Python Exception Types Reference

These two are the most common and the most often confused:

int("unknown")        # ValueError: invalid literal for int() with base 10: 'unknown'
"Tickets: " + 12      # TypeError: can only concatenate str (not "int") to str
len(42)               # TypeError: object of type 'int' has no len()

A ValueError means the function accepts that type, but not that particular value: int() accepts strings, but not this one. A TypeError means the operation doesn't support the type at all, or a function was called with the wrong number or kind of arguments.

The distinction suggests the fix. A ValueError usually comes from data, such as user input or a file, so validate it and report the bad record. A TypeError usually comes from code that combines the wrong things, so fix the code, often with a conversion or by correcting a function call. When you raise exceptions in your own functions, follow the same convention.

KeyError and IndexError both inherit from LookupError and mean "the thing you asked for isn't there":

prices = {"stalls": 45}
prices["balcony"]           # KeyError: 'balcony'
seats = ["A1", "A2"]
seats[5]                    # IndexError: list index out of range

A related one is AttributeError, raised for a missing attribute or method. The most common form is 'NoneType' object has no attribute ..., meaning a variable you expected to hold an object holds None, often because a function returned nothing.

Prevent lookups from failing where absence is normal: prices.get("balcony", 0), if index < len(seats), getattr(obj, "name", None). Where a missing key indicates a bug or bad data, let the exception happen, or catch it and raise a clearer one. Wrapping large blocks in except LookupError: can hide a real mistake in your data model, such as a misspelt key.

File and network operations raise subclasses of OSError, each describing a different external condition:

try:
    with open("sales.csv", encoding="utf-8") as file:
        rows = file.readlines()
except FileNotFoundError:
    print("sales.csv is missing: export it from the box office first")
except PermissionError:
    print("sales.csv is locked: close it in Excel and try again")

Different causes need different remedies, which is why catching the specific subclass is useful. Retrying can help with a timeout, but never with a missing file or a permission problem. except OSError as e: catches the whole family and e.errno and e.filename give details.

Every exception class inherits from BaseException. Ordinary errors inherit from its subclass Exception; a few special ones don't:

BaseException
 ├── KeyboardInterrupt      (Ctrl+C)
 ├── SystemExit             (sys.exit())
 └── Exception
      ├── ValueError
      ├── TypeError
      ├── LookupError
      │    ├── KeyError
      │    └── IndexError
      ├── OSError
      │    ├── FileNotFoundError
      │    └── PermissionError
      └── ...

Catching a parent catches all its children, so except LookupError handles both KeyError and IndexError. Order except clauses from most specific to most general, because Python uses the first one that matches.

Catch the narrowest exception you expect, around the smallest piece of code that can raise it. except Exception: should be rare and should log the error; a bare except: or except BaseException: also swallows Ctrl+C and sys.exit(), making the program hard to stop.

Task: classify ticket import failures and handle only the expected conversion and filesystem cases.

from pathlib import Path

def read_quantities(path):
    try:
        lines = Path(path).read_text(encoding="utf-8").splitlines()
    except FileNotFoundError:
        print(f"{path} not found")
        return []
    quantities = []
    for line in lines:
        try:
            quantities.append(int(line))
        except ValueError:
            print(f"Skipping {line!r}")
    return quantities

Path("qty.txt").write_text("4\nfour\n6\n", encoding="utf-8")
print(read_quantities("qty.txt"))     # Skipping 'four', then [4, 6]
print(read_quantities("none.txt"))    # none.txt not found, then []
Path("qty.txt").unlink()

Each try wraps one kind of operation, so a bug elsewhere, such as a typo, still raises normally. For the worked check, the exception that normally describes invalid integer text is ValueError.

The documentation lists every built-in exception with the full hierarchy. Continue with debugging and logging. Practise with the Python exercises and Python quiz.