howto-argparse
Argparse Tutorial
Release2.7.11
Guido van Rossum
and the Python development team
January08,2016
Python Software Foundation
Email:docs@https://www.wendangku.net/doc/6112601604.html, Contents
1Concepts1 2The basics2 3Introducing Positional arguments3 4Introducing Optional arguments4
4.1Short options (5)
5Combining Positional and Optional arguments6 6Getting a little more advanced9
6.1Con?icting options (10)
7Conclusion12
author Tshepang Lekhonkhobe
This tutorial is intended to be a gentle introduction to argparse,the recommended command-line parsing module in the Python standard library.This was written for argparse in Python3.A few details are different in 2.x,especially some exception messages,which were improved in3.x.
Note:There are two other modules that ful?ll the same task,namely getopt(an equivalent for getopt() from the C language)and the deprecated optparse.Note also that argparse is based on optparse,and therefore very similar in terms of usage.
1Concepts
Let’s show the sort of functionality that we are going to explore in this introductory tutorial by making use of the ls command:
$ls
cpython devguide prog.py pypy rm-unused-function.patch
$ls pypy
ctypes_configure demo dotviewer include lib_pypy lib-python...
$ls-l
total20
drwxr-xr-x19wena wena4096Feb1818:51cpython
drwxr-xr-x4wena wena4096Feb812:04devguide
-rwxr-xr-x1wena wena535Feb1900:05prog.py
drwxr-xr-x14wena wena4096Feb700:59pypy
-rw-r--r--1wena wena741Feb1801:01rm-unused-function.patch
$ls--help
Usage:ls[OPTION]...[FILE]...
List information about the FILEs(the current directory by default).
Sort entries alphabetically if none of-cftuvSUX nor--sort is specified. ...
A few concepts we can learn from the four commands:
?The ls command is useful when run without any options at all.It defaults to displaying the contents of the current directory.
?If we want beyond what it provides by default,we tell it a bit more.In this case,we want it to display
a different directory,pypy.What we did is specify what is known as a positional argument.It’s named
so because the program should know what to do with the value,solely based on where it appears on the command line.This concept is more relevant to a command like cp,whose most basic usage is cp SRC DEST.The?rst position is what you want copied,and the second position is where you want it copied to.
?Now,say we want to change behaviour of the program.In our example,we display more info for each?le instead of just showing the?le names.The-l in that case is known as an optional argument.
?That’s a snippet of the help text.It’s very useful in that you can come across a program you have never used before,and can?gure out how it works simply by reading its help text.
2The basics
Let us start with a very simple example which does(almost)nothing:
import argparse
parser=argparse.ArgumentParser()
parser.parse_args()
Following is a result of running the code:
$python prog.py
$python prog.py--help
usage:prog.py[-h]
optional arguments:
-h,--help show this help message and exit
$python prog.py--verbose
usage:prog.py[-h]
prog.py:error:unrecognized arguments:--verbose
$python prog.py foo
usage:prog.py[-h]
prog.py:error:unrecognized arguments:foo
Here is what is happening:
?Running the script without any options results in nothing displayed to stdout.Not so useful.
?The second one starts to display the usefulness of the argparse module.We have done almost nothing, but already we get a nice help message.
?The--help option,which can also be shortened to-h,is the only option we get for free(i.e.no need to specify it).Specifying anything else results in an error.But even then,we do get a useful usage message, also for free.
3Introducing Positional arguments
An example:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("echo")
args=parser.parse_args()
print args.echo
And running the code:
$python prog.py
usage:prog.py[-h]echo
prog.py:error:the following arguments are required:echo
$python prog.py--help
usage:prog.py[-h]echo
positional arguments:
echo
optional arguments:
-h,--help show this help message and exit
$python prog.py foo
foo
Here is what’s happening:
?We’ve added the add_argument()method,which is what we use to specify which command-line op-tions the program is willing to accept.In this case,I’ve named it echo so that it’s in line with its function.
?Calling our program now requires us to specify an option.
?The parse_args()method actually returns some data from the options speci?ed,in this case,echo.
?The variable is some form of‘magic’that argparse performs for free(i.e.no need to specify which variable that value is stored in).You will also notice that its name matches the string argument given to the method,echo.
