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Python Command Line Arguments – 3 Ways to Read/Parse

Python command-line arguments are the parameters provided to the script while executing it. The command-line arguments are used to provide specific inputs to the program.

What is the benefit of Python Command Line Arguments?

Python command-line arguments help us to keep our program generic in nature. For example, we can write a program to process a CSV file. If we pass the CSV file name from the command-line, then our program will work for any CSV file. This will make our program loosely coupled and it will be easy to maintain it.

Another benefit of command-line arguments is the additional security that comes with it. Let’s say we have a program to save data into the database. If we store the database credentials in the script or some configuration file, it can be accessed and executed by anyone having access to the files. But, if the user/password is provided as a command-line argument, then it’s not present in the file system and our program is more secured.

How to Pass Command-line Arguments in Python?

If you are running the python script from the terminal, just pass the arguments after the script name. The arguments are separated with white space characters.

$ python script.py arg1 arg2 . argN

Passing Command-line arguments in PyCharm

PyCharm is the most popular IDE for Python programming. If you want to pass command-line arguments to a python program, go to “Run > Edit Configurations” and set the Parameters value and save it.

How to Read Command-line arguments in Python Script?

There are three popular modules to read and parse command-line arguments in the Python script.

1. Reading Python Command-line arguments using the sys module

The command-line arguments are stored in the sys module argv variable, which is a list of strings. We can read the command-line arguments from this list and use it in our program.

Note that the script name is also part of the command-line arguments in the sys.argv variable.

import sys if len(sys.argv) != 2: raise ValueError('Please provide email-id to send the email.') print(f'Script Name is ') email = sys.argv[1] print(f'Sending test email to ')

$ python3.7 command-line-args.py [email protected] Script Name is command-line-args.py Sending test email to [email protected] $ python3.7 command-line-args.py Traceback (most recent call last): File "command-line-args.py", line 4, in raise ValueError('Please provide email-id to send the email.') ValueError: Please provide email-id to send the email. $

2. Parsing Command-line arguments using the getopt module

Python getopt module works in a similar way as the Unix getopt() function. This module is helpful when you want the script to accept options and their values, similar to the many Unix commands.

This module works in conjunction with the sys.argv to parse the command-line arguments and extract the options values in a list of tuples.

import getopt import sys argv = sys.argv[1:] opts, args = getopt.getopt(argv, 'x:y:') # list of options tuple (opt, value) print(f'Options Tuple is ') # list of remaining command-line arguments print(f'Additional Command-line arguments list is ')

$ python3.7 command-line-args.py -x 1 -y 2 A B Options Tuple is [('-x', '1'), ('-y', '2')] Additional Command-line arguments list is ['A', 'B'] $

3. Parsing Command-line arguments using argparse module

We can use Python argparse module also to parse command-line arguments. There are a lot of options with argparse module.

• positional arguments
• the help message
• the default value for arguments
• specifying the data type of argument and many more.

import argparse # create parser parser = argparse.ArgumentParser() # add arguments to the parser parser.add_argument("language") parser.add_argument("name") # parse the arguments args = parser.parse_args() # get the arguments value if args.language == 'Python': print("I love Python too") else: print("Learn Python, you will like it") print(f'Hello , this was a simple introduction to argparse module')

$ python3.7 command-line-args.py Python David I love Python too Hello David, this was a simple introduction to argparse module $ $ python3.7 command-line-args.py Java Lisa Learn Python, you will like it Hello Lisa, this was a simple introduction to argparse module $ $ python3.7 command-line-args.py -h usage: command-line-args.py [-h] language name positional arguments: language name optional arguments: -h, --help show this help message and exit $

Notice that the help message is automatically generated by the argparse module.

Conclusion

If your script requires simple command-line arguments, you can go with sys.argv. But, if your program accepts a lot of positional arguments, default argument values, help messages, etc, then you should use argparse module. The getopt module works too but it’s confusing and hard to understand.

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2. Using the Python Interpreter¶

The Python interpreter is usually installed as /usr/local/bin/python3.11 on those machines where it is available; putting /usr/local/bin in your Unix shell’s search path makes it possible to start it by typing the command:

to the shell. 1 Since the choice of the directory where the interpreter lives is an installation option, other places are possible; check with your local Python guru or system administrator. (E.g., /usr/local/python is a popular alternative location.)

