Counting Number of Lines in a File Without Opening It in Python

If you want to find the number of lines (records) in a file using Python, you can open and read it using Python functions to determine the number of lines. However, there could be scenarios where you have a very large file with millions of records and you need to find the number of records before opening the file for processing.

In such cases, you can use the subprocess module to leverage system commands like wc (word count) on Unix-based systems (Linux, macOS). The wc -l command counts the number of lines in a file. The subprocess.run() function allows you to run external or shell commands within your Python script. It executes the command, captures the output, and then you can parse it to extract the line count.

In this post, I will provide you with a Python script that utilizes the subprocess module to return the number of lines in a file without explicitly opening and reading it. This method is efficient for Unix-based systems.

Here is the Python code:

import argparse
import subprocess

def count_records(file_path):
    try:
        # Use the 'wc -l' command to count lines
        result = subprocess.run(['wc', '-l', file_path], capture_output=True, text=True)
        # Extract the line count from the output
        line_count = int(result.stdout.split()[0])
        return line_count
    except FileNotFoundError:
        print(f"File not found: {file_path}")
        return None
    except Exception as e:
        print(f"An error occurred: {e}")
        return None


def main():
    """
    This program returns the number of lines in a file without explicitly opening and reading it.
    """
    # get file path from the command line
    parser = argparse.ArgumentParser(description="Find number of lines in a file")
    parser.add_argument("-file_path", required=True, help="provide full path to the file")
    args = parser.parse_args()

    # count the number of lines
    num_records = count_records(args.file_path)
    if num_records is not None:
        print(f"Number of records in the file: {num_records}")

if __name__ == "__main__":
    main()

To run this code, save it as “file_rec_count.py” (or whatever name you prefer) and execute it from the command line with the full path to your file, as shown below:

python file_rec_count.py -file_path "path_to_your_file"

Please let me know in the comments if you encounter any issues with the code.