Note however that,although the help display looks nice and all,it currently is not as helpful as it can be.For example we see that we got echo as a positional argument,but we don’t know what it does,other than by guessing or by reading the source code.So,let’s make it a bit more useful:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("echo",help="echo the string you use here")
args=parser.parse_args()
print args.echo
And we get:
$python prog.py-h
usage:prog.py[-h]echo
positional arguments:
echo echo the string you use here
optional arguments:
-h,--help show this help message and exit
Now,how about doing something even more useful:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",help="display a square of a given number") args=parser.parse_args()
print args.square**2
Following is a result of running the code:
$python prog.py4
Traceback(most recent call last):
File"prog.py",line5,in
print args.square**2
TypeError:unsupported operand type(s)for**or pow():'str'and'int'
That didn’t go so well.That’s because argparse treats the options we give it as strings,unless we tell it otherwise.So,let’s tell argparse to treat that input as an integer:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",help="display a square of a given number",
type=int)
args=parser.parse_args()
print args.square**2
Following is a result of running the code:
$python prog.py4
16
$python prog.py four
usage:prog.py[-h]square
prog.py:error:argument square:invalid int value:'four'
That went well.The program now even helpfully quits on bad illegal input before proceeding.
4Introducing Optional arguments
So far we,have been playing with positional arguments.Let us have a look on how to add optional ones:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("--verbosity",help="increase output verbosity")
args=parser.parse_args()
if args.verbosity:
print"verbosity turned on"
And the output:
$python prog.py--verbosity1
verbosity turned on
$python prog.py
$python prog.py--help
usage:prog.py[-h][--verbosity VERBOSITY]
optional arguments:
-h,--help show this help message and exit
--verbosity VERBOSITY
increase output verbosity
$python prog.py--verbosity
usage:prog.py[-h][--verbosity VERBOSITY]
prog.py:error:argument--verbosity:expected one argument
Here is what is happening:
?The program is written so as to display something when--verbosity is speci?ed and display nothing when not.
?To show that the option is actually optional,there is no error when running the program without it.Note that by default,if an optional argument isn’t used,the relevant variable,in this case args.verbosity,is given None as a value,which is the reason it fails the truth test of the if statement.
?The help message is a bit different.
?When using the--verbosity option,one must also specify some value,any value.
The above example accepts arbitrary integer values for--verbosity,but for our simple program,only two values are actually useful,True or False.Let’s modify the code accordingly:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("--verbose",help="increase output verbosity",
action="store_true")
args=parser.parse_args()
if args.verbose:
print"verbosity turned on"
And the output:
$python prog.py--verbose
verbosity turned on
$python prog.py--verbose1
usage:prog.py[-h][--verbose]
prog.py:error:unrecognized arguments:1
$python prog.py--help
usage:prog.py[-h][--verbose]
optional arguments:
-h,--help show this help message and exit
--verbose increase output verbosity
Here is what is happening:
?The option is now more of a?ag than something that requires a value.We even changed the name of the option to match that idea.Note that we now specify a new keyword,action,and give it the value "store_true".This means that,if the option is speci?ed,assign the value True to args.verbose.
Not specifying it implies False.
?It complains when you specify a value,in true spirit of what?ags actually are.
?Notice the different help text.
4.1Short options
If you are familiar with command line usage,you will notice that I haven’t yet touched on the topic of short versions of the options.It’s quite simple:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("-v","--verbose",help="increase output verbosity",
action="store_true")
args=parser.parse_args()
if args.verbose:
print"verbosity turned on"
And here goes:
$python prog.py-v
verbosity turned on
$python prog.py--help
usage:prog.py[-h][-v]
optional arguments:
-h,--help show this help message and exit
-v,--verbose increase output verbosity
Note that the new ability is also re?ected in the help text.
5Combining Positional and Optional arguments
Our program keeps growing in complexity:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",type=int,
help="display a square of a given number")
parser.add_argument("-v","--verbose",action="store_true",
help="increase output verbosity")
args=parser.parse_args()
answer=args.square**2
if args.verbose:
print"the square of{}equals{}".format(args.square,answer)
else:
print answer
And now the output:
$python prog.py
usage:prog.py[-h][-v]square
prog.py:error:the following arguments are required:square
$python prog.py4
16
$python prog.py4--verbose
the square of4equals16
$python prog.py--verbose4
the square of4equals16
?We’ve brought back a positional argument,hence the complaint.
?Note that the order does not matter.
