When working with files in Python, it’s common to encounter situations where you need to append new lines to an existing file without overwriting its current content. Additionally, managing whitespace correctly is crucial to avoid unwanted spaces in your output. In this post, we’ll address both of these issues with a practical example.

The Problem

Suppose you have a piece of code that processes lines from a file and appends certain lines to another file. Initially, your code might look something like this:

if line.startswith("START"):
    data = process_line(line, field_list)
    data_list.append(data)

    with open(self.report_type + ".txt", 'w') as fp:
        fp.write("%s\n" % line)

The problem with the above code is that it opens the file in write mode ('w'), which overwrites the file every time a new line is added. To append lines instead of overwriting, we need to open the file in append mode ('a').

Solution: Appending Lines

Here’s the corrected code that opens the file in append mode and removes extra whitespace:

def process_file(input_file, report_type):
    data_list = []

    with open(input_file, 'r') as infile:
        for line in infile:
            if line.startswith("START"):
                data = process_line(line, field_list)
                data_list.append(data)

                with open(report_type + ".txt", 'a') as fp:
                    fp.write(line.strip() + "\n")

    return data_list

# Example usage
data_list = process_file("input.txt", "report")
print("Processing complete. Data list:", data_list)

Explanation

1. Opening the File in Append Mode:
   • By using 'a' instead of 'w', we ensure that each new line is added to the end of the file without deleting its existing content.
2. Removing Extra Whitespace:
   • The line.strip() method is used to remove any leading or trailing whitespace, including newline characters.
   • After stripping the whitespace, we add a newline character "\n" explicitly to ensure that each line is correctly terminated.

Conclusion

Managing file operations in Python requires careful attention to how files are opened and how lines are written. By using append mode and stripping unnecessary whitespace, you can ensure that your file operations are efficient and produce the desired output.