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Writing to file in Python

Last Updated : 19 Dec, 2024

Writing to a file in Python means saving data generated by your program into a file on your system. This article will cover the how to write to files in Python in detail.

Creating a file is the first step before writing data to it. In Python, we can create a file using the following three modes:

• Write ("w") Mode: This mode creates a new file if it doesn't exist. If the file already exists, it truncates the file (i.e., deletes the existing content) and starts fresh.
• Append ("a") Mode: This mode creates a new file if it doesn't exist. If the file exists, it appends new content at the end without modifying the existing data.
• Exclusive Creation ("x") Mode: This mode creates a new file only if it doesn't already exist. If the file already exists, it raises a FileExistsError.

Example:

# Write mode: Creates a new file or truncates an existing file

with open("file.txt", "w") as f:
    f.write("Created using write mode.")

f = open("file.txt","r")
print(f.read())

# Append mode: Creates a new file or appends to an existing file

with open("file.txt", "a") as f:
    f.write("Content appended to the file.")

f = open("file.txt","r")
print(f.read())

# Exclusive creation mode: Creates a new file, raises error if file exists

try:
    with open("file.txt", "x") as f:
        f.write("Created using exclusive mode.")
except FileExistsError:
    print("Already exists.")

Created using write mode.
Created using write mode.Content appended to the file.
Already exists.

If we want to modify or add new content to an already existing file, we can use two methodes:

write mode ("w"): This will overwrite any existing content,

writelines(): Allows us to write a list of string to the file in a single call.

Example:

# Writing to an existing file (content will be overwritten)
with open("file1.txt", "w") as f:
    f.write("Written to the file.")
    
f = open("file1.txt","r")
print(f.read())

# Writing multiple lines to an existing file using writelines()
s = ["First line of text.\n", "Second line of text.\n", "Third line of text.\n"]

with open("file1.txt", "w") as f:
    f.writelines(s)
    
f = open("file1.txt","r")
print(f.read())

Written to the file.
First line of text.
Second line of text.
Third line of text.

Explanation:

• open("example.txt", "w"): Opens the file example.txt in write mode. If the file exists, its content will be erased and replaced with the new data.
• file.write("Written to the file."): Writes the new content into the file.
• file.writelines(lines): This method takes a list of strings and writes them to the file. Unlike write() which writes a single string writelines() writes each element in the list one after the other. It does not automatically add newline characters between lines, so the \n needs to be included in the strings to ensure proper line breaks.

When dealing with non-text data (e.g., images, audio, or other binary data), Python allows you to write to a file in binary mode. Binary data is not encoded as text, and using binary write mode ("wb") ensures that the file content is handled as raw bytes.

Example:

# Writing binary data to a file
bin = b'\x00\x01\x02\x03\x04'

with open("file.bin", "wb") as f:
    f.write(bin)

f = open("file.bin","r")
print(f.read())

Explanation:

• bin= b'\x00\x01\x02\x03\x04': The b before the string indicates that this is binary data. Each pair represents a byte value.
• open("file.bin", "wb"): Opens the file file.bin in binary write mode. If the file doesn't exist, Python will create it.
• file.write(bin): Writes the binary data to the file as raw bytes.

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