How about we give this program of ours back the ability to have multiple verbosity values,and actually get to use them:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",type=int,
help="display a square of a given number")
parser.add_argument("-v","--verbosity",type=int,
help="increase output verbosity")
args=parser.parse_args()
answer=args.square**2
if args.verbosity==2:
print"the square of{}equals{}".format(args.square,answer)
elif args.verbosity==1:
print"{}^2=={}".format(args.square,answer)
else:
print answer
And the output:
$python prog.py4
16
$python prog.py4-v
usage:prog.py[-h][-v VERBOSITY]square
prog.py:error:argument-v/--verbosity:expected one argument
$python prog.py4-v1
4^2==16
$python prog.py4-v2
the square of4equals16
$python prog.py4-v3
16
These all look good except the last one,which exposes a bug in our program.Let’s?x it by restricting the values
the--verbosity option can accept:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",type=int,
help="display a square of a given number")
parser.add_argument("-v","--verbosity",type=int,choices=[0,1,2],
help="increase output verbosity")
args=parser.parse_args()
answer=args.square**2
if args.verbosity==2:
print"the square of{}equals{}".format(args.square,answer)
elif args.verbosity==1:
print"{}^2=={}".format(args.square,answer)
else:
print answer
And the output:
$python prog.py4-v3
usage:prog.py[-h][-v{0,1,2}]square
prog.py:error:argument-v/--verbosity:invalid choice:3(choose from0,1,2) $python prog.py4-h
usage:prog.py[-h][-v{0,1,2}]square
positional arguments:
square display a square of a given number
optional arguments:
-h,--help show this help message and exit
-v{0,1,2},--verbosity{0,1,2}
increase output verbosity
Note that the change also re?ects both in the error message as well as the help string.
Now,let’s use a different approach of playing with verbosity,which is pretty common.It also matches the way
the CPython executable handles its own verbosity argument(check the output of python--help):
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",type=int,
help="display the square of a given number")
parser.add_argument("-v","--verbosity",action="count",
help="increase output verbosity")
args=parser.parse_args()
answer=args.square**2
if args.verbosity==2:
print"the square of{}equals{}".format(args.square,answer)
elif args.verbosity==1:
print"{}^2=={}".format(args.square,answer)
else:
print answer
We have introduced another action,“count”,to count the number of occurrences of a speci?c optional arguments: $python prog.py4
16
$python prog.py4-v
4^2==16
$python prog.py4-vv
the square of4equals16
$python prog.py4--verbosity--verbosity
the square of4equals16
$python prog.py4-v1
usage:prog.py[-h][-v]square
prog.py:error:unrecognized arguments:1
$python prog.py4-h
usage:prog.py[-h][-v]square
positional arguments:
square display a square of a given number
optional arguments:
-h,--help show this help message and exit
-v,--verbosity increase output verbosity
$python prog.py4-vvv
16
?Yes,it’s now more of a?ag(similar to action="store_true")in the previous version of our script.
That should explain the complaint.
?It also behaves similar to“store_true”action.
?Now here’s a demonstration of what the“count”action gives.You’ve probably seen this sort of usage before.
?And,just like the“store_true”action,if you don’t specify the-v?ag,that?ag is considered to have None value.
?As should be expected,specifying the long form of the?ag,we should get the same output.
?Sadly,our help output isn’t very informative on the new ability our script has acquired,but that can always be?xed by improving the documentation for out script(e.g.via the help keyword argument).
?That last output exposes a bug in our program.
Let’s?x:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",type=int,
help="display a square of a given number")
parser.add_argument("-v","--verbosity",action="count",
help="increase output verbosity")
args=parser.parse_args()
answer=args.square**2
#bugfix:replace==with>=
if args.verbosity>=2:
print"the square of{}equals{}".format(args.square,answer)
elif args.verbosity>=1:
print"{}^2=={}".format(args.square,answer)
else:
print answer
And this is what it gives:
$python prog.py4-vvv
the square of4equals16
$python prog.py4-vvvv
the square of4equals16
$python prog.py4
Traceback(most recent call last):
File"prog.py",line11,in
if args.verbosity>=2:
TypeError:unorderable types:NoneType()>=int()
?First output went well,and?xes the bug we had before.That is,we want any value>=2to be as verbose as possible.
?Third output not so good.
Let’s?x that bug:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("square",type=int,
help="display a square of a given number")
parser.add_argument("-v","--verbosity",action="count",default=0,
help="increase output verbosity")
args=parser.parse_args()
answer=args.square**2
if args.verbosity>=2:
print"the square of{}equals{}".format(args.square,answer)
elif args.verbosity>=1:
print"{}^2=={}".format(args.square,answer)
else:
print answer
We’ve just introduced yet another keyword,default.We’ve set it to0in order to make it comparable to the other int values.Remember that by default,if an optional argument isn’t speci?ed,it gets the None value,and that cannot be compared to an int value(hence the TypeError exception).
