EC
RuntimeError
PythonERRORCriticalRuntime ErrorMEDIUM confidence

Error doesn't fall in other categories

What this means

Raised when an error is detected that doesn’t fall in any of the other categories. The associated value is a string indicating what precisely went wrong.

Why it happens
  1. 1An operation in a C extension for Python fails without setting a more specific error.
  2. 2Some libraries raise it in situations that are hard to classify, for example, modifying a collection while iterating over it in some contexts.
  3. 3Asynchronous code might raise it if an event loop is managed incorrectly (e.g., it's already running).
How to reproduce

This error can be triggered in some Python versions when a dictionary's size changes during iteration.

trigger — this will error
trigger — this will error
my_dict = {1: 'a', 2: 'b', 3: 'c'}
for key, value in my_dict.items():
    if key == 1:
        my_dict[4] = 'd'  # Modifying dict during iteration

expected output

Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
RuntimeError: dictionary changed size during iteration

Fix 1

Iterate over a copy of the object

WHEN You need to modify a collection (like a list or dict) while iterating over it.

Iterate over a copy of the object
my_dict = {1: 'a', 2: 'b', 3: 'c'}
# Iterate over a copy of the keys
for key in list(my_dict.keys()):
    if key == 1:
        my_dict[4] = 'd'

Why this works

By creating a copy of the dictionary's keys (`list(my_dict.keys())`), you iterate over a static list, which is unaffected by modifications to the original dictionary.

Fix 2

Create a new collection

WHEN You are building a modified version of the original collection.

Create a new collection
original_dict = {1: 'a', 2: 'b', 3: 'c'}
new_dict = {}
for key, value in original_dict.items():
    new_dict[key] = value
    if key == 1:
        new_dict[4] = 'd'

Why this works

This approach avoids modifying the source of the iteration altogether by populating a new dictionary, which is safer and often clearer.

Code examples
Triggerpython
d = {1: "a", 2: "b"}
for k in d:
    d[k + 10] = "x"  # RuntimeError: dictionary changed size during iteration
Handle with try/exceptpython
try:
    result = run_operation()
except RuntimeError as e:
    print(f"Runtime error: {e}")
Avoid by iterating a copypython
d = {1: "a", 2: "b"}
for k in list(d.keys()):
    d[k + 10] = "x"  # safe — iterating a snapshot
What not to do

✕ Catching `RuntimeError` generically

It's a very broad exception. If you are getting one, you should investigate the specific cause and handle it as precisely as possible, as it often points to a serious logic flaw.

Version notes

Sources
Official documentation ↗

cpython/Objects/exceptions.c

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