howto-argparse.pdf

Argparse Tutorial

Release3.4.0

Guido van Rossum

Fred L. Drake, Jr., editor

April 17, 2014

PythonSoftwareFoundation

Email:docs@python.org

Contents

1Concepts

2Thebasics

3IntroducingPositionalarguments

4IntroducingOptionalarguments

4.1

Short options

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5CombiningPositionalandOptionalarguments

6Gettingalittlemoreadvanced

6.1

Conﬂicting options

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

7Conclusion

authorTshepang Lekhonkhobe

This tutorial is intended to be a gentle introduction to argparse , the recommended command-line parsing

module in the Python standard library.

Note: There are two other modules that fulﬁll 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.

1 Concepts

Let’s show the sort of functionality that we are going to explore in this introductory tutorial by making use of the

lscommand:

$ ls

cpythondevguideprog.pypypyrm-unused-function.patch

$ lspypy

ctypes_configuredemodotviewerincludelib_pypylib-python...

$ ls-l

total20

drwxr-xr-x19wenawena4096Feb1818:51cpython

drwxr-xr-x4wenawena4096Feb812:04devguide

-rwxr-xr-x1wenawena535Feb1900:05prog.py

drwxr-xr-x14wenawena4096Feb700:59pypy

-rw-r--r--1wenawena741Feb1801:01rm-unused-function.patch

$ ls--help

Usage:ls [ OPTION ] ... [ FILE ] ...

ListinformationabouttheFILEs ( thecurrentdirectorybydefault ) .

Sortentriesalphabetically if noneof-cftuvSUXnor--sortisspecified.

...

A few concepts we can learn from the four commands:

• Thelscommand 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 likecp, whose most basic usage is cpSRC

DEST . The ﬁrst position iswhatyouwantcopied,and the second position iswhereyouwantitcopiedto.

• Now, say we want to change behaviour of the program. In our example, we display more info for each ﬁle

instead of just showing the ﬁle 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 ﬁgure out how it works simply by reading its help text.

2 The 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:

$ python3prog.py

$ python3prog.py--help

usage:prog.py [ -h ]

optionalarguments:

-h,--helpshowthis help messageand exit

$ python3prog.py--verbose

usage:prog.py [ -h ]

prog.py:error:unrecognizedarguments:--verbose

$ python3prog.pyfoo

usage:prog.py [ -h ]

prog.py:error:unrecognizedarguments: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.

3 Introducing Positional arguments

An example:

import argparse

parser = argparse . ArgumentParser()

parser . add_argument( "echo" )

args = parser . parse_args()

print (args . echo)

And running the code:

$ python3prog.py

usage:prog.py [ -h ] echo

prog.py:error:thefollowingargumentsarerequired: echo

$ python3prog.py--help

usage:prog.py [ -h ] echo

positionalarguments:

echo

optionalarguments:

-h,--helpshowthis help messageand exit

$ python3prog.pyfoo

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 speciﬁed, 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 = "echothestringyouusehere" )

args = parser . parse_args()

print (args . echo)

And we get:

$ python3prog.py-h

usage:prog.py [ -h ] echo

positionalarguments:

echo echo thestringyouusehere

optionalarguments:

-h,--helpshowthis help messageand exit

Now, how about doing something even more useful:

import argparse

parser = argparse . ArgumentParser()

parser . add_argument( "square" ,help = "displayasquareofagivennumber" )

args = parser . parse_args()

print (args . square ** 2 )

Following is a result of running the code:

$ python3prog.py4

Traceback ( mostrecentcalllast ) :

File "prog.py" ,line5,in<module>

print ( args.square ** 2 )

TypeError:unsupportedoperand type ( s ) for ** orpow () : ’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 = "displayasquareofagivennumber" ,

type = int )

args = parser . parse_args()

print (args . square ** 2 )

Following is a result of running the code:

$ python3prog.py4

$ python3prog.pyfour

usage:prog.py [ -h ] square

prog.py:error:argumentsquare:invalidintvalue: ’four’

That went well. The program now even helpfully quits on bad illegal input before proceeding.

4 Introducing 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 = "increaseoutputverbosity" )

args = parser . parse_args()

if args . verbosity:

print ( "verbosityturnedon" )

And the output:

$ python3prog.py--verbosity1

verbosityturnedon

$ python3prog.py

$ python3prog.py--help

usage:prog.py [ -h ][ --verbosityVERBOSITY ]

optionalarguments:

-h,--help showthis help messageand exit

--verbosityVERBOSITY

increaseoutputverbosity

$ python3prog.py--verbosity

usage:prog.py [ -h ][ --verbosityVERBOSITY ]

prog.py:error:argument--verbosity:expectedoneargument

Here is what is happening:

• The program is written so as to display something when --verbosity is speciﬁed 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 = "increaseoutputverbosity" ,

action = "store_true" )

args = parser . parse_args()

if args . verbose:

print ( "verbosityturnedon" )

And the output:

$ python3prog.py--verbose

verbosityturnedon

$ python3prog.py--verbose1

usage:prog.py [ -h ][ --verbose ]

prog.py:error:unrecognizedarguments:1

$ python3prog.py--help

usage:prog.py [ -h ][ --verbose ]

optionalarguments:

-h,--helpshowthis help messageand exit

--verbose increaseoutputverbosity

Here is what is happening:

• The option is now more of a ﬂag 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 speciﬁed, assign the value True to args.verbose .

Not specifying it implies False .

• It complains when you specify a value, in true spirit of what ﬂags actually are.

• Notice the different help text.

4.1 Short 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 = "increaseoutputverbosity" ,

action = "store_true" )

args = parser . parse_args()

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