pitfalls.md

Chapter 18 — Pitfalls: Control Flow

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Pitfall 1: The Missing finally in @contextmanager

When using @contextmanager, the code before the yield is setup, and the code after is teardown. If an exception occurs inside the user's with block, the exception is propagated directly out of the yield expression inside your generator.

from contextlib import contextmanager

@contextmanager
def lock_manager():
    print("Acquiring lock...")
    yield
    # BUG: If an exception occurs in the with block, this generator
    # crashes at the yield, and the lock is NEVER released!
    print("Releasing lock...")

with lock_manager():
    raise ValueError("Oops!")

Why it fails: The generator crashes, skipping the rest of its body. Fix: You must always wrap the yield in a try/finally block.

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Pitfall 2: Confusing for/else Readability

The keyword else on a for loop is historically a terrible naming choice. Most programmers intuitively read it as "execute this if the loop didn't run (because the list was empty)".

results = []
for user in results:
    print(f"Processing {user}")
else:
    # BUG (Logical): Developer thinks this runs because results is empty.
    # It actually runs because the loop completed naturally (without breaking)!
    print("No users found.")

Fix: Because for/else is so counter-intuitive to developers coming from other languages, it is highly discouraged in modern style guides unless it is strictly used with a break statement inside the loop (e.g., a search loop where else acts as "not found").

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Pitfall 3: Wrapping Too Much in try/except

A massive anti-pattern is putting code that should not be caught into the try block.

# BAD
try:
    config = load_json()     # We expect JSONDecodeError here
    run_server(config)       # If this raises JSONDecodeError due to a bug, it gets caught accidentally!
except JSONDecodeError:
    print("Bad config file")

# GOOD
try:
    config = load_json()
except JSONDecodeError:
    print("Bad config file")
else:
    # Only executes if the try succeeded. 
    # Any exception raised here bubbles up properly!
    run_server(config)