Python RuntimeError — What It Means & How to Fix It

What Does This Error Mean?

A RuntimeError is a general-purpose error that Python raises when something goes wrong while the program is running, and no more specific error type applies. Unlike a SyntaxError (which happens before the program runs), a RuntimeError happens during execution. It often means an unexpected condition was detected that makes it impossible to continue safely.

Affected Models

Python 2.x
Python 3.x
All Python versions

Common Causes

Modifying a list, dictionary, or set while iterating over it in a loop (the 'dictionary changed size during iteration' error)
Exceeding the maximum recursion depth — a function that calls itself too many times
Trying to use a generator after it has already been fully consumed
Thread-safety issues — multiple threads accessing shared data at the same time without proper locking
A library or framework raising a RuntimeError to signal an internal unexpected state

How to Fix It

Read the full error message and traceback. RuntimeError messages are often descriptive — 'dictionary changed size during iteration' or 'maximum recursion depth exceeded' tells you exactly what went wrong.

The traceback shows the chain of function calls that led to the error.
If the error is 'dictionary changed size during iteration', do not modify a dictionary while looping over it. Instead, loop over a copy: for key in list(my_dict):

Same applies to sets and lists. Collect items to add or remove in a separate list, then apply changes after the loop.
If the error involves recursion depth, your recursive function is calling itself too many times. Make sure the base case (the stopping condition) is correct and is always reached.

See the RecursionError page for more detailed help on fixing recursive functions.
If you are using threads or async code, make sure shared data is protected with appropriate locks (threading.Lock) or that you are using thread-safe data structures.

Concurrency bugs are hard to reproduce reliably. Adding proper locking is the right long-term fix.
If the RuntimeError comes from a library (like Flask, Django, or SQLAlchemy), search the exact error message online along with the library name. Library-specific RuntimeErrors usually have well-known solutions.

Include the full error message in your search — the more specific, the better.

When to Call a Professional

RuntimeErrors usually require careful code review to diagnose. The error message often provides a clue about the specific cause. If you are working with threads or async code and cannot identify the source, consider asking for help in a programming forum with a clear code example.

Frequently Asked Questions

Why does Python have a generic RuntimeError instead of more specific errors?

Python does have many specific error types (IndexError, KeyError, etc.) for common situations. RuntimeError is the fallback — it is used when the problem does not fit into any existing specific category. Library authors also use RuntimeError when they want to signal an unexpected state that is specific to their library.

What does 'dictionary changed size during iteration' mean?

It means you added or removed keys from a dictionary while a for loop was going through it. Python cannot handle this safely — if items are added or removed mid-loop, the loop would produce unpredictable results. The fix is to loop over a copy of the dictionary, or collect changes and apply them after the loop.

Is a RuntimeError always a programming mistake?

Usually yes — it means something unexpected happened that the code did not account for. But sometimes it comes from external conditions, like a library reaching an internal limit. In either case, the right approach is to read the full error message and traceback to understand the root cause.