How Can I Efficiently Add a Line to the Beginning of a File in Python?

Appending a Line to the Beginning of a File

Modifying the beginning of a file can be challenging, especially if the file is large. Traditionally, a separate file was used as a workaround. However, there are more efficient ways to achieve this using Python.

Understanding File Modes

When opening a file in append mode ('a'), any writing is directed to the end of the file, regardless of the current position of the file pointer. This is problematic when trying to modify the beginning of the file.

Solution 1: Loading the File into Memory

For smaller files that can be loaded into memory, the following code snippet can be used:

<code class="python">def line_prepender(filename, line):
    with open(filename, 'r+') as f:
        content = f.read()
        f.seek(0, 0)
        f.write(line.rstrip('\r\n') + '\n' + content)</code>

This method reads the entire file into memory, appends the new line to the beginning, and rewrites the file.

Solution 2: Using fileinput

For larger files, the fileinput module can be useful:

<code class="python">def line_pre_adder(filename, line_to_prepend):
    f = fileinput.input(filename, inplace=1)
    for xline in f:
        if f.isfirstline():
            print(line_to_prepend.rstrip('\r\n') + '\n' + xline, end='')
        else:
            print(xline, end='')</code>

This method is more efficient for large files as it does not load the entire file into memory. It iterates through the file line by line, printing the new line at the beginning of the first line and then the remaining lines unmodified. Note that this method requires the file to be writable.

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