Writing to file in Python

Writing to a file means saving data produced by your program so it can be accessed even after the program finishes running. This guide walks you through most common write tasks (overwrite, append, create-if-not-exists etc.) with clear examples, expected outputs and concise explanations.

Before writing to a file, let’s first understand the different ways to create one.

Creating a File

Creating a file is the first step before writing data. In Python you control creation behaviour with the mode passed to open() (or with pathlib helpers). Below are the common modes, minimal examples and a line-by-line explanation for each pattern:

Note: You can combine flags like "wb" (write + binary) or "a+" (append + read). Always pick a mode based on whether you want to overwrite, append, or strictly create a new file.

Now let's understand each mode one by one.

Overwrite an existing file

Opening a file in write mode ("w") clears any existing content before writing new data. This is useful when you want to start fresh and replace old information with new output.

Example: This code writes two lines to file.txt, overwriting any previous content.

with open("file.txt", "w", encoding="utf-8") as f:
    f.write("Created using write mode.\n")
    f.write("Second line.\n")

with open("file.txt", "r", encoding="utf-8") as f:
    print(f.read())

Output

Created using write mode.
Second line.

Explanation: Here "w" truncates file.txt and writes fresh content. The with block ensures the file is properly saved and closed.

Append to an existing file

Using "a" mode (append), we can add new data at the end of a file without removing what’s already there.

Example: This writes an extra line to file.txt without removing existing content.

with open("file.txt", "a", encoding="utf-8") as f:
    f.write("Appended line.\n")

with open("file.txt", "r", encoding="utf-8") as f:
    print(f.read())

Output

Created using write mode.
Second line.
Appended line.

Explanation: "a" always writes at the end. Useful for logs, reports, or when you want to keep history intact.

Create only if it does not exist

With "x" mode, Python creates a new file but raises FileExistsError if the file already exists. This prevents accidental overwrites.

Example: This tries to create file.txt only if it doesn’t exist, otherwise prints an informative message.

try:
    with open("file.txt", "x", encoding="utf-8") as f:
        f.write("Created using exclusive mode.\n")
except FileExistsError:
    print("file.txt already exists, exclusive creation aborted.")

Output

file.txt already exists, exclusive creation aborted.

Explanation: "x" mode is useful for safe file creation. It guarantees no overwriting if the file exists, Python raises a FileExistsError instead.

Writing multiple lines

Sometimes you need to write several lines at once instead of one by one. Python provides two common approaches: writelines() for lists of strings and join() for building a single text block.

Example 1: This writes a list of lines; each element includes a \n.

lines = ["First line\n", "Second line\n", "Third line\n"]
with open("file1.txt", "w", encoding="utf-8") as f:
    f.writelines(lines)

with open("file1.txt", "r", encoding="utf-8") as f:
    print(f.read())

Output

First line
Second line
Third line

Explanation: writelines() writes each string in the list directly. It does not add newlines automatically you must include \n in each string yourself.

Example 2: join + single write (often cleaner)

This creates the final text with '\n'.join(...) and writes once.

lines = ["Line A", "Line B", "Line C"]
text = "\n".join(lines) + "\n"
with open("file2.txt", "w", encoding="utf-8") as f:
    f.write(text)

with open("file2.txt", "r", encoding="utf-8") as f:
    print(f.read())

Output

Line A
Line B
Line C

Explanation: Building a single string and calling write() once can be more efficient and less error-prone, especially when you must ensure consistent newlines.

Writing to a Binary File

For non-text data (like images, audio, or other binary content), use binary write mode ("wb"). This treats the file as raw bytes instead of text.

Example: The following code writes a small sequence of bytes into a file called file.bin and then reads it back in binary mode.

data = b'\x00\x01\x02\x03\x04'
with open("file.bin", "wb") as f:
    f.write(data)

Output

b'\x00\x01\x02\x03\x04'

Explanation:

• data = b'\x00\x01\x02\x03\x04': Creates a bytes object (raw binary data).
• open("file.bin", "wb"): Opens file in binary write mode (creates/overwrites).
• f.write(data): Saves the bytes to the file.
• open("file.bin", "rb"): Opens file in binary read mode.
• print(f.read()): Reads and prints the stored bytes.