How to change a normal Python‐level GNU Radio variable after the flowgraph has started

You can’t usually change a normal Python‐level GNU Radio variable after the flowgraph has started, but there are workarounds = you can make a variable externally adjustable while the script is running by using one of the mechanisms GNU Radio supports for runtime parameter control.

Here are the practical methods you can use:

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✅ 1. Use a QT GUI Variable (for manual on-screen control)

If you’re already using the QT GUI:

self.variable = variable = 42

self._variable_range = Range(0, 100, 1, 42, 200)

self._variable_win = RangeWidget(self._variable_range, self.set_variable, "Variable", "counter", float)

This lets you change the variable during execution with a slider or dial.

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✅ 2. Add the “Parameter Block” (GNU Radio 3.10+ / 3.11)

GNU Radio now has an official Parameter block (in GRC under Core → Misc).
This allows runtime changes via:

• CLI gr-ctrl
• REST API
• ZMQ message
• Native GUI element

You add:

1. Parameter block
2. Set its ID (e.g., center_freq)
3. Bind other blocks to it (e.g. Osmocom Source → Center Frequency parameter becomes center_freq).

Now you can change it at runtime using:

gr-ctrl set center_freq 1420405000

Your running script will update instantly.

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✅ 3. Use a Python Setter Method (set_variable()) and message ports

If you create a variable like:

self.my_gain = 20

GRC will auto-generate:

def set_my_gain(self, my_gain):

    self.my_gain = my_gain

    self.some_block.set_gain(self.my_gain)

Then you can attach:

• a Message Strobe
• a ZMQ SUB socket
• a REST interface
• your own external script

and send a value at any time.

Example: ZMQ PUSH → Message Debug → Variable Setter.

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✅ 4. Use ZMQ to push parameters from an external script

Flowgraph side:

ZMQ SUB → Python Block → set_variable()

External script:

import zmq, json

ctx = zmq.Context()

s = ctx.socket(zmq.PUSH)

s.connect("tcp://127.0.0.1:5557")

s.send_json({"my_gain": 30})

This updates your running flowgraph.

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✅ 5. Use GNU Radio’s Python API to expose controls

If running headless and you want a terminal interface:

Add something like this to your script:

def run_controller(self):

    while True:

        text = input("New freq: ")

        self.set_center_freq(float(text))

Run it in a thread:

_thread.start_new_thread(self.run_controller, ())

Now you can type values while the flowgraph runs.

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⭐ Recommended (Most Robust): Parameter Block + gr-ctrl

If you want a clean, scalable setup (especially for interferometry work), use:

Parameter block with gr-ctrl.

It gives you:

• No GUI needed
• Script remains running
• Update safely via terminal or another program
• Works across network
• Supports many parameters

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