path: root/devdocs/python~3.12/howto%2Fargparse.html

 <span id="id1"></span><h1>Argparse Tutorial</h1> <dl class="field-list simple"> <dt class="field-odd">author</dt> <dd class="field-odd">
<p>Tshepang Mbambo</p> </dd> </dl> <p>This tutorial is intended to be a gentle introduction to <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a>, the recommended command-line parsing module in the Python standard library.</p> <div class="admonition note"> <p class="admonition-title">Note</p> <p>There are two other modules that fulfill the same task, namely <a class="reference internal" href="../library/getopt#module-getopt" title="getopt: Portable parser for command line options; support both short and long option names."><code>getopt</code></a> (an equivalent for <code>getopt()</code> from the C language) and the deprecated <a class="reference internal" href="../library/optparse#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code>optparse</code></a>. Note also that <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> is based on <a class="reference internal" href="../library/optparse#module-optparse" title="optparse: Command-line option parsing library. (deprecated)"><code>optparse</code></a>, and therefore very similar in terms of usage.</p> </div> <section id="concepts"> <h2>Concepts</h2> <p>Let’s show the sort of functionality that we are going to explore in this introductory tutorial by making use of the <strong class="command">ls</strong> command:</p> <pre data-language="shell">$ ls
cpython  devguide  prog.py  pypy  rm-unused-function.patch
$ ls pypy
ctypes_configure  demo  dotviewer  include  lib_pypy  lib-python ...
$ ls -l
total 20
drwxr-xr-x 19 wena wena 4096 Feb 18 18:51 cpython
drwxr-xr-x  4 wena wena 4096 Feb  8 12:04 devguide
-rwxr-xr-x  1 wena wena  535 Feb 19 00:05 prog.py
drwxr-xr-x 14 wena wena 4096 Feb  7 00:59 pypy
-rw-r--r--  1 wena wena  741 Feb 18 01:01 rm-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.
...
</pre> <p>A few concepts we can learn from the four commands:</p> <ul class="simple"> <li>The <strong class="command">ls</strong> command is useful when run without any options at all. It defaults to displaying the contents of the current directory.</li> <li>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, <code>pypy</code>. 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 <strong class="command">cp</strong>, whose most basic usage is <code>cp SRC DEST</code>. The first position is <em>what you want copied,</em> and the second position is <em>where you want it copied to</em>.</li> <li>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 <code>-l</code> in that case is known as an optional argument.</li> <li>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.</li> </ul> </section> <section id="the-basics"> <h2>The basics</h2> <p>Let us start with a very simple example which does (almost) nothing:</p> <pre data-language="python">import argparse
parser = argparse.ArgumentParser()
parser.parse_args()
</pre> <p>Following is a result of running the code:</p> <pre data-language="shell">$ python prog.py
$ python prog.py --help
usage: prog.py [-h]

options:
  -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
</pre> <p>Here is what is happening:</p> <ul class="simple"> <li>Running the script without any options results in nothing displayed to stdout. Not so useful.</li> <li>The second one starts to display the usefulness of the <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> module. We have done almost nothing, but already we get a nice help message.</li> <li>The <code>--help</code> option, which can also be shortened to <code>-h</code>, 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.</li> </ul> </section> <section id="introducing-positional-arguments"> <h2>Introducing Positional arguments</h2> <p>An example:</p> <pre data-language="python">import argparse
parser = argparse.ArgumentParser()
parser.add_argument("echo")
args = parser.parse_args()
print(args.echo)
</pre> <p>And running the code:</p> <pre data-language="shell">$ 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

options:
  -h, --help  show this help message and exit
$ python prog.py foo
foo
</pre> <p>Here is what’s happening:</p> <ul class="simple"> <li>We’ve added the <a class="reference internal" href="../library/argparse#argparse.ArgumentParser.add_argument" title="argparse.ArgumentParser.add_argument"><code>add_argument()</code></a> method, which is what we use to specify which command-line options the program is willing to accept. In this case, I’ve named it <code>echo</code> so that it’s in line with its function.</li> <li>Calling our program now requires us to specify an option.</li> <li>The <a class="reference internal" href="../library/argparse#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code>parse_args()</code></a> method actually returns some data from the options specified, in this case, <code>echo</code>.</li> <li>The variable is some form of ‘magic’ that <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> 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, <code>echo</code>.</li> </ul> <p>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 <code>echo</code> 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:</p> <pre data-language="python">import argparse
parser = argparse.ArgumentParser()
parser.add_argument("echo", help="echo the string you use here")
args = parser.parse_args()
print(args.echo)
</pre> <p>And we get:</p> <pre data-language="shell">$ python prog.py -h
usage: prog.py [-h] echo

