Writing a class with two special methods is a lot of ceremony for a simple job. The contextlib module lets you write a context manager as a generator function instead: the code before yield is the setup, the yielded value is what as receives, and the code after is the tidy-up. Wrap the yield in try/finally so the tidy-up runs even on errors:

from contextlib import contextmanager
import time

@contextmanager
def timer(label):
    start = time.perf_counter()
    try:
        yield
    finally:
        print(f"{label}: {time.perf_counter() - start:.4f}s")

with timer("adding"):
    total = sum(range(500000))