Basic cleanup
Input
make_csv([' Apple ', 'Banana', '', None, 'Cherry'])
Output
apple,banana,cherry
Whitespace trimmed, empty and None removed, and values lowercased then joined.
Full lesson preview
Map, filter, and join strings into CSV
Clean a list of mixed values using map and filter, then join them into a CSV string.
Python practice13 minFunctions as Objects (Lambda, Map/Filter)IntermediateLast updated March 20, 2026
Problem statement
Write a function make_csv(items) that takes a list of values and returns a single CSV string. The function should: 1) convert each item to a string, 2) strip surrounding whitespace, 3) discard values that become empty after stripping or are None, 4) convert remaining strings to lowercase, and 5) join them with commas. Use map, filter, and join (and lambda where appropriate).
Task
Use map and filter with lambda to normalize values, remove empty entries, and join into a comma-separated string.
Examples
Input format
A Python list of values (strings, numbers, booleans, or None).
Output format
A single string containing comma-separated cleaned values, or an empty string if nothing remains.
Constraints
Do not use loops explicitly; prefer map and filter with lambdas. The function must handle non-string values by converting them using str().
Samples
Sample 1
Input
make_csv([' a ',' B '])
Output
a,b
Both values trimmed and lowercased, then joined.