Python Exception Handling Cheatsheet

Master Python's try/except/finally syntax and common exceptions.

Exception Hierarchy

Exception	When it occurs
ValueError	Wrong value type
TypeError	Wrong data type
KeyError	Dict key missing
IndexError	List index out of range
FileNotFoundError	File doesn't exist
ZeroDivisionError	Division by zero
AttributeError	Object has no attribute
ImportError	Module not found
PermissionError	Insufficient permissions

Code Examples

# Basic try/except
try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")

# Multiple exceptions
try:
    data = int("not a number")
except (ValueError, TypeError) as e:
    print(f"Error: {e}")

# Full pattern with else and finally
def read_file(filename):
    try:
        with open(filename) as f:
            return f.read()
    except FileNotFoundError:
        print(f"File {filename} not found")
        return None
    except PermissionError:
        print("No permission to read")
        return None
    else:
        print("File read successfully")  # runs if no exception
    finally:
        print("Always runs")  # cleanup code

# Custom exceptions
class ValidationError(ValueError):
    def __init__(self, field, message):
        self.field = field
        super().__init__(f"{field}: {message}")

def validate_age(age):
    if not isinstance(age, int):
        raise TypeError(f"Expected int, got {type(age).__name__}")
    if age < 0 or age > 150:
        raise ValidationError("age", f"Must be 0-150, got {age}")
    return True

# Context manager for exceptions
from contextlib import suppress
with suppress(FileNotFoundError):
    os.remove("nonexistent.txt")  # silently ignored

---

Follow me for more Python cheatsheets! 🐍

Follow for more Python content!

---

喜欢这篇文章？关注获取更多Python自动化内容！

---

If you found this useful, you might like Python Interview Prep Guide — a practical resource that takes things a step further. At $24.99 it's a solid investment for your toolkit.