What is the difference between a list comprehension, a generator expression, and map or filter?

All four build a new sequence from an existing one; they differ in what they return and when the work happens.

• List comprehension — [x*2 for x in data if x > 0]. Builds the whole list immediately. Readable, and the right default for anything of modest size.
• Generator expression — the same with parentheses, (x*2 for x in data). Returns a generator; nothing is computed until consumed and nothing is stored. Use it for large or infinite inputs, and whenever the result is fed straight into sum, any, max, or a loop.
• map and filter — return lazy iterators in Python 3. map(str.upper, names) is clean when you are applying an existing named function, but map(lambda x: x*2, data) is both slower and harder to read than the comprehension.

The idiomatic guidance: comprehensions are preferred in Python; reach for map only when passing a function that already exists. Dict and set comprehensions follow the same pattern with braces.

Note: Nested comprehensions past two levels become unreadable. A plain loop is better code than a comprehension nobody can parse.