And:
$python prog.py4
16
You can go quite far just with what we’ve learned so far,and we have only scratched the surface.The argparse module is very powerful,and we’ll explore a bit more of it before we end this tutorial.
6Getting a little more advanced
What if we wanted to expand our tiny program to perform other powers,not just squares:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("x",type=int,help="the base")
parser.add_argument("y",type=int,help="the exponent")
parser.add_argument("-v","--verbosity",action="count",default=0)
args=parser.parse_args()
answer=args.x**args.y
if args.verbosity>=2:
print"{}to the power{}equals{}".format(args.x,args.y,answer)
elif args.verbosity>=1:
print"{}^{}=={}".format(args.x,args.y,answer)
else:
print answer
Output:
$python prog.py
usage:prog.py[-h][-v]x y
prog.py:error:the following arguments are required:x,y
$python prog.py-h
usage:prog.py[-h][-v]x y
positional arguments:
x the base
y the exponent
optional arguments:
-h,--help show this help message and exit
-v,--verbosity
$python prog.py42-v
4^2==16
Notice that so far we’ve been using verbosity level to change the text that gets displayed.The following example instead uses verbosity level to display more text instead:
import argparse
parser=argparse.ArgumentParser()
parser.add_argument("x",type=int,help="the base")
parser.add_argument("y",type=int,help="the exponent")
parser.add_argument("-v","--verbosity",action="count",default=0)
args=parser.parse_args()
answer=args.x**args.y
if args.verbosity>=2:
print"Running'{}'".format(__file__)
if args.verbosity>=1:
print"{}^{}==".format(args.x,args.y),
print answer
Output:
$python prog.py42
16
$python prog.py42-v
4^2==16
$python prog.py42-vv
Running'prog.py'
4^2==16
6.1Con?icting options
So far,we have been working with two methods of an argparse.ArgumentParser instance.Let’s introduce a third one,add_mutually_exclusive_group().It allows for us to specify options that con?ict with each
other.Let’s also change the rest of the program so that the new functionality makes more sense:we’ll introduce the--quiet option,which will be the opposite of the--verbose one:
import argparse
parser=argparse.ArgumentParser()
group=parser.add_mutually_exclusive_group()
group.add_argument("-v","--verbose",action="store_true")
group.add_argument("-q","--quiet",action="store_true")
parser.add_argument("x",type=int,help="the base")
parser.add_argument("y",type=int,help="the exponent")
args=parser.parse_args()
answer=args.x**args.y
if args.quiet:
print answer
elif args.verbose:
print"{}to the power{}equals{}".format(args.x,args.y,answer)
else:
print"{}^{}=={}".format(args.x,args.y,answer)
Our program is now simpler,and we’ve lost some functionality for the sake of demonstration.Anyways,here’s the output:
$python prog.py42
4^2==16
$python prog.py42-q
16
$python prog.py42-v
4to the power2equals16
$python prog.py42-vq
usage:prog.py[-h][-v|-q]x y
prog.py:error:argument-q/--quiet:not allowed with argument-v/--verbose $python prog.py42-v--quiet
usage:prog.py[-h][-v|-q]x y
prog.py:error:argument-q/--quiet:not allowed with argument-v/--verbose That should be easy to follow.I’ve added that last output so you can see the sort of?exibility you get,i.e.mixing long form options with short form ones.
Before we conclude,you probably want to tell your users the main purpose of your program,just in case they don’t know:
import argparse
parser=argparse.ArgumentParser(description="calculate X to the power of Y") group=parser.add_mutually_exclusive_group()
group.add_argument("-v","--verbose",action="store_true")
group.add_argument("-q","--quiet",action="store_true")
parser.add_argument("x",type=int,help="the base")
parser.add_argument("y",type=int,help="the exponent")
args=parser.parse_args()
answer=args.x**args.y
if args.quiet:
print answer
elif args.verbose:
print"{}to the power{}equals{}".format(args.x,args.y,answer)
else:
print"{}^{}=={}".format(args.x,args.y,answer)
Note that slight difference in the usage text.Note the[-v|-q],which tells us that we can either use-v or -q,but not both at the same time:
$python prog.py--help
usage:prog.py[-h][-v|-q]x y
calculate X to the power of Y
positional arguments:
x the base
y the exponent
optional arguments:
-h,--help show this help message and exit
-v,--verbose
-q,--quiet
7Conclusion
The argparse module offers a lot more than shown here.Its docs are quite detailed and thorough,and full of examples.Having gone through this tutorial,you should easily digest them without feeling overwhelmed.