Exception Handling — Notes

Syntax errors break Python's grammar (missing colon, unmatched bracket, = for == in a condition). Python finds them while parsing, before running, so no statement runs and they cannot be handled in the same program.

Exceptions (runtime errors) occur while a valid program runs, e.g. 10 / 0. Python creates an exception object and looks for a handler; with none, the program stops and prints a traceback whose last line is ExceptionName: message.

Logical errors give wrong results with no message; only testing reveals them.

Each built-in exception is a class named after the problem:

• ValueError — right type, unsuitable value: int("7k")
• TypeError — unsuitable type for an operation: "Rs " + 50
• NameError — name not defined
• IndexError — sequence index out of range
• KeyError — dictionary key missing
• ZeroDivisionError — divisor zero in /, // or %
• FileNotFoundError (an OSError, also called IOError) — file missing
• ImportError (and ModuleNotFoundError) — import fails
• EOFError — input() reaches end of input
• OverflowError — result too large
• SyntaxError — grammar error

The error lines in the tables were produced by Python 3.12; older versions word some messages slightly differently, but the exception names are the same.

raise throws a named exception deliberately: raise ValueError("fee cannot be negative"). The rest of the block is skipped, and a matching handler (or the end of the program) follows.

assert checks a condition: assert cond, "message" raises AssertionError with the message only when the condition is False; it is mainly used to check assumptions during development.

limit = 3
try:
    assert limit >= 5, "limit too small"
except AssertionError as e:
    print("Check failed:", e)

Output: Check failed: limit too small

Risky statements go in try; handlers go in except clauses.

• The try block stops at the first failing statement; the rest of it is skipped.
• Python compares the exception with the except clauses top to bottom and runs the first match; specific handlers first, except Exception last.
• except (ValueError, TypeError): handles either; except ValueError as e: keeps the object, print(e) shows the message.
• A bare except: catches everything and hides bugs.
• After the handler, execution continues after the whole try statement.

Order: try, except …, else, finally.

• else runs only if the try block raised no exception: use it for work that needs the risky step to succeed.
• finally runs in every case (success, handled error, unhandled error, break): use it for clean-up such as closing files.

try:
    qty = int("3")
except ValueError:
    print("bad")
else:
    print("qty", qty)
finally:
    print("done")

Output:

qty 3
done

Put the input step inside a loop with try-except, and break when the input is valid; use raise inside try for values outside the allowed range so that one handler deals with all invalid entries. Keep try blocks small, catch specific exceptions, show helpful messages, and avoid a bare except or a silent pass.