Python File Handling

File handling lets your program store information outside the program run. This is how apps create logs, save reports, and manage data files.

The most important rule: use `with` so the file closes automatically.

## Write to a file

```python
with open("notes.txt", "w") as file:
    file.write("Hello from San Diego!")
```

Now `notes.txt` exists and contains one line.

## Read the file

```python
with open("notes.txt", "r") as file:
    content = file.read()

print(content)
```

Expected output:

```
Hello from San Diego!
```

## Append to the file

```python
with open("notes.txt", "a") as file:
    file.write("\nNew line added!")
```

Read again:

```python
with open("notes.txt", "r") as file:
    print(file.read())
```

Expected output:

```
Hello from San Diego!
New line added!
```

## Read line by line (best for large files)

```python
with open("notes.txt", "r") as file:
    for line in file:
        print(line.strip())
```

Expected output:

```
Hello from San Diego!
New line added!
```

## Handling missing file error

```python
try:
    with open("missing.txt", "r") as file:
        print(file.read())
except FileNotFoundError:
    print("The file does not exist.")
```

Expected output:

```
The file does not exist.
```

## Graph: safe file workflow

```mermaid
flowchart TD
  A[Open with 'with'] --> B{File exists?}
  B -->|Yes| C[Read/Write]
  B -->|No| D[Handle error]
  C --> E[Auto close]
  D --> E[Auto close]
```

In the next lesson, you will start intermediate Python with list comprehensions, which help you write cleaner data transformations.