How do I delete items from a dictionary while iterating over it?

python

dictionary

comprehension

best-practices

byAnton Shumikhin·Sep 11, 2024

Here is a quick and dirty solution. We take a snapshot of the dictionary keys and iterate over it:

my_dict = {'a': 1, 'b': 2, 'c': 3}
# Snap, I got a list of keys. Iterate over it, no dictionary was hurt during the process. 
for key in list(my_dict):
    if my_dict[key] > 1:
        del my_dict[key]   # Under 1? Stay. Over 1? Bye bye!

With this piece of magic, no runtime errors occur, and my_dict keeps only items where the value is 1 or less.

Choose your weapon: Safe deletion methods

You've got choices when it comes to removing items. Make your pick based on clarity or performance.

Let's get Pythonic: Dictionary comprehension

The one-liner:

my_dict = {key: val for key, val in my_dict.items() if val <= 1}  # Bouncer at work: No entry if value > 1!

Swifty and swift, a new dictionary, purged from undesired items.

Old school copy and iterate with `items()`

When the original is sacred:

original_dict = my_dict.copy()   # Invoke cloning magic, keeps the original safe
for key, value in original_dict.items():
    if value > 1:
        del my_dict[key]    # Keys unlocked by copy, originals stay untouched

Hungry for efficiency: Post-iteration deletion

Save memory, kick out keys post-iteration:

keys_to_remove = [key for key in my_dict if my_dict[key] > 1]   # Grant keys to their doom!
for key in keys_to_remove:
    del my_dict[key]  # Who has the keys now?

All these methods, intricately designed, ensures your dictionary is mutated safely during iteration.

Gentleman's guide: Best practices and common pitfalls

Play safe with dictionaries. Here's rules of the game and common dirt tracks!

The `.copy()` privilege

.copy() is safe, but beware of memory usage. Use it when original integrity matters.

Leverage Python powers: Built-in methods

Use.keys()`, it's your friend. In Python 3:

for key in list(my_dict.keys()):  # list() clones the keys
    # The real my_dict is taking a break while we play with its keys

Watch for the size

When dictionary size bloats, copies or lists may not be your friend. Try out streaming or batch processing.

Play safe in crowds: Concurrency and locks

In multi-threaded environments, dictionary tweaks need attention. Use locks to prevent data glitches.

Playing by the rules make your code stable, safe, and reliable. Winning is in understanding the ground rules, isn't it?

explain-codes / Python / How do I delete items from a dictionary while iterating over it?

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Choose your weapon: Safe deletion methods Gentleman's guide: Best practices and common pitfalls