✍️ Python Write/Create Files – write(), writelines(), Modes Explained

🧲 Introduction – Why Learn File Writing in Python?

Writing to files is a vital part of real-world programming. Whether you’re logging data, exporting reports, or generating configuration files, Python’s built-in functions make file creation and manipulation seamless and powerful.

🎯 In this tutorial, you’ll learn:

• How to create and write to files in Python
• File modes like w, a, and x
• Writing strings vs writing lists
• Appending vs overwriting data
• Best practices for efficient file handling

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📂 1. Create or Write to a File using open()

✅ Syntax:

file = open("output.txt", "w")
file.write("This is a new file.")
file.close()

✅ This creates a new file if it doesn’t exist, or overwrites if it does.

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🔐 Best Practice – Use with Statement

with open("output.txt", "w") as file:
    file.write("Hello, Python File Writer!")

✅ Why use with?

• Automatically closes the file
• Cleaner and safer code

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✏️ 2. Python File Write Modes Explained

Mode	Description
'w'	Write (overwrite file if exists)
'a'	Append (add to end of file)
'x'	Create (fail if file already exists)
'w+'	Write + Read
'a+'	Append + Read
'b'	Binary mode (e.g., 'wb')
't'	Text mode (default)

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🧾 3. Writing with write() – Single String

with open("log.txt", "w") as f:
    f.write("Log Entry 1\n")
    f.write("Log Entry 2\n")

✅ Writes string data to file.

⚠️ Warning: This overwrites the file every time it’s opened in 'w' mode.

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📋 4. Writing with writelines() – List of Strings

lines = ["First Line\n", "Second Line\n", "Third Line\n"]
with open("data.txt", "w") as f:
    f.writelines(lines)

✅ Best for multiple lines stored in a list.

💡 Tip: Make sure each string ends with \n.

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➕ 5. Appending to a File – a Mode

with open("data.txt", "a") as f:
    f.write("Appended line\n")

✅ Adds content at the end without erasing existing data.

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🚫 6. Creating a New File – x Mode

with open("newfile.txt", "x") as f:
    f.write("This file is created only if it doesn't exist.")

✅ Fails if file already exists.

💡 Tip: Use it to avoid overwriting by mistake.

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💡 7. Writing Variables and Loops

names = ["Alice", "Bob", "Charlie"]
with open("names.txt", "w") as f:
    for name in names:
        f.write(name + "\n")

✅ Simple way to write lists with formatting.

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🧪 8. Real-World Example: Save User Input to File

name = input("Enter your name: ")
with open("users.txt", "a") as f:
    f.write(name + "\n")

✅ Appends user entries into a file like a guestbook.

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⚠️ 9. Common Mistakes to Avoid

❌ Forgetting to close file (fixed by using with)

❌ Using w when you meant to append with a

❌ Writing lists directly without joining strings

✅ Use:

f.write("\n".join(mylist))

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📌 Summary – Recap & Next Steps

Python’s open() function combined with write(), writelines(), and file modes like w, a, and x allows flexible, efficient file output. Use the with statement to avoid manual resource handling.

🔍 Key Takeaways:

• Use 'w' to overwrite, 'a' to append, 'x' to create new files
• Use write() for strings and writelines() for lists
• Always prefer with open() for safety
• Use loops or join() to handle lists or dynamic data

⚙️ Real-World Relevance:
Used in logging, data export, report generation, user input storage, and more.

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❓ FAQ – Python Write/Create Files

❓ How do I create a file only if it doesn’t exist?

✅ Use:

open("file.txt", "x")

Raises FileExistsError if file already exists.

❓ What’s the difference between write() and writelines()?

• write() → Writes a single string
• writelines() → Writes a list of strings

❓ How do I prevent overwriting files?

✅ Use 'a' mode to append
✅ Or 'x' mode to create file only if it doesn’t exist

❓ Can I write and read from the same file?

✅ Yes, use 'w+' or 'a+' mode:

with open("file.txt", "w+") as f:
    f.write("test")
    f.seek(0)
    print(f.read())

❓ Do I need to add \n manually?

✅ Yes. Python does not add newlines automatically with write() or writelines().

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