Zip parallel labels and values with strict length checking

Zip parallel labels and values with strict length checking

By Batu.

A pair of parallel sequences is often a compact way to carry field names and their values. Here, zip(labels, values, strict=True) produces two-item tuples, and dict() consumes those tuples to create settings. The resulting mapping contains server: "api", retries: 3, and enabled: True in the input order.

The strict=True argument changes an important default: ordinary zip() silently stops when its shortest input ends. Strict zip() instead raises ValueError when iteration discovers unequal lengths. This recipe deliberately consumes the mismatched zip with list() so the exception is observed and tested. strict was added in Python 3.10; on Python 3.9 or earlier, passing that keyword raises TypeError, so use Python 3.10+ for this form.

Strictness checks counts, not meaning. Duplicate labels are still accepted by zip(), while dict() retains only the last value for a duplicate key. It also cannot validate an iterator without advancing it: both inputs are consumed as the result is built. If values are costly or one-shot, materialize or otherwise manage them according to the surrounding workflow.

The Python zip() documentation describes its iterator behavior and strict mode.

AI assistance disclosure: Batu used AI assistance to draft this article.

labels = ["server", "retries", "enabled"]
values = ["api", 3, True]

settings = dict(zip(labels, values, strict=True))
assert settings == {"server": "api", "retries": 3, "enabled": True}

try:
    list(zip(["left"], [1, 2], strict=True))
except ValueError:
    print("length mismatch")

print(settings)

Expected stdout:

length mismatch
{'server': 'api', 'retries': 3, 'enabled': True}

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