Why You Should Never Modify a Dictionary While Looping in Python 🐍

Have you ever tried to modify a dictionary while looping over it in Python—only to get hit with a confusing RuntimeError? You're not alone. This is a classic Python trap that even experienced developers can fall into.

❗ The Problem

my_dict = {'a': 1, 'b': 2, 'c': 3}

You might try this:

for key, value in my_dict.items():
    if value == 2:
        del my_dict[key]

But Python will throw:

RuntimeError: dictionary changed size during iteration

🔍 Why This Happens

Python loops through the internal structure of the dictionary. If you change that structure mid-loop by adding or removing keys, Python raises an error to protect your program from unpredictable behavior.

✅ The Right Way: Loop Over a Copy

for key in list(my_dict.keys()):
    if my_dict[key] == 2:
        del my_dict[key]

By using list(my_dict.keys()), you're making a safe copy of the keys before the loop begins. Now you can modify the dictionary freely inside the loop.

⚠️ What About Adding New Keys?

for key in list(my_dict.keys()):
    my_dict['new_key'] = 999

This is now perfectly safe because you're looping through a fixed list, not the live dictionary.

🧠 Takeaway

• ❌ Never modify a dictionary while looping through it.
• ✅ Always loop over a copy using list(my_dict.keys()).
• 💡 This avoids runtime errors and makes your code more robust.

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