Enumerate, zip, and lazy processing

enumerate pairs each value with an index. zip pairs values from multiple iterables and stops at the shortest input by default.

Worked example

subjects = ["Python", "SQL"]
minutes = [20, 30]
for index, (subject, value) in enumerate(zip(subjects, minutes), start=1):
    print(index, subject, value)

1 Python 20
2 SQL 30

Why this works

zip creates pairs as the loop requests them. enumerate numbers those pairs starting at 1. With strict=True, consuming mismatched inputs raises ValueError instead of discarding an unmatched value.

Another case

print(list(zip([1, 2], ["a"], strict=True)))

Traceback (most recent call last):
  File "zip_strict.py", line 1, in <module>
    print(list(zip([1, 2], ["a"], strict=True)))
ValueError: zip() argument 2 is shorter than argument 1

Your terminal may show the full path to the file instead of just its name. Python 3.11 and newer also add ^ or ~ marker lines under the part of the line that failed, and recent versions colour the output. The last line, with the error type and message, is the same.

Practice

Predict list(zip([1, 2], ["a"])) and explain the missing second pair.

Try another case

What does list(enumerate(["a", "b"])) produce?