How do I access command line arguments?

Question

I use python to create my project settings setup, but I need help getting the command line arguments.

I tried this on the terminal:

$python myfile.py var1 var2 var3

In my Python file, I want to use all variables that are input.

Solution 1

Python tutorial explains it:

import sys

print(sys.argv)

More specifically, if you run python example.py one two three:

>>> import sys
>>> print(sys.argv)
['example.py', 'one', 'two', 'three']

Solution 2

To get only the command line arguments

(not including the name of the Python file)

import sys

sys.argv[1:]

The [1:] is a slice starting from the second element (index 1) and going to the end of the arguments list. This is because the first element is the name of the Python file, and we want to remove that.

Solution 3

I highly recommend argparse which comes with Python 2.7 and later.

The argparse module reduces boiler plate code and makes your code more robust, because the module handles all standard use cases (including subcommands), generates the help and usage for you, checks and sanitize the user input - all stuff you have to worry about when you are using sys.argv approach. And it is for free (built-in).

Here a small example:

import argparse

parser = argparse.ArgumentParser("simple_example")
parser.add_argument("counter", help="An integer will be increased by 1 and printed.", type=int)
args = parser.parse_args()
print(args.counter + 1)

and the output for python prog.py -h

usage: simple_example [-h] counter
    
positional arguments:
  counter     counter will be increased by 1 and printed.
    
optional arguments:
  -h, --help  show this help message and exit

and the output for python prog.py 1 As one would expect:

2

Solution 4

Python code:

import sys

# main
param_1= sys.argv[1] 
param_2= sys.argv[2] 
param_3= sys.argv[3]  
print 'Params=', param_1, param_2, param_3

Invocation:

$python myfile.py var1 var2 var3

Output:

Params= var1 var2 var3 

Solution 5

You can use sys.argv to get the arguments as a list.

If you need to access individual elements, you can use

sys.argv[i]  

where i is index, 0 will give you the python filename being executed. Any index after that are the arguments passed.

View more solutions

Comments

[Kellen Stuart]

What does [1:] mean?

[Kamil Jarosz]

@KolobCanyon it means "take a sublist starting from index 1 till the end", i.e. skip the first element

[Bassinator]

Quality answer, not really sure why anyone here would use any of the other approaches given here. Especially since it's built in! Don't you guys have better things to do than parsing?

[look]

sys.argv[0] is not the current working directory. It is the name of the file that is executing.

[Samuel Nde]

@KolobCanyon take the the second argument [1:] upwards because the first argument of sys.argv is by default the name of the python file that you are running for example testrun.py.

[MikeL]

Just to mention how you should access the specific argument in args namespace: args.counter

[reducing activity]

@Bassinator In cases when no parsing of arguments is needed.

[Albicocco]

to use default value, without specific value in the command line, you can use something like: parser.add_argument('var3', nargs='?', type=int, default=3) var3 is a optional param, if you don't specify a value in the command line you can access to var3 in your code, default value for var3=3

[alper]

Can type be int or float?

[Michael Dorner]

Yes, snd many more: docs.python.org/3/library/argparse.html#type

[Jason Harrison]

@Bassinator sometimes you want to record the exact command used. For example in machine learning experiments, logging sys.argv is an easy way to log the command line without having to update the code after any improvements to the argument definitions for argparse.ArgumentParser

[Josep Escobar]

You are the boss! Thanks man!

[Michael Dorner]

I feel flattered. :) Thanks man! :)