Python Exceptions

An exception is a run-time error — unlike a syntax error, it can’t be detected until the offending line actually executes. Left uncaught, it aborts the program.

Common built-in exceptions

Exception	Raised when…
AssertionError	an assert fails
IOError	a file does not exist
IndexError	a sequence index is out of range
KeyError	a dict key does not exist
NameError	a variable/name does not exist
TypeError	an unexpected type is given to a function/operator
ValueError	correct type, but an inappropriate value
ZeroDivisionError	division by zero

Catching exceptions

try:
    <code that may raise>
except SomeError:
    <handle SomeError>
except AnotherError:
    <handle AnotherError>
else:
    <runs only if no exception occurred>
finally:
    <always runs>

• Specific exceptions must be listed before a bare except: — Python enforces this ordering.
• A bare except: catches any exception, but hides genuine bugs — prefer catching specific exception types.
• Keep try blocks short: once an exception is raised inside one, the rest of the block is skipped.

Raising exceptions

raise SomeError                  # bare
raise SomeError("message")       # with an explanatory message

LBYL vs. EAFP

Two equally valid philosophies for guarding against errors:

• LBYL (Look Before You Leap) — check the condition with an if before attempting the risky operation.
• EAFP (Easier to Ask for Forgiveness than Permission) — attempt the operation inside a try, and handle the resulting exception if it fails.