positional arguments:
  echo        echo the string you use here

options:
  -h, --help  show this help message and exit
</pre> <p>Now, how about doing something even more useful:</p> <pre data-language="python">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)
</pre> <p>Following is a result of running the code:</p> <pre data-language="shell">$ python prog.py 4
Traceback (most recent call last):
  File "prog.py", line 5, in &lt;module&gt;
    print(args.square**2)
TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'
</pre> <p>That didn’t go so well. That’s because <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> treats the options we give it as strings, unless we tell it otherwise. So, let’s tell <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> to treat that input as an integer:</p> <pre data-language="python">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)
</pre> <p>Following is a result of running the code:</p> <pre data-language="shell">$ python prog.py 4
16
$ python prog.py four
usage: prog.py [-h] square
prog.py: error: argument square: invalid int value: 'four'
</pre> <p>That went well. The program now even helpfully quits on bad illegal input before proceeding.</p> </section> <section id="introducing-optional-arguments"> <h2>Introducing Optional arguments</h2> <p>So far we have been playing with positional arguments. Let us have a look on how to add optional ones:</p> <pre data-language="python">import argparse
parser = argparse.ArgumentParser()
parser.add_argument("--verbosity", help="increase output verbosity")
args = parser.parse_args()
if args.verbosity:
    print("verbosity turned on")
</pre> <p>And the output:</p> <pre data-language="shell">$ python prog.py --verbosity 1
verbosity turned on
$ python prog.py
$ python prog.py --help
usage: prog.py [-h] [--verbosity VERBOSITY]

options:
  -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
</pre> <p>Here is what is happening:</p> <ul class="simple"> <li>The program is written so as to display something when <code>--verbosity</code> is specified and display nothing when not.</li> <li>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 <code>args.verbosity</code>, is given <code>None</code> as a value, which is the reason it fails the truth test of the <a class="reference internal" href="../reference/compound_stmts#if"><code>if</code></a> statement.</li> <li>The help message is a bit different.</li> <li>When using the <code>--verbosity</code> option, one must also specify some value, any value.</li> </ul> <p>The above example accepts arbitrary integer values for <code>--verbosity</code>, but for our simple program, only two values are actually useful, <code>True</code> or <code>False</code>. Let’s modify the code accordingly:</p> <pre data-language="python">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")
</pre> <p>And the output:</p> <pre data-language="shell">$ python prog.py --verbose
verbosity turned on
$ python prog.py --verbose 1
usage: prog.py [-h] [--verbose]
prog.py: error: unrecognized arguments: 1
$ python prog.py --help
usage: prog.py [-h] [--verbose]

options:
  -h, --help  show this help message and exit
  --verbose   increase output verbosity
</pre> <p>Here is what is happening:</p> <ul class="simple"> <li>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, <code>action</code>, and give it the value <code>"store_true"</code>. This means that, if the option is specified, assign the value <code>True</code> to <code>args.verbose</code>. Not specifying it implies <code>False</code>.</li> <li>It complains when you specify a value, in true spirit of what flags actually are.</li> <li>Notice the different help text.</li> </ul> <section id="short-options"> <h3>Short options</h3> <p>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:</p> <pre data-language="python">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")
</pre> <p>And here goes:</p> <pre data-language="shell">$ python prog.py -v
verbosity turned on
$ python prog.py --help
usage: prog.py [-h] [-v]

options:
  -h, --help     show this help message and exit
  -v, --verbose  increase output verbosity
</pre> <p>Note that the new ability is also reflected in the help text.</p> </section> </section> <section id="combining-positional-and-optional-arguments"> <h2>Combining Positional and Optional arguments</h2> <p>Our program keeps growing in complexity:</p> <pre data-language="python">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(f"the square of {args.square} equals {answer}")
else:
    print(answer)
</pre> <p>And now the output:</p> <pre data-language="shell">$ python prog.py
usage: prog.py [-h] [-v] square
prog.py: error: the following arguments are required: square
$ python prog.py 4
16
$ python prog.py 4 --verbose
the square of 4 equals 16
$ python prog.py --verbose 4
the square of 4 equals 16
</pre> <ul class="simple"> <li>We’ve brought back a positional argument, hence the complaint.</li> <li>Note that the order does not matter.</li> </ul> <p>How about we give this program of ours back the ability to have multiple verbosity values, and actually get to use them:</p> <pre data-language="python">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(f"the square of {args.square} equals {answer}")
elif args.verbosity == 1:
    print(f"{args.square}^2 == {answer}")
else:
    print(answer)
</pre> <p>And the output:</p> <pre data-language="shell">$ python prog.py 4
16
$ python prog.py 4 -v
usage: prog.py [-h] [-v VERBOSITY] square
prog.py: error: argument -v/--verbosity: expected one argument
$ python prog.py 4 -v 1
4^2 == 16
$ python prog.py 4 -v 2
the square of 4 equals 16
$ python prog.py 4 -v 3
16
</pre> <p>These all look good except the last one, which exposes a bug in our program. Let’s fix it by restricting the values the <code>--verbosity</code> option can accept:</p> <pre data-language="python">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(f"the square of {args.square} equals {answer}")
elif args.verbosity == 1:
    print(f"{args.square}^2 == {answer}")
else:
    print(answer)
</pre> <p>And the output:</p> <pre data-language="shell">$ python prog.py 4 -v 3
usage: prog.py [-h] [-v {0,1,2}] square
prog.py: error: argument -v/--verbosity: invalid choice: 3 (choose from 0, 1, 2)
$ python prog.py 4 -h
usage: prog.py [-h] [-v {0,1,2}] square

