Use ArgumentParser module to add command line type argument
parsing capabilities to python programs.
Let’s start with an example.
countposts.py -h
usage: countposts.py [-h] [-v] [-o] [-a] [-s] [-g] [-t]
countposts.py: Count posts
optional arguments:
-h, --help show this help message and exit
-v, --verbose Verbose mode [Default=Off]
-o, --headless Headless mode [Default=Off]
-a, --countall Count posts for all [Default=Off]
-s, --saveasyougo Save data as the code runs [Default=Off]
-g, --dash Generate dash only [Default=Off]
-t, --test Test mode [Default=Off]
This is the output of countposts.py -h. We do not need to write any extra lines of code
to display the help menu. It is automatically generated by the parser. All we need to do is
add the arguments, and set the variable types and values they can take. While the possibilities
are endless and a lot of complex arguments can be generated, we will focus on simple
implementations. Almost always the simplest ones are sufficient. For a more in-depth discussion
see the docs.
The above output is generated by the following code.
import argparse
def getargs():
# parse command-line arguments using argparse()
description = """countposts.py: Count posts"""
parser = argparse.ArgumentParser(description=description)
parser.add_argument('-v', '--verbose', help='Verbose mode [Default=Off]', action='store_true')
parser.add_argument('-o', '--headless', help='Headless mode [Default=Off]', action='store_true')
parser.add_argument('-a', '--countall', help='Count posts for all [Default=Off]', action='store_true')
parser.add_argument('-s', '--saveasyougo', help='Save data as the code runs [Default=Off]', action='store_true')
parser.add_argument('-g', '--dash', help='Generate dash only [Default=Off]', action='store_true')
parser.add_argument('-t', '--test', help='Test mode [Default=Off]', action='store_true')
args = parser.parse_args()
return args
def main():
args = getargs()
verbose = args.verbose
testing = args.test
saveasyougo = args.saveasyougo
countall = args.countall
headless = args.headless
dash = args.dash
if __name__ == "__main__":
main()
Most of the lines are self-explanatory. All variables (arguments) are boolean and take True or False values.
The default value is False. Let’s dissect the getargs() method.
parser = argparse.ArgumentParser(description=description)
This line creates the ArgumentParser instance and sets the description parameter. The
description parameter describes the script (what it does). If we want to add a use case, we can use the
epilog keyword.
parser.add_argument('-v', '--verbose', help='Verbose mode [Default=Off]', action='store_true')
This line adds a new argument. Both the short and long forms of the switch are defined. The verbose switch can be
invoked either by the short-form -v switch or the long-form --verbose. These are also called options.
As we will see below, ArgumentParser will return a namespace where the long-form argument names will be used
as variables.
The help keyword explains the argument (switch) does. It is good practice to specify the default values.
The action keyword specifies what action is taken when the argument (switch) is invoked.
action='store_true' informs ArgumentParser to store TRUE in the variable verbose.
To sum it up, ArgumentParser creates a variable named verbose and sets its type as Boolean. Then it stores
FALSE as the default value. When this option is invoked in the command line by the user, ArgumentParser
will change its value to TRUE.
The rest of the lines have identical functionality.
args = parser.parse_args()
This line collects all the values, packs them in a namepace, and returns them.
Let’s look at the main() method where these arguments are unpacked.
def main():
args = getargs()
verbose = args.verbose
The packed values are fetched into args. We can access individual variables by refering to the long-form names
of the arguments.
We can easily extend the above implementation to add arguments which can take numerical values.
def getargs():
# parse command-line arguments using argparse()
description = """countposts.py: Count posts"""
parser = argparse.ArgumentParser(description=description)
parser.add_argument('-v', '--verbose', help='Verbose mode [Default=Off]', action='store_true')
parser.add_argument('-o', '--headless', help='Headless mode [Default=Off]', action='store_true')
parser.add_argument('-a', '--countall', help='Count posts for all [Default=Off]', action='store_true')
parser.add_argument('-d', '--timedelta', help='Counts posts within [n] days [Default=0]', type=int)
parser.add_argument('-s', '--saveasyougo', help='Save data as the code runs [Default=Off]', action='store_true')
parser.add_argument('-g', '--dash', help='Generate dash only [Default=Off]', action='store_true')
parser.add_argument('-t', '--test', help='Test mode [Default=Off]', action='store_true')
args = parser.parse_args()
return args
We have added an extra argument.
parser.add_argument('-d', '--timedelta', help='Counts posts within [n] days [Default=0]', type=int)
The type=int keyword tells ArgumentParser to treat the argument timedelta as an integer. If we now
type countposts.py -h, we will get the following.
countposts.py -h
usage: countposts.py [-h] [-v] [-o] [-a] [-d TIMEDELTA] [-s] [-g] [-t]
countposts.py: Count posts
optional arguments:
-h, --help show this help message and exit
-v, --verbose Verbose mode [Default=Off]
-o, --headless Headless mode [Default=Off]
-a, --countall Count posts for all [Default=Off]
-d TIMEDELTA, --timedelta TIMEDELTA
Counts posts within [n] days [Default=0]
-s, --saveasyougo Save data as the code runs [Default=Off]
-g, --dash Generate dash only [Default=Off]
-t, --test Test mode [Default=Off]
There so many other possibilities which we didn’t explore. Feel free to check out https://docs.python.org/3/library/argparse.html for more options.