Writing Files¶

Programs often need to write data to files.

Examples include:

• saving program output
• generating reports
• storing logs
• exporting processed data

flowchart TD
    A[Program data]
    A --> B[open file in write mode]
    B --> C[write()]
    C --> D[data stored on disk]
````

---

## 1. Opening Files for Writing

To write to a file, specify mode `"w"`.

```python
f = open("output.txt", "w")

This creates the file if it does not exist.

If the file exists, it is overwritten.

2. Writing Text¶

f = open("output.txt", "w")

f.write("Hello\n")
f.write("Python\n")

f.close()

3. Appending to Files¶

Mode "a" appends to the end of a file.

f = open("log.txt", "a")

f.write("New entry\n")

f.close()

4. Writing Multiple Lines¶

lines = ["a\n", "b\n", "c\n"]

f = open("letters.txt", "w")
f.writelines(lines)
f.close()

5. File Modes¶

Example:

f = open("data.txt", "r")

6. Worked Example¶

numbers = [1, 2, 3]

f = open("numbers.txt", "w")

for n in numbers:
    f.write(str(n) + "\n")

f.close()

7. Common Pitfalls¶

Overwriting files accidentally¶

Using "w" replaces existing content.

Writing non-string objects¶

write() expects strings.

Convert values first.

f.write(str(x))

8. Summary¶

Key ideas:

• files can be opened in write or append mode
• write() stores text data
• writelines() writes multiple lines
• writing often requires converting values to strings

File writing allows programs to persist data beyond program execution.