positional arguments:
  square                display a square of a given number

options:
  -h, --help            show this help message and exit
  -v {0,1,2}, --verbosity {0,1,2}
                        increase output verbosity
</pre> <p>Note that the change also reflects both in the error message as well as the help string.</p> <p>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 <code>python --help</code>):</p> <pre data-language="python">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(f"the square of {args.square} equals {answer}")
elif args.verbosity == 1:
    print(f"{args.square}^2 == {answer}")
else:
    print(answer)
</pre> <p>We have introduced another action, “count”, to count the number of occurrences of specific options.</p> <pre data-language="shell">$ python prog.py 4
16
$ python prog.py 4 -v
4^2 == 16
$ python prog.py 4 -vv
the square of 4 equals 16
$ python prog.py 4 --verbosity --verbosity
the square of 4 equals 16
$ python prog.py 4 -v 1
usage: prog.py [-h] [-v] square
prog.py: error: unrecognized arguments: 1
$ python prog.py 4 -h
usage: prog.py [-h] [-v] square

positional arguments:
  square           display a square of a given number

options:
  -h, --help       show this help message and exit
  -v, --verbosity  increase output verbosity
$ python prog.py 4 -vvv
16
</pre> <ul class="simple"> <li>Yes, it’s now more of a flag (similar to <code>action="store_true"</code>) in the previous version of our script. That should explain the complaint.</li> <li>It also behaves similar to “store_true” action.</li> <li>Now here’s a demonstration of what the “count” action gives. You’ve probably seen this sort of usage before.</li> <li>And if you don’t specify the <code>-v</code> flag, that flag is considered to have <code>None</code> value.</li> <li>As should be expected, specifying the long form of the flag, we should get the same output.</li> <li>Sadly, our help output isn’t very informative on the new ability our script has acquired, but that can always be fixed by improving the documentation for our script (e.g. via the <code>help</code> keyword argument).</li> <li>That last output exposes a bug in our program.</li> </ul> <p>Let’s fix:</p> <pre data-language="python">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 &gt;=
if args.verbosity &gt;= 2:
    print(f"the square of {args.square} equals {answer}")
elif args.verbosity &gt;= 1:
    print(f"{args.square}^2 == {answer}")
else:
    print(answer)
</pre> <p>And this is what it gives:</p> <pre data-language="shell">$ python prog.py 4 -vvv
the square of 4 equals 16
$ python prog.py 4 -vvvv
the square of 4 equals 16
$ python prog.py 4
Traceback (most recent call last):
  File "prog.py", line 11, in &lt;module&gt;
    if args.verbosity &gt;= 2:
TypeError: '&gt;=' not supported between instances of 'NoneType' and 'int'
</pre> <ul class="simple"> <li>First output went well, and fixes the bug we had before. That is, we want any value &gt;= 2 to be as verbose as possible.</li> <li>Third output not so good.</li> </ul> <p>Let’s fix that bug:</p> <pre data-language="python">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 &gt;= 2:
    print(f"the square of {args.square} equals {answer}")
elif args.verbosity &gt;= 1:
    print(f"{args.square}^2 == {answer}")
else:
    print(answer)
</pre> <p>We’ve just introduced yet another keyword, <code>default</code>. We’ve set it to <code>0</code> in order to make it comparable to the other int values. Remember that by default, if an optional argument isn’t specified, it gets the <code>None</code> value, and that cannot be compared to an int value (hence the <a class="reference internal" href="../library/exceptions#TypeError" title="TypeError"><code>TypeError</code></a> exception).</p> <p>And:</p> <pre data-language="shell">$ python prog.py 4
16
</pre> <p>You can go quite far just with what we’ve learned so far, and we have only scratched the surface. The <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> module is very powerful, and we’ll explore a bit more of it before we end this tutorial.</p> </section> <section id="getting-a-little-more-advanced"> <h2>Getting a little more advanced</h2> <p>What if we wanted to expand our tiny program to perform other powers, not just squares:</p> <pre data-language="python">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 &gt;= 2:
    print(f"{args.x} to the power {args.y} equals {answer}")
elif args.verbosity &gt;= 1:
    print(f"{args.x}^{args.y} == {answer}")
else:
    print(answer)
</pre> <p>Output:</p> <pre data-language="shell">$ 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

