If you are a developer or system administrator, you may have encountered the RuntimeError: Can’t Start New Thread error at some point.
This error occurs when your application or system attempts to create a new thread yet it fails due to a lack of resources. In this article, we will explore the causes of this error and provide some solutions to help you fix it.
Why the Runtimeerror Can’t Start New Thread Error Occur?
This runtimeerror can’t start new thread error message typically occurs because the process has reached the limit of available threads and it will not create a new thread.
What are the Causes of the Error?
The following are the common causes of the error why it occurs:
- Exceeding the maximum number of threads
- Limitations of the resource
- Using too many libraries
- Other processes running in the background
- Code Errors
How to Fix Runtimeerror: Can’t Start New Thread Error?
Here are the solutions to fix the error Fix Runtimeerror: Can’t Start New Thread:
Solution 1: Limit the Number of Threads
The first solution to fix this error is you need to limit the number of threads to create your program. You can use the threading.active_count() function to get the current number of threads and limit the number of threads your program creates accordingly.
Solution 2: Increase System Resources
The second solution to fix this error is you need to increase the resources available to your system. You can do this by upgrading your hardware or optimizing your program to use fewer resources.
Solution 3: Optimize Your Program
To fix this error, you need to optimize your program and use only the required libraries. You can also check the libraries you are using and make sure they are not creating irrelevant threads.
Solution 4: Close Other Processes
The other way to fix this error is you need to close any unneeded processes that running in the background. You can use the Task Manager or Activity Monitor to identify the processes and close them.
Solution 5: Debug Your Code
The last solution to fix this error is you need to debug your code and fix any errors. You can use a debugger or print statements to identify the errors and fix them.
Additional Resources
The following articles will be able to help you to understand more about Runtimeerror:
Python RuntimeError debugging checklist
- Read the full error message. It usually names the specific violation.
- Check RuntimeError subclass. RecursionError, NotImplementedError, StopIteration are common subclasses with more specific meaning.
- Print state before the failing call. Insert breakpoint() or print statements.
- Rule out library API changes. Especially for PyTorch, TensorFlow, asyncio between versions.
Common RuntimeError sources
- Dictionary/set modified during iteration. Iterate over a copy.
- PyTorch device mismatch or OOM. Move tensors, lower batch size.
- asyncio event loop misuse. Use asyncio.run() or TaskGroup.
- Maximum recursion depth exceeded. Add base case or convert to iteration.
- NotImplementedError from abstract method. Subclass forgot to override.
Modern tooling to prevent RuntimeError
- Type hints + mypy. Catches many signatures before runtime.
- Ruff. Catches many runtime-adjacent bugs.
- pytest with fixtures. Test each function with edge inputs.
- logger.exception(). Captures traceback + context in structured logs.
Official documentation
Frequently Asked Questions
What is Python RuntimeError and what causes it?
RuntimeError is a generic catch-all for errors that don’t fit other specific categories. Common 2026 sources: PyTorch CUDA out of memory, asyncio event-loop conflicts, Flask ‘working outside of application context,’ mutating a dict/list during iteration, and threading deadlocks. The error message usually points to the underlying cause.
How do I fix PyTorch CUDA out of memory RuntimeError?
Three options: (1) Reduce batch size (the most direct fix). (2) Clear cache: torch.cuda.empty_cache() between epochs. (3) Use mixed precision (torch.cuda.amp.autocast) to halve memory. (4) If on a shared GPU, check nvidia-smi to see other processes hogging memory.
How do I fix ‘dictionary changed size during iteration’?
You’re modifying a dict (adding/removing keys) inside ‘for k in my_dict’. Two fixes: (1) iterate over a copy: for k in list(my_dict.keys()). (2) Build a new dict and assign: my_dict = {k: v for k, v in my_dict.items() if keep(k)}. Same applies to set and list mutations during iteration.
How do I fix Flask ‘Working outside of application context’?
Wrap the code in app.app_context(): with app.app_context(): db.create_all(). This usually happens in scripts run outside of a Flask request (CLI tools, background jobs). For test code, use the test client which auto-creates context.
Where can I find more RuntimeError fixes?
Browse the RuntimeError reference hub for 49+ specific fixes (PyTorch CUDA, asyncio, Flask context, dict iteration). For Python fundamentals see the Python Tutorial hub.
Conclusion
The “RuntimeError: can’t start new ‘thread‘” error can be frustrating to deal with, yet it is a common issue that developers encounter.
Through understanding the causes of the error and applying the solutions we’ve discussed, you can fix the error and prevent it from occurring in the future.
FAQs
Yes, this error can occur in any programming language that supports threading.
Yes, insufficient memory is one of the causes of this error. When your system runs out of memory, it cannot create new threads.
Yes, a large number of open file descriptors can cause this error. When you have too many file descriptors, the system cannot create new threads.
Yes, this error can occur on a single-core processor. When a program creates too many threads, it can exhaust the resources available on a single-core processor and result in an error.
Adones Evangelista
Programmer & Technical Writer at PIES IT Solution
Adones Evangelista is a programmer and writer at PIES IT Solution, author of over 900 tutorials and error-fix guides at itsourcecode.com. Specializes in JavaScript, Django, Laravel, and Python error debugging covering ValueError, TypeError, AttributeError, ModuleNotFoundError, and RuntimeError, plus C/C++ and PHP capstone projects for BSIT students.
Expertise: JavaScript · Python · Django · Laravel · Error Debugging · C/C++
· View all posts by Adones Evangelista →