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title: How to Pass Arguments to QRunnable Worker Threads in PyQt6
description: Writing 'self.model.do_this("message")' when creating a QRunnable Worker calls the function immediately on the main thread, freezing the GUI until it finishes....
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# How to Pass Arguments to QRunnable Worker Threads in PyQt6

**[Planet Python](https://daily.dev/sources/planetpython)** · 6 min read · 0 upvotes · 0 comments

## Summary

Writing 'self.model.do_this("message")' when creating a QRunnable Worker calls the function immediately on the main thread, freezing the GUI until it finishes. The fix is to pass the function reference and its arguments separately, like 'Worker(self.model.do_this, "message")'. The Worker's __init__ stores fn, args and kwargs, then calls 'self.fn(*self.args, **self.kwargs)' inside run(), which executes on the background thread. This keeps the GUI responsive while the task runs.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://www.pythonguis.com/faq/delay-in-signal-from-thread>

## Questions this post answers

### Why does my PyQt6 GUI freeze even though I'm using QThreadPool and QRunnable?

The GUI freezes when you call the worker function with parentheses, like Worker(self.model.do_this("msg")), because Python executes it immediately on the main thread before Worker ever runs. The function completes fully, including any long-running work, and only its return value (often None) gets passed to Worker. Pass the function reference without parentheses instead.

_Developers debugging PyQt6 thread freezes track fixes like this on daily.dev._

### How do I pass arguments to a function running in a QRunnable worker thread in PyQt6?

Pass the function and its arguments as separate items to the Worker constructor: Worker(self.model.do_this, "This is a message"). The Worker's __init__ accepts fn, *args, and **kwargs, storing them, then its run() method calls self.fn(*self.args, **self.kwargs) on the background thread so the GUI stays responsive.

_daily.dev helps developers building PyQt6 apps find patterns like this QRunnable fix._

### What is the difference between Worker(self.model.do_this) and Worker(self.model.do_this("hello")) in PyQt6?

Worker(self.model.do_this) passes the function itself, which then runs on the worker thread when run() is called. Worker(self.model.do_this("hello")) calls the function immediately on the main thread and passes its None return value to Worker instead, blocking the GUI for the task's full duration.

_Anyone wiring up QThreadPool workers can bookmark distinctions like this on daily.dev._

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---

Tags: [#python](https://daily.dev/tags/python), [#qt](https://daily.dev/tags/qt)

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