python-crash-course-book/chap10_file_exceptions.md

# Chapter 10. Files and Exceptions

Files operation skipped here. Directly head to Exception

## Exceptions

* **exceptions** = objects python use to manage errors that arise during a program's execution. 
* When python unsure what to do, exception object is created.
  * If exceptions are handled correctly, program can continue run
  * If don't handle exception, program will halt and show *traceback*

Syntax of exception handling: `try-except`

* `try` tells python do sth
* `except` tell python what to do if an exception is raised.

### e.g. ZeroDivisionError Exception

* Simple error, python will create `ZeroDivisionError` in response to do "Divide by Zero".
* When you think an error may occur, you can write a `try-except` block to handle exception

```python
try:
    print(5/0)
except ZeroDivisionError:
    print("You can't divide by zero!")
```

If more code followed `try-except` block, the program would continue running. e.g. as shown below

### Using Exceptions to Prevent Crashes

Handling errors correctly is important when the program has more work to do after the error occurs.

```python
print("Give me two numbers, and I'll divide them.")
print("Enter 'q' to quit.")

while True:
    first_number = input("\nFirst number: ")
    if first_number == 'q':
        break
    second_number = input("Second number: ")
    if second_number == 'q':
        break
    
    answer = int(first_number) / int(second_number)
    print(answer)
```

This function ask for numbers and will calculate division. To properly cover potential failure, use `try-except-else` block

* `try` block: Python attempts to run code
* `except` block: catch possible error
* `else` block: if division operation is successful, carry on!

Hence, program can be more robust

```python
    try:
        answer = int(first_number) / int(second_number)
    except ZeroDivisionError:
        print("You can't divide by 0!")
    else:
        print(answer)
```

### Handling FileNotFoundError Exception

```python
filename = 'alice.txt'

with open(filename, encoding='utf-8') as f:
    contents = f.read()
```

If python cannot find `alice.txt`, it will raise halt, and show traceback

```
Traceback (most recent call last):
  File "alice.py", line 6, in <module>
    with open(filename, encoding='utf-8') as f:
FileNotFoundError: [Errno 2] No such file or directory: 'alice.txt'
```

After changing python code to

```python
filename = 'alice.txt'

try:
    with open(filename, encoding='utf-8') as f:
        contents = f.read()
except FileNotFoundError:
    print(f"Sorry, the file {filename} does not exist")
```

Python will show "Sorry, the file alice.txt does not exist"

### Failing Silently

If you want the program to fail silently and carry on, use `pass` to tell Python to do nothing, and skip the try-except-else blocks

```python
def count_words(filename):
    """Count the approximate number of words in a file."""
    try:
        --snip--
    except FileNotFoundError:
        pass
    else:
        --snip--

    filenames = ['alice.txt', 'siddhartha.txt', 'moby_dick.txt', 'little_women.txt']
    for filename in filenames:
        count_words(filename)
```

## Storing Data

`json.dump()` & `json.load`