RuntimeError in Python: Causes and Fixes with Simple Examples

 What is RuntimeError in Python?

A RuntimeError in Python occurs when an error happens while the program is running, and Python cannot classify it into a more specific error type.

In simple words, Python is saying:

“Something went wrong during execution, but I don’t know exactly which error category it fits into.”

This error usually appears when your program logic breaks at runtime, not during syntax checking.

---

Basic Example of RuntimeError

raise RuntimeError("Something went wrong!")

Output:

RuntimeError: Something went wrong!

Here, we are manually raising a RuntimeError using raise.

---

Common Situations Where RuntimeError Occurs

1️⃣ Infinite Recursion (Recursion Limit Exceeded)

def test():
    test()

test()

Output:

RuntimeError: maximum recursion depth exceeded

🔹 Python stops the program to avoid crashing the system.

---

2️⃣ Modifying a Collection While Iterating

numbers = [1, 2, 3, 4]

for n in numbers:
    if n == 2:
        numbers.remove(n)

Output:

RuntimeError: list changed size during iteration

---

3️⃣ Generator Errors

def gen():
    raise RuntimeError("Generator failed")

g = gen()
next(g)

---

4️⃣ Thread-Related Runtime Errors

RuntimeError may occur when:

• Using threads incorrectly

• Calling thread methods in the wrong state

import threading

t = threading.Thread(target=lambda: None)
t.start()
t.start()

Output:

RuntimeError: threads can only be started once

---

RuntimeError vs Other Errors

Error Type	When It Occurs
SyntaxError	Before execution
TypeError	Wrong data type
ValueError	Invalid value
RuntimeError	Error during execution (general)

---

How to Handle RuntimeError

Using try-except

try:
    raise RuntimeError("Unexpected issue")
except RuntimeError as e:
    print("Handled RuntimeError:", e)

---

Best Practices to Avoid RuntimeError

✔ Avoid infinite recursion
✔ Don’t modify lists while looping
✔ Use proper thread handling
✔ Validate logic before runtime
✔ Use specific exceptions when possible

---

When Should You Raise RuntimeError?

Use RuntimeError when:

• No specific built-in exception fits the problem

• You want to signal a generic execution failure

if not condition:
    raise RuntimeError("Condition failed at runtime")

---

Final Thoughts

RuntimeError is a general-purpose exception that signals something went wrong during program execution.
Understanding where and why it occurs helps you debug faster and write safer Python code.