Python RuntimeError Reference: 49+ PyTorch, CUDA, asyncio, Flask Fixes (2026)

itsourcecode.com hosts 49+ documented fixes for Python RuntimeError messages: the catch-all error class Python raises when something goes wrong at runtime that doesn’t fit a more specific category. In modern Python, most RuntimeErrors come from PyTorch/CUDA (GPU memory, scalar type, cudnn), asyncio (event loop already running), Flask/Django (working outside request context), or iteration mutation (set/dict changed size during iteration). Browse the fixes below.
What is a Python RuntimeError?
A RuntimeError is the base class for errors that don’t fit a more specific built-in exception. Libraries raise RuntimeError when something happens that the standard exception hierarchy doesn’t anticipate, PyTorch raises it for GPU/tensor problems, asyncio for event loop state issues, Flask for missing application context. Because it’s a catch-all, the fix depends heavily on which library raised it and what the message says.
How to debug any RuntimeError in 4 steps

Read the full message. RuntimeError messages are library-specific and usually descriptive. “CUDA out of memory” means GPU memory; “event loop is already running” means asyncio context; “working outside of request context” means Flask scope.
Identify the library. The traceback above the error tells you which library raised it. PyTorch → tensor/dtype/GPU. asyncio → event loop. Flask/Django → request context. PySpark → Java gateway. Each has a different fix pattern.
Check resource state. For CUDA: nvidia-smi to see GPU memory. For asyncio: check if you’re inside an existing event loop. For dict/set iteration: never mutate while iterating, copy first or use a comprehension.
Search this hub by library category below. Most RuntimeErrors have well-documented patterns. Find your exact message.

Featured RuntimeError fixes by library
🔥 PyTorch: scalar type / dtype mismatches
PyTorch tensor operations require specific dtypes; mixing float16, float32, bfloat16, and double raises RuntimeError.

Found dtype Long but expected Float
Expected scalar type Half but found Float
Expected scalar type BFloat16 but found Float
Expected scalar type Float but found Double
mat1 and mat2 shapes cannot be multiplied

🎮 PyTorch / CUDA: GPU errors
The most painful RuntimeErrors in ML, usually GPU memory, driver, or device assertion issues.

CUDA out of memory
No CUDA GPUs are available
CUDA error: invalid device ordinal
CUDA error: device-side assert triggered
All CUDA-capable devices are busy or unavailable
cuDNN error: CUDNN_STATUS_MAPPING_ERROR
cuDNN error: CUDNN_STATUS_NOT_INITIALIZED
cuDNN error: CUDNN_STATUS_EXECUTION_FAILED
Couldn’t install torch

⏱️ asyncio: event loop state

This event loop is already running
No running event loop

🌐 Flask / Django: request context

Working outside of request context (Flask)
SQLALCHEMY_DATABASE_URI or SQLALCHEMY_BINDS must be set
Cannot add middleware after an application has started

🔁 Mutation-during-iteration

Dictionary changed size during iteration
Set changed size during iteration

☕ PySpark: Java gateway

Java gateway process exited before sending its port number

📦 Build / Install / Config

Poetry configuration is invalid
Ninja is required to load C++ extensions
Product license has not been initialized
Python raise RuntimeError (manual raise)

Related error categories
RuntimeError is one of 10 hubs in our Python & JavaScript error reference cluster, 980+ documented fixes total:

TypeError Reference, 220+ Python & JS TypeError fixes
ModuleNotFoundError Reference, 198+ Python import errors
AttributeError Reference, 173+ “object has no attribute X” fixes
ValueError Reference, 100+ pandas/NumPy/sklearn fixes
ImportError Reference, 67+ “cannot import name X from Y” fixes
NameError Reference, 49+ “name X is not defined” fixes
SyntaxError Reference, 48+ Python & JavaScript syntax errors
ReferenceError Reference, 34+ JavaScript “is not defined” fixes
HTTP Error Reference, 35+ HTTP status code fixes (4xx, 5xx)
Python Tutorial, beginner-to-intermediate Python lessons

About this RuntimeError reference
Built since 2015 by PIES Information Technology Solutions, Binalbagan, Negros Occidental, Philippines. Every post comes from a real error encountered in production. Used by 12,000+ Python developers monthly.