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(common-exceptions)=
# Common Python exceptions 

Python has dozens of specific errors that can be raised when code fails to run. Below are a few common ones that you may encounter in [Activity 3](clean-code-activity-3). 
	
## `TypeError`

Occurs when an operation is applied to an object of an inappropriate type.

```{code-cell} ipython3
---
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slideshow:
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tags: [raises-exception]
---
# Example: Trying to add a number and a string
1 + 'string'  # This will raise a TypeError
```

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(value_error)=
### `ValueError`

- **Raised when** a function receives an argument of the right type but an invalid value.
- **Example:** `int('abc')` (trying to convert an invalid string to an integer).

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
tags: [raises-exception]
---
int("abc")
```

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### `KeyError`

- **Raised when** a dictionary key is not found.
- **Example:** `my_dict['nonexistent_key']` (trying to access a key that doesn’t exist in the dictionary).

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
tags: [raises-exception]
---
# Example: Accessing a nonexistent key in a dictionary
my_dict = {"a": 1, "b": 2}
my_dict['nonexistent_key']
```

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### `IndexError`

- **Raised when** an invalid index is used to access a list or tuple.
- **Example:** `my_list[10]` (trying to access the 11th element of a list with fewer elements).

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
tags: [raises-exception]
---
my_list = [1, 2, 3]
my_list[10] 
```

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### `AttributeError`

Raised when an object does not have a specific attribute or method.

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
tags: [raises-exception]
---
my_string = "Hello"
my_string.nonexistent_method()
```

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(file_error)=
### `FileNotFoundError`

A `FileNotFoundError` occurs in Python when the code attempts to open or access a file that does not exist at the specified path.

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
tags: [raises-exception]
---
with open("data/nonexistent_file.json", "r") as file:
    data = file.read()
```

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By catching this exception, you can 

1. Raise a kinder and more informative message.
2. Direct the user toward the next steps
3. FUTURE: write tests for this step of the workflow (if you are creating a package!) that make sure that it handles a bad file path properly.

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
tags: [raises-exception]
---
from pathlib import Path

file_path = Path("data") / "nonexistent_file.json"
try:
    with open(file_path, "r") as file:
        data = file.read()
except FileNotFoundError as fe:
    raise FileNotFoundError(f"Oops! it looks like you provided a path to a file that doesn't exist. You provided: {file_path}. Make sure the file path exists. ")
```

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If you don't raise the error but instead provide a print statement, you can provide a simple, clean output without the full "stack" or set of Python messages that provides the full "tracking" or traceback of where the error originated. 

The challenge with not raising a FileNotFound error is that it will be a bit trickier to test the output. 

:::{todo}
* you could do `sys.exit` too... but I've run into issues with that in the past (I wish I could remember what they were).
:::

```{code-cell} ipython3
---
editable: true
slideshow:
  slide_type: ''
---
file_path = Path("data") / "nonexistent_file.json"
try:
    with open(file_path, "r") as file:
        data = file.read()
except FileNotFoundError as fe:
    print(f"Oops! it looks like you provided a path to a file that doesn't exist. You provided: {file_path}. Make sure the file path exists. ")
```