How to read a file from command line using Python?

Reading files from the command line is a common task in many Python scripts and automation workflows. Whether we're building a simple utility to process text files or developing a complex data analysis tool, being able to accept file input directly from the command line enhances the flexibility and usability of our program.

Python provides built-in modules such as sys and argparse that make this task straightforward and efficient. In this article, we'll explore different methods to read a file from the command line using Python.

Using sys.argv

In Python, sys.argv is a list provided by the built-in sys module that stores the command-line arguments passed to a script when it is run.

sys.argv

Where:

• sys.argv is a list containing command-line arguments passed to the script
• sys.argv[0] is the script name
• sys.argv[1] is the first argument, typically the file path

Example

Below is an example that uses sys.argv to receive the file path from the command line and read the file ?

import sys

def read_file_from_command_line():
    # Check if the filename is provided
    if len(sys.argv) < 2:
        print("Usage: python script.py <filename>")
        sys.exit(1)
    
    # sys.argv[1] holds the filename
    filename = sys.argv[1]
    
    try:
        with open(filename, 'r') as file:
            content = file.read()
            print("File Content:")
            print(content)
    except FileNotFoundError:
        print(f"Error: File '{filename}' not found.")
    except IOError:
        print(f"Error: Cannot read file '{filename}'.")

# Call the function
if __name__ == "__main__":
    read_file_from_command_line()

Run the script from the command line using:

python script.py data.txt

The output will be:

File Content:
This is the content of data.txt

Using argparse

The argparse module is a built-in Python module for parsing command-line arguments. It provides more flexible and user-friendly ways to define and process arguments, automatically generates help messages, and supports both positional and optional arguments.

Basic syntax to create an ArgumentParser object ?

import argparse

parser = argparse.ArgumentParser()
parser.add_argument("filename")
args = parser.parse_args()

Where:

• argparse.ArgumentParser() creates a new argument parser object
• add_argument("filename") defines a required positional argument named filename
• args.filename retrieves the value passed to that argument

Example

Below is an example that uses argparse to receive the file path from the command line and read the file ?

import argparse

def read_file_with_argparse():
    parser = argparse.ArgumentParser(description="Read a file from command line.")
    parser.add_argument("filename", help="The path to the file to read")
    args = parser.parse_args()
    
    try:
        with open(args.filename, 'r') as file:
            content = file.read()
            print("File Content:")
            print(content)
    except FileNotFoundError:
        print(f"Error: File '{args.filename}' not found.")
    except IOError:
        print(f"Error: Cannot read file '{args.filename}'.")

# Call the function
if __name__ == "__main__":
    read_file_with_argparse()

Run the script from the command line using:

python script.py data.txt

The output will be:

File Content:
This is the content of data.txt

Comparison

Conclusion

Use sys.argv for simple scripts that need basic command-line file input. Use argparse for more robust applications that require better error handling and automatic help generation.