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Python

RuntimeError: What It Means and How to Fix It

Error reference

RuntimeError

Language

Python

Severity

Medium

When it happens

RuntimeError is a catch-all exception for errors that don't fit a more specific category.

Potential fixes

To safely modify a dict during iteration, iterate over a copy (list(d.keys())) or collect changes and apply them after the loop. For other RuntimeErrors, read the message carefully as it describes the specific invalid operation.

Related errors

RecursionErrorStopIterationKeyError

Deep dive

RuntimeError is a catch-all exception for errors that don't fit a more specific category. The most common instance is 'dictionary changed size during iteration', but it also covers thread-related errors and generator issues.

Why it happens

  • •Adding or removing keys from a dictionary while iterating over it.
  • •Calling a coroutine or generator in an invalid state.
  • •Recursive call detected in a context that prohibits recursion.
  • •Using a thread primitive incorrectly, such as releasing an unlocked lock.

Potential fixes

To safely modify a dict during iteration, iterate over a copy (list(d.keys())) or collect changes and apply them after the loop. For other RuntimeErrors, read the message carefully as it describes the specific invalid operation.

Examples

Python

Code that triggers the error

d = {"a": 1, "b": 2}
for key in d:
    if key == "a":
        d["c"] = 3

Error output

RuntimeError: dictionary changed size during iteration

Fixed code

d = {"a": 1, "b": 2}
to_add = {}
for key in d:
    if key == "a":
        to_add["c"] = 3
d.update(to_add)

Practice in English

How would you explain a RuntimeError to a fellow dev? Choose the right phrase:

"I hit an error...

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