Lessons · Python · join (pieces to text)
From pieces to one string
",".join(pieces) puts the separator between every item and returns one string. The separator is what you call it on.
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What it is for
CSV lines, sentences from words, paths from parts, SQL from fragments. Concatenating in a loop with + is slow and leaves a stray separator; join is the right tool.
How to think about it
What are the pieces, and what goes between them? Get the pieces into a list of strings first (convert numbers with str). Then decide the separator, and call join on the separator.
Worked example
parts = ["2026", "09", "04"]All strings.
print("-".join(parts))2026-09-04.nums = [1, 2, 3]Numbers: join needs strings.
print(", ".join(str(n) for n in nums))1, 2, 3.Your turn
Make a sentence from words.
sentence = "".join(words)
Solve one with the tests running
The trap
Calling join on the list: parts.join(","). It is the other way round; the string owns join.
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