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

# Python Files I/O

# File Handling, CSV, and Context Managers

## 1. Reading and Writing Text Files with `open()`

The `open()` function is used to open a file for reading, writing, or appending.

### Syntax

```python theme={null}
file = open("filename.txt", "mode")
```

### Common File Modes

| Mode   | Description                                     |
| ------ | ----------------------------------------------- |
| `"r"`  | Read (default)                                  |
| `"w"`  | Write (creates new file or overwrites existing) |
| `"a"`  | Append (adds content to end of file)            |
| `"x"`  | Create a new file; error if file exists         |
| `"r+"` | Read and write                                  |

### Reading a File

```python theme={null}
file = open("sample.txt", "r")
content = file.read()
print(content)
file.close()
```

### Read Line by Line

```python theme={null}
file = open("sample.txt", "r")

for line in file:
    print(line)

file.close()
```

### Writing to a File

```python theme={null}
file = open("sample.txt", "w")
file.write("Hello, Python!")
file.close()
```

### Appending to a File

```python theme={null}
file = open("sample.txt", "a")
file.write("\nWelcome!")
file.close()
```

***

# 2. CSV Handling Using the `csv` Module

CSV (Comma-Separated Values) files store tabular data.

### Import Module

```python theme={null}
import csv
```

### Reading a CSV File

```python theme={null}
import csv

with open("students.csv", "r") as file:
    reader = csv.reader(file)

    for row in reader:
        print(row)
```

**Output Example**

```text theme={null}
['Name', 'Age']
['Alice', '20']
['Bob', '21']
```

### Writing to a CSV File

```python theme={null}
import csv

with open("students.csv", "w", newline="") as file:
    writer = csv.writer(file)

    writer.writerow(["Name", "Age"])
    writer.writerow(["Alice", 20])
    writer.writerow(["Bob", 21])
```

### Writing Multiple Rows

```python theme={null}
rows = [
    ["John", 22],
    ["Emma", 19],
    ["David", 21]
]

with open("students.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerows(rows)
```

### Using `DictReader`

```python theme={null}
import csv

with open("students.csv", "r") as file:
    reader = csv.DictReader(file)

    for row in reader:
        print(row["Name"], row["Age"])
```

***

# 3. Context Managers (`with` Statement)

A context manager automatically handles opening and closing files.

### Syntax

```python theme={null}
with open("file.txt", "r") as file:
    content = file.read()
    print(content)
```

### Advantages

* Automatically closes the file.
* Prevents resource leaks.
* Cleaner and more readable code.
* Handles exceptions safely.

### Without `with`

```python theme={null}
file = open("file.txt", "r")
content = file.read()
file.close()
```

### With `with`

```python theme={null}
with open("file.txt", "r") as file:
    content = file.read()
```

No need to call `close()`.

***

# Common File Methods

| Method         | Purpose                     |
| -------------- | --------------------------- |
| `read()`       | Reads the entire file       |
| `readline()`   | Reads one line              |
| `readlines()`  | Reads all lines into a list |
| `write()`      | Writes text                 |
| `writelines()` | Writes multiple lines       |
| `close()`      | Closes the file             |

***

# Example Program

```python theme={null}
import csv

# Write to a text file
with open("message.txt", "w") as file:
    file.write("Hello World!")

# Read the text file
with open("message.txt", "r") as file:
    print(file.read())

# Write CSV data
with open("marks.csv", "w", newline="") as file:
    writer = csv.writer(file)
    writer.writerow(["Name", "Marks"])
    writer.writerow(["Alice", 95])
    writer.writerow(["Bob", 88])

# Read CSV data
with open("marks.csv", "r") as file:
    reader = csv.reader(file)
    for row in reader:
        print(row)
```

## Quick Revision

* **`open()`**: Opens a file.
* **Modes**: `r` (read), `w` (write), `a` (append), `x` (create).
* **`csv.reader()`**: Reads CSV files.
* **`csv.writer()`**: Writes CSV files.
* **`csv.DictReader()`**: Reads CSV rows as dictionaries.
* **`with` statement**: Automatically closes files and makes code safer and cleaner.