Gotcha: Python dictionary iteration during modification

RuntimeError when modifying a dictionary while iterating over it.

Never modify a dict while iterating. Use these patterns instead:

# BAD: RuntimeError!
for key in my_dict:
    if should_remove(key):
        del my_dict[key]  # RuntimeError: dictionary changed size

# GOOD: Create list of keys to remove
keys_to_remove = [k for k in my_dict if should_remove(k)]
for key in keys_to_remove:
    del my_dict[key]

# GOOD: Dictionary comprehension (creates new dict)
my_dict = {k: v for k, v in my_dict.items() if not should_remove(k)}

# GOOD: Copy keys first
for key in list(my_dict.keys()):
    if should_remove(key):
        del my_dict[key]

# Same applies to sets:
my_set = {x for x in my_set if not should_remove(x)}

Python dictionaries use hash tables that can be reorganized during modification. Iterating while modifying would produce unpredictable results, so Python raises RuntimeError.

Filtering or cleaning up Python dictionaries