Python Morsels: Accessing command-line arguments in Python

Transcript

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.

Passing command-line arguments

When you run a program from your system command-line, you can pass in arguments:

Here we have a program called greet.py:

print("Hello world")

Here we're calling this program:

$ python3 greet.py
Hello world

Function arguments vs command-line arguments

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.

Python accepts whatever command-line arguments we give it

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

Our greet.py program doesn't use command-line arguments at all. It's just a single line of code:

print("Hello world")

So what's your guess? Will python print Hello world? Will it print Hello Trey? Will we get an error? Something else?

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.

Where are command-line arguments stored?

Python stores command-line arguments in the sys module within a list called argv:

import sys

print(sys.argv)
print("Hello world")

The sys.argv list has all of the arguments passed to our Python program:

$ python3 greet.py Trey
['greet.py', 'Trey']
Hello world

Our 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.

Accessing command-line arguments

If we wanted to grab the argument after our program name we could read the second item (index 1) from sys.argv:

import sys

print("Hello", sys.argv[1])

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[1])
IndexError: list index out of range

If we pass in two few arguments we'll get an IndexError (because sys.argv doesn't have an index 1 in this case).

Processing 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.

Python provides all command-line arguments as strings

Let's take a look at another example. Here we have a program called add.py:

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 add.py with 2 and 3?

$ python3 add.py 2 3

Take a guess.

6?

5?

100?

Here's what we see when we run add.py with 2 and 3:

$ 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.

If we print out sys.argv:

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 2 and 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

Summary

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 argparse module).

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