Argparse

Many Python programs receive arguments from the command-line: this reduces the needs for constants and settings files. With argparse, a module, we can handle these arguments.

To use argparse, we must first call ArgumentParser. Then we call add_argument to set up the arguments, and finally we call parse_args to parse them.

Example

This program receives a "file" argument and then opens the specified file and prints out its contents. The file name is not stored in the program source code.

Step 1 We create an ArgumentParser and specify its description. The description is shown when the help command is specified.

Step 2 For each argument we want to handle, we must call add_argument. We specify "store" as the action, and also make "file" required.

Step 3 We parse the arguments at this stage by calling parse_args. The "file" argument will be available at args.file.

Step 4 The goal of the program is to print out the specified file, so here we call open() and read() to with args.file as the file name.

import argparse

# Step 1: create argument parser with description.
parser = argparse.ArgumentParser(description="Print file contents.")

# Step 2: add file argument, and use store for the action.
parser.add_argument("--file", action="store",
                    help="print the file contents",
                    required=True)

# Step 3: parse the arguments.
args = parser.parse_args()

# Step 4: print out the specified file's contents.
with open(args.file, "r") as f:
    print(f.read())
python3 program.py --file example.txt
Some example text.

Using argument parsing allows Python programs to be configured from the command-line, without requiring edits to the source code or settings files. Argparse leads to more robust programs.