Python list comprehension examples

The core four

nums = [1, 2, 3, 4, 5]

[n * n for n in nums]                # transform      → [1,4,9,16,25]
[n for n in nums if n % 2 == 0]     # filter         → [2,4]
[n*n for n in nums if n > 2]        # both           → [9,16,25]
["even" if n%2==0 else "odd" for n in nums]  # if-else → LEFT side

The if-else position trip-up: filtering if goes on the right (drops items); choosing if-else goes on the left (keeps every item, picks its value). Mixing these up is the #1 comprehension syntax error.

Real-world one-liners

# clean a list of strings
[s.strip().lower() for s in raw if s.strip()]

# pull one field out of a list of dicts (API responses!)
[u["email"] for u in users if u["active"]]

# parse numbers from a CSV row
[float(x) for x in line.split(",")]

# filenames matching a pattern
[f for f in os.listdir(".") if f.endswith(".csv")]

Dict and set flavors

{w: len(w) for w in words}            # dict: {"hi": 2, "hello": 5}
{u["id"]: u for u in users}          # index a list by id — daily move
{n % 3 for n in nums}                # set: unique values only
{v: k for k, v in d.items()}         # invert a dict

Flattening nested lists

matrix = [[1, 2], [3, 4], [5]]
[x for row in matrix for x in row]   # → [1,2,3,4,5]
# read the fors left-to-right, same order as nested loops.
# this is also the ceiling — one level. deeper = write a loop.