Lessons · Python · looping over a dict

Walking a dict: keys, values, or both

for k in d gives keys; d.values() gives values; d.items() gives (key, value) pairs.

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What it is for

Config, counts, records by id: dicts hold most structured data, and you constantly need to walk them to report, total, or transform.

How to think about it

Do I need the keys, the values, or both? Ask what you need inside the loop. Only the names: keys. Only the numbers: values. Both, which is most of the time: items() with two loop variables.

Worked example

stock = {"pen": 3, "ink": 0}

Product to quantity.

for name, qty in stock.items():

Two names, because items() gives pairs.

    if qty == 0:

Use the value.

        print(name, "is out")

Use the key. 'ink is out'.

Your turn

Add up every quantity.

total = 0
for q in stock.():
    total += q

The trap

Changing a dict's size while looping over it raises an error. Collect the keys to delete first, then delete after the loop.

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