On Windows machines where you have installed Python from the Microsoft Store , the python3.11 command will be available. If you have the py.exe launcher installed, you can use the py command. See Excursus: Setting environment variables for other ways to launch Python.

Typing an end-of-file character ( Control — D on Unix, Control — Z on Windows) at the primary prompt causes the interpreter to exit with a zero exit status. If that doesn’t work, you can exit the interpreter by typing the following command: quit() .

The interpreter’s line-editing features include interactive editing, history substitution and code completion on systems that support the GNU Readline library. Perhaps the quickest check to see whether command line editing is supported is typing Control — P to the first Python prompt you get. If it beeps, you have command line editing; see Appendix Interactive Input Editing and History Substitution for an introduction to the keys. If nothing appears to happen, or if ^P is echoed, command line editing isn’t available; you’ll only be able to use backspace to remove characters from the current line.

The interpreter operates somewhat like the Unix shell: when called with standard input connected to a tty device, it reads and executes commands interactively; when called with a file name argument or with a file as standard input, it reads and executes a script from that file.

A second way of starting the interpreter is python -c command [arg] . , which executes the statement(s) in command, analogous to the shell’s -c option. Since Python statements often contain spaces or other characters that are special to the shell, it is usually advised to quote command in its entirety.

Some Python modules are also useful as scripts. These can be invoked using python -m module [arg] . , which executes the source file for module as if you had spelled out its full name on the command line.

When a script file is used, it is sometimes useful to be able to run the script and enter interactive mode afterwards. This can be done by passing -i before the script.

All command line options are described in Command line and environment .

2.1.1. Argument Passing¶

When known to the interpreter, the script name and additional arguments thereafter are turned into a list of strings and assigned to the argv variable in the sys module. You can access this list by executing import sys . The length of the list is at least one; when no script and no arguments are given, sys.argv[0] is an empty string. When the script name is given as ‘-‘ (meaning standard input), sys.argv[0] is set to ‘-‘ . When -c command is used, sys.argv[0] is set to ‘-c’ . When -m module is used, sys.argv[0] is set to the full name of the located module. Options found after -c command or -m module are not consumed by the Python interpreter’s option processing but left in sys.argv for the command or module to handle.

2.1.2. Interactive Mode¶

When commands are read from a tty, the interpreter is said to be in interactive mode. In this mode it prompts for the next command with the primary prompt, usually three greater-than signs ( >>> ); for continuation lines it prompts with the secondary prompt, by default three dots ( . ). The interpreter prints a welcome message stating its version number and a copyright notice before printing the first prompt:

$ python3.11 Python 3.11 (default, April 4 2021, 09:25:04) [GCC 10.2.0] on linux Type "help", "copyright", "credits" or "license" for more information. >>> 

Continuation lines are needed when entering a multi-line construct. As an example, take a look at this if statement:

>>> the_world_is_flat = True >>> if the_world_is_flat: . print("Be careful not to fall off!") . Be careful not to fall off! 

For more on interactive mode, see Interactive Mode .

2.2. The Interpreter and Its Environment¶

2.2.1. Source Code Encoding¶

By default, Python source files are treated as encoded in UTF-8. In that encoding, characters of most languages in the world can be used simultaneously in string literals, identifiers and comments — although the standard library only uses ASCII characters for identifiers, a convention that any portable code should follow. To display all these characters properly, your editor must recognize that the file is UTF-8, and it must use a font that supports all the characters in the file.

To declare an encoding other than the default one, a special comment line should be added as the first line of the file. The syntax is as follows:

where encoding is one of the valid codecs supported by Python.

For example, to declare that Windows-1252 encoding is to be used, the first line of your source code file should be:

One exception to the first line rule is when the source code starts with a UNIX “shebang” line . In this case, the encoding declaration should be added as the second line of the file. For example:

#!/usr/bin/env python3 # -*- coding: cp1252 -*- 

On Unix, the Python 3.x interpreter is by default not installed with the executable named python , so that it does not conflict with a simultaneously installed Python 2.x executable.

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