Exception hierarchy

Learn ValueError, TypeError, KeyError and others.

Exception hierarchy

In Python all errors form a tree. The root is BaseException; Exception is its subclass. Common errors like ValueError, TypeError and KeyError are subclasses of Exception:

BaseException
├── Exception
│   ├── ValueError
│   ├── TypeError
│   └── KeyError
└── KeyboardInterrupt

except Exception catches all program errors but not system errors like KeyboardInterrupt. This is safer than a bare except:.

Knowing the hierarchy helps you write the right except block.

Multiple except blocks

After a single try block you can write multiple except blocks. Python reads top to bottom and runs the first match:

try:
    value = int("abc")
except ValueError:
    print("değer hatası")
except TypeError:
    print("tür hatası")
except Exception as e:
    print(f"diğer: {e}")

With as e you bind the error object to a variable and access its message. Always put the most general one (Exception) last.

This lets you react differently to different error types.

Exercise Catch value error with multiple except

Inside a try block write value = int("abc"). Inside an except ValueError: block write print("değer hatası"). Inside an except Exception as e: block write print(f"diğer: {e}").

Show example solution

try:
    value = int("abc")
except ValueError:
    print("değer hatası")
except Exception as e:
    print(f"diğer: {e}")

Test yourself

Which is NOT a subclass of Exception?
- KeyboardInterrupt
- TypeError
- KeyError
- ValueError
Show answer

KeyboardInterrupt — KeyboardInterrupt is a direct subclass of BaseException, not Exception.
Match the hierarchy

BaseException

Root class
Exception

Program errors
ValueError

Subclass
as e

Binds the error object

Complete the except as e pattern

try:
    value = int("abc")
____ as e:
    print(f"hata: {e}")

Show answer

try:
    value = int("abc")
except Exception as e:
    print(f"hata: {e}")

Next lesson: Custom exceptions

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