of the flowgraph? Thank you in advance! Thanasis ---
**Author:** ![Thanasis\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/7ea924/32.png) [@Thanasis\_B](https://www.ruby-forum.com/u/Thanasis_B)\ **Post date:** [January 22, 2015, 11:55pm UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928/3 "2015-01-22T23:55:16Z")
Thank you Doug! return -1 indeed cause the flowgraph to exit. Is there any trick to stop also the execution on my program (after 1 sec)? Another problem I have is that the output is always 32768 bytes def work(self, input\_items, output\_items): … return len(output\_items[0]) If I change it to this: return len(output\_items[0][:len(test)]) my output becomes empty Is it possible to get in the output the exact number of bytes of my helloMsg parameter? 2015-01-22 23:31 GMT+02:00 Anderson, Douglas J. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): ---
**Author:** ![Thanasis\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/7ea924/32.png) [@Thanasis\_B](https://www.ruby-forum.com/u/Thanasis_B)\ **Post date:** [January 23, 2015, 10:55am UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928/4 "2015-01-23T10:55:53Z")
On 01/22/2015 11:53 PM, Thanasis B. wrote: > Thank you Doug! > > return -1 indeed cause the flowgraph to exit. Is there any trick to stop > also the execution on my program (after 1 sec)? You can exit whenever you like. E.g. run a timer until 1 sec is over and then return -1. When you return -1, the FG _should_ shut down by itself. > Another problem I have is that the output is always 32768 bytes You have no problem: Your output _buffer_ is of this size. Your output size is whatever you return (something like return len(test) would be what you want). M ---
**Author:** ![Thanasis\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/7ea924/32.png) [@Thanasis\_B](https://www.ruby-forum.com/u/Thanasis_B)\ **Post date:** [April 9, 2015, 4:00am UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928/5 "2015-04-09T04:00:14Z")
Hi, > I think having the work fn return “-1” will cause the flowgraph to exit, > so you could potentially have a “self.count = 1” in **init** and then > when you’ve output \>= self.count, have work return -1 What exactly do you mean by output \>= self.count? I understand the count part obviously, but I didn’t quite get what we are comparing to the count. Is it the len(output\_items[0])? And if this is greater than or equal to the count then work returns -1? Is that what you mean? I’m trying to code a python source block myself and I’m facing a similar issue, where I am getting a repeated output infinite times. thanks a lot AKY ---
**Author:** ![Thanasis\_B](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/7ea924/32.png) [@Thanasis\_B](https://www.ruby-forum.com/u/Thanasis_B)\ **Post date:** [March 25, 2015, 1:04am UTC](https://www.ruby-forum.com/t/fwd-write-a-source-block-in-python/233928/6 "2015-03-25T01:04:20Z")
Hi, I am trying to create a similar source block using python. I am new to Python and I have couple questions here: 1. How do you change your work function in order to output the message only once? I get either 0 output (output file with 0 Bytes) or repeated output, I don’t know where should i put “return -1”. 2. what is the difference between “for j in range(len(test)): out[j] = test[j]” and “out[:len(test)] = test” ? 3. what is the difference between “return len(test)” and “return len(output\_items[0])”? So the return value only determines the output size? Thank you so much! Hai Martin B. wrote in post #1167125: > On 01/22/2015 11:53 PM, Thanasis B. wrote: > > > Thank you Doug! > > > > return -1 indeed cause the flowgraph to exit. Is there any trick to stop > > also the execution on my program (after 1 sec)? > > You can exit whenever you like. E.g. run a timer until 1 sec is over and > then return -1. > > When you return -1, the FG _should_ shut down by itself. > > > Another problem I have is that the output is always 32768 bytes > > You have no problem: Your output _buffer_ is of this size. Your output > size is whatever you return (something like return len(test) would be > what you want). > > M