What if we wanted to pass information to our Python program to change the way that it runs? One of the most common ways to do this is with command-line arguments.
When you run a program from your system command-line, you can pass in arguments:
Here we have a program called
Here we're calling this program:
$ python3 greet.py Hello world
The word "argument" is a loaded term.
In Python we have function arguments which are inputs to a function. But command-line arguments are inputs to an entire program*.
So function arguments and command-line arguments are not very related, except that they're both inputs into something.
What do you think will happen if we passed an argument to our
greet.py program, as we run it through Python?
$ python3 greet.py Trey
greet.py program doesn't use command-line arguments at all.
It's just a single line of code:
So what's your guess?
Will python print
Will it print
Will we get an error?
When we run this
greet.py program with the argument
Trey we see
Hello world printed out:
$ python3 greet.py Trey Hello world
That's a little bit odd.
Python is just ignoring whatever argument we passed to this program. Python stores all the arguments that we give it, but it doesn't do anything with the arguments: it's up to us (the implementer of this program) to do something with them.
Python stores command-line arguments in the
sys module within a list called
import sys print(sys.argv) print("Hello world")
sys.argv list has all of the arguments passed to our Python program:
$ python3 greet.py Trey ['greet.py', 'Trey'] Hello world
sys.argv list here has two things in it, our program name and the argument we've passed in.
$ python3 greet.py Trey ['greet.py', 'Trey']
The first thing in
sys.argv is always going to be our program name.
If we wanted to grab the argument after our program name we could read the second item (index
import sys print("Hello", sys.argv)
We're assuming we get an argument passed in and we're printing that argument out:
$ python3 greet.py Trey Hello Trey $ python3 greet.py Carol Hello Carol
If I pass more than one argument our program is currently just ignoring everything after that first argument:
$ python3 greet.py Trey Hunner Hello Trey
What if we pass too few arguments?
$ python3 greet.py Traceback (most recent call last): File "/home/trey/greet.py", line 3, in <module> print("Hello", sys.argv) IndexError: list index out of range
If we pass in two few arguments we'll get an
sys.argv doesn't have an index
1 in this case).
sys.argv manually is something that you should only do for very simple Python programs: programs that don't need much complex command-line argument processing and ideally only do this for programs that are just for your use.
Let's take a look at another example.
Here we have a program called
import sys program, x, y = sys.argv print(x + y)
This program takes
sys.argv and unpacks it into three variables: the program name, and whatever two arguments are given after that:
program, x, y = sys.argv
Then it adds those arguments together and prints them out:
print(x + y)
What should we see when we run
$ python3 add.py 2 3
Take a guess.
Here's what we see when we run
$ python3 add.py 2 3 23
Weird, right? Clearly 2 plus 3 is not 23, or at least it shouldn't be 23, and yet that's what we see.
import sys program, x, y = sys.argv print(sys.argv)
We'll see that it's a list of strings:
$ python3 add.py 2 3 ['add.py', '2', '3']
Python stores all command-line arguments as strings because all command-line arguments are given to Python as text by the operating system. Python doesn't do any processing with them; it's up to us to make meaning of these arguments.
So if we want
3 to be numbers, we'll have to convert them to numbers ourselves:
import sys program, x, y = sys.argv print(float(x) + float(y))
Now we finally see the result we're expecting:
$ python3 add.py 2 3 5.0
If you need to make a very simple command-line interface (one that's just for you) and it doesn't need to be friendly, you can read
sys.argv to manually process the arguments coming into your program.
But if you need something more complex, you should probably use a proper command-line argument processing tool (like Python's
Hello friendly web visitor! 👋
This page is part of Python Morsels, an online Python skill-building service.
The best way to learn is by doing. In the case of Python that means writing Python code. If you'd like to improve your Python skills every week, try out Python Morsels by entering your email below to create an account.
Python Morsels topics pages are free and the first month's worth of exercises is free as well. You don't need to enter payment details to sign up.
You can find explanations of many other Python topics by signing up below.