> ## Documentation Index
> Fetch the complete documentation index at: https://python4ai.codewithsiva.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Common errors

> Understand Python's error types

## Errors you'll encounter

Here are the most common Python errors and how to fix them. Understanding these will save you hours of debugging.

## FileNotFoundError

Happens when a file doesn't exist:

```python theme={null}
# This fails if file doesn't exist
with open('missing.txt', 'r') as f:
    content = f.read()
```

**Fix:**

```python theme={null}
import os

# Check first
if os.path.exists('data.txt'):
    with open('data.txt', 'r') as f:
        content = f.read()
else:
    print("File not found")

# Or use try-except
try:
    with open('data.txt', 'r') as f:
        content = f.read()
except FileNotFoundError:
    content = ""  # Default value
```

## ValueError

Happens when a value is wrong type or format:

```python theme={null}
# These cause ValueError
int("hello")        # Can't convert to number
int("12.5")         # int() doesn't accept decimals
list.remove("x")    # Item not in list
```

**Fix:**

```python theme={null}
# Validate user input
try:
    age = int(input("Age: "))
except ValueError:
    print("Please enter a number")

# Convert carefully
float_str = "12.5"
number = int(float(float_str))  # Convert to float first
```

## KeyError

Happens when a dictionary key doesn't exist:

```python theme={null}
user = {"name": "Alice", "age": 25}
print(user["email"])  # KeyError - no email key
```

**Fix:**

```python theme={null}
# Check if key exists
if "email" in user:
    print(user["email"])

# Use get() with default
email = user.get("email", "no-email@example.com")

# Or handle the error
try:
    print(user["email"])
except KeyError:
    print("Email not found")
```

## IndexError

Happens when accessing invalid list position:

```python theme={null}
numbers = [1, 2, 3]
print(numbers[5])  # IndexError - only 3 items
```

**Fix:**

```python theme={null}
# Check length first
if len(numbers) > 5:
    print(numbers[5])

# Use negative indexing carefully
last_item = numbers[-1] if numbers else None

# Handle the error
try:
    print(numbers[5])
except IndexError:
    print("List too short")
```

## TypeError

Happens when operations use wrong types:

```python theme={null}
# These cause TypeError
"hello" + 5            # Can't add string and number
len(42)                # Numbers don't have length
int([1, 2, 3])         # Can't convert list to int
```

**Fix:**

```python theme={null}
# Convert types explicitly
"hello" + str(5)       # "hello5"
str(42)                # "42"

# Check type first
if isinstance(value, str):
    print(len(value))
```

## AttributeError

Happens when accessing non-existent attributes:

```python theme={null}
text = "hello"
text.append("!")  # AttributeError - strings don't have append
```

**Fix:**

```python theme={null}
# Use correct methods
text = "hello"
text = text + "!"  # Strings are immutable

# Check if attribute exists
if hasattr(obj, 'append'):
    obj.append(item)
```

<Note>
  Error messages are your friends! They tell you:

  * What went wrong (error type)
  * Where it happened (file and line number)
  * Why it happened (error message)

  Always read the full error message before trying to fix it.
</Note>

## Reading error messages

Python error messages show a "traceback":

```
Traceback (most recent call last):
  File "script.py", line 5, in <module>
    result = 10 / 0
ZeroDivisionError: division by zero
```

Read from bottom to top:

1. **Error type**: `ZeroDivisionError`
2. **Error message**: `division by zero`
3. **Location**: `script.py`, line 5
4. **Code that failed**: `result = 10 / 0`

## Common mistake patterns

<AccordionGroup>
  <Accordion title="Off-by-one errors">
    ```python theme={null}
    # Wrong - lists start at 0
    items = ["a", "b", "c"]
    third_item = items[3]  # IndexError

    # Right
    third_item = items[2]  # "c"
    ```
  </Accordion>

  <Accordion title="Modifying while iterating">
    ```python theme={null}
    # Wrong - changes list size during iteration
    numbers = [1, 2, 3, 4, 5]
    for num in numbers:
        if num % 2 == 0:
            numbers.remove(num)  # Dangerous!

    # Right - create new list
    odd_numbers = [n for n in numbers if n % 2 != 0]
    ```
  </Accordion>

  <Accordion title="Mutable default arguments">
    ```python theme={null}
    # Wrong - same list for all calls!
    def add_item(item, items=[]):
        items.append(item)
        return items

    # Right - create new list each time
    def add_item(item, items=None):
        if items is None:
            items = []
        items.append(item)
        return items
    ```
  </Accordion>
</AccordionGroup>

## What's next?

Now that you understand error handling, let's learn about classes - Python's way of organizing code and data together.

<Card title="Classes" icon="shapes" href="/advanced/classes">
  Object-oriented programming
</Card>