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6.1 Errors and Exceptions

Python distinguishes syntax errors from runtime exceptions.

Syntax errors vs exceptions​

  • Syntax errors: detected while parsing code.
  • Exceptions: raised during execution (e.g., TypeError, ValueError, OSError).

Handling with try/except​

try:
    value = int(input("Enter number: "))
except ValueError:
    print("Invalid integer")

else and finally​

  • else runs only if no exception occurred in try.
  • finally runs regardless of success/failure, useful for cleanup.
try:
    f = open("data.txt")
except OSError:
    print("Could not open file")
else:
    print(f.readline())
    f.close()

Raising and chaining​

try:
    connect_db()
except ConnectionError as exc:
    raise RuntimeError("Database startup failed") from exc

Context managers (with)​

Prefer with to guarantee cleanup for resources like files and sockets.

with open("data.txt") as f:
    data = f.read()

Official references​