List Comprehensions

[expr for x in seq if cond] — read it left to right.

Why bother

• One line, clear intent, faster than for + .append()
• Works for dicts ({k: v for ...}) and sets ({x for ...}) too

When NOT to

• If the expression is complex, write the loop. Readability > cleverness.
• If you don't need the list (just want side effects), use a regular loop.

Try it

1. Build a list of (word, len(word)) for words longer than 4 letters.
2. Convert [1, 2, None, 3, None, 4] into [1, 2, 3, 4].