Common Runtime Errors¶

Programs may encounter various runtime errors during execution.

Understanding the most common errors helps programmers debug code quickly.

flowchart TD
    A[Runtime Errors]
    A --> B[TypeError]
    A --> C[ValueError]
    A --> D[IndexError]
    A --> E[KeyError]
    A --> F[ZeroDivisionError]
````

---

## 1. TypeError

Occurs when an operation is applied to incompatible types.

Example:

```python
"hello" + 5

Output:

TypeError: can only concatenate str (not "int") to str

2. ValueError¶

Occurs when a function receives a value of the correct type but an inappropriate value.

Example:

int("hello")

Output:

ValueError: invalid literal for int()

3. IndexError¶

Occurs when accessing an invalid index in a sequence.

numbers = [1, 2, 3]
print(numbers[10])

Output:

IndexError: list index out of range

4. KeyError¶

Occurs when accessing a missing dictionary key.

data = {"a": 1}
print(data["b"])

Output:

KeyError: 'b'

5. ZeroDivisionError¶

Occurs when dividing by zero.

print(10 / 0)

Output:

ZeroDivisionError

6. NameError¶

Occurs when a variable name is not defined.

print(x)

Output:

NameError: name 'x' is not defined

7. AttributeError¶

Occurs when accessing a nonexistent attribute.

x = 5
x.append(3)

Output:

AttributeError: 'int' object has no attribute 'append'

8. Worked Examples¶

Example 1: safe dictionary access¶

data = {"a": 1}

print(data.get("b"))

Output:

None

Example 2: checking length before indexing¶

values = [1, 2]

if len(values) > 3:
    print(values[3])

9. Summary¶

Key ideas:

• runtime errors occur during execution
• different exception types describe different problems
• recognizing common exceptions helps with debugging
• careful checks can prevent some errors