'too many values to unpack', iterating over a dict. key=>string, value=>list

python

dictionary-iteration

python-3

unpacking-in-for-loops

byNikita Barsukov·Dec 25, 2024

The notorious 'too many values to unpack' error surfaces when you naively try to unpack a dict with a loop. To befriend both keys and values of a dict, enlist the help of the .items() method. It cheerfully presents pairs of keys and their corresponding lists (or cliques, as they prefer to be called) of values:

my_dict = {'key1': ['value1', 'value2'], 'key2': ['value3', 'value4']}
for key, value_list in my_dict.items():
    print(f"{key} => {value_list}")

In this special friendship, key prefers to stick with the dictionary key, and value_list is a party-goer, hanging out with the list. No more unpacking dramas!

Deeper dive into dictionary iteration

Dictionaries in Python and their iteration methods are something we can't escape from. If you're a fan of Python 2, you've probably heard of iteritems(), but if Python 3 is your jam, keep items() on your speed dial since iteritems() moved to a farm upstate living happily in Python 2 land.

Key tips for error-dodging iteration

Remember your iteration etiquette. Use the method that suits your Python version.
For Python 3 users, my_dict.items() is your night in shining armor, generating key-value pairs.
If Python 2 feels like home, my_dict.iteritems() is a more efficient pal, cause he doesn't like wasting memory.
Lest we forget that items() in Python 3 returns an iterator, not a list. Memory conservators rejoice!

Unpacking in for loops

To get the unpacking right in the loop, refer to this cheat sheet:

# Correct method for dictionary iteration and unpacking
for key, value_list in my_dict.items():
    for value in value_list:
        # Disclaimer: No dictionaries were harmed in the making of this code.

Just think of the structure fitting neatly into the for loop like tetris blocks, different pieces but part of the whole game.

Be aware! Pitfalls and lifelines

Let's talk about those sneaky issues that we might miss and their trusty solutions when iterating over dictionaries.

Accessing nested lists

If your dictionary's values run in packs (or lists), have an inner loop in your arsenal to iterate through each friend in the list:

for key, value_list in my_dict.items():
    for value in value_list:
        # Congrats! You just made a list of new friends.

Misusing dict.keys() or dict.values()

At times, all we need are keys or values. Using these incorrectly with unpacking will cause a kerfuffle. Use them when you need one component, and .items() when both want to play.

Adapting foreign code

Looking for answers from dependable resources like Stack Overflow? Always mold the code examples to fit your Python version and target structure. Focus on accepted answers, they are community-vetted and hold the golden ticket to your solution.

explain-codes / Python / 'too many values to unpack', iterating over a dict. key=>string, value=>list

Linked

Iterating over dictionaries using 'for' loops



Error: " 'dict' object has no attribute 'iteritems' "



How can I avoid "RuntimeError: dictionary changed size during iteration" error?



Create a dictionary with comprehension



Access an arbitrary element in a dictionary in Python