options:
  -h, --help       show this help message and exit
  -v, --verbosity
$ python prog.py 4 2 -v
4^2 == 16
</pre> <p>Notice that so far we’ve been using verbosity level to <em>change</em> the text that gets displayed. The following example instead uses verbosity level to display <em>more</em> text instead:</p> <pre data-language="python">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 &gt;= 2:
    print(f"Running '{__file__}'")
if args.verbosity &gt;= 1:
    print(f"{args.x}^{args.y} == ", end="")
print(answer)
</pre> <p>Output:</p> <pre data-language="shell">$ python prog.py 4 2
16
$ python prog.py 4 2 -v
4^2 == 16
$ python prog.py 4 2 -vv
Running 'prog.py'
4^2 == 16
</pre> <section id="specifying-ambiguous-arguments"> <span id="id2"></span><h3>Specifying ambiguous arguments</h3> <p>When there is ambiguity in deciding whether an argument is positional or for an argument, <code>--</code> can be used to tell <a class="reference internal" href="../library/argparse#argparse.ArgumentParser.parse_args" title="argparse.ArgumentParser.parse_args"><code>parse_args()</code></a> that everything after that is a positional argument:</p> <pre data-language="python">&gt;&gt;&gt; parser = argparse.ArgumentParser(prog='PROG')
&gt;&gt;&gt; parser.add_argument('-n', nargs='+')
&gt;&gt;&gt; parser.add_argument('args', nargs='*')

&gt;&gt;&gt; # ambiguous, so parse_args assumes it's an option
&gt;&gt;&gt; parser.parse_args(['-f'])
usage: PROG [-h] [-n N [N ...]] [args ...]
PROG: error: unrecognized arguments: -f

&gt;&gt;&gt; parser.parse_args(['--', '-f'])
Namespace(args=['-f'], n=None)

&gt;&gt;&gt; # ambiguous, so the -n option greedily accepts arguments
&gt;&gt;&gt; parser.parse_args(['-n', '1', '2', '3'])
Namespace(args=[], n=['1', '2', '3'])

&gt;&gt;&gt; parser.parse_args(['-n', '1', '--', '2', '3'])
Namespace(args=['2', '3'], n=['1'])
</pre> </section> <section id="conflicting-options"> <h3>Conflicting options</h3> <p>So far, we have been working with two methods of an <a class="reference internal" href="../library/argparse#argparse.ArgumentParser" title="argparse.ArgumentParser"><code>argparse.ArgumentParser</code></a> instance. Let’s introduce a third one, <a class="reference internal" href="../library/argparse#argparse.ArgumentParser.add_mutually_exclusive_group" title="argparse.ArgumentParser.add_mutually_exclusive_group"><code>add_mutually_exclusive_group()</code></a>. It allows for us to specify options that conflict with each other. Let’s also change the rest of the program so that the new functionality makes more sense: we’ll introduce the <code>--quiet</code> option, which will be the opposite of the <code>--verbose</code> one:</p> <pre data-language="python">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(f"{args.x} to the power {args.y} equals {answer}")
else:
    print(f"{args.x}^{args.y} == {answer}")
</pre> <p>Our program is now simpler, and we’ve lost some functionality for the sake of demonstration. Anyways, here’s the output:</p> <pre data-language="shell">$ python prog.py 4 2
4^2 == 16
$ python prog.py 4 2 -q
16
$ python prog.py 4 2 -v
4 to the power 2 equals 16
$ python prog.py 4 2 -vq
usage: prog.py [-h] [-v | -q] x y
prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose
$ python prog.py 4 2 -v --quiet
usage: prog.py [-h] [-v | -q] x y
prog.py: error: argument -q/--quiet: not allowed with argument -v/--verbose
</pre> <p>That should be easy to follow. I’ve added that last output so you can see the sort of flexibility you get, i.e. mixing long form options with short form ones.</p> <p>Before we conclude, you probably want to tell your users the main purpose of your program, just in case they don’t know:</p> <pre data-language="python">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(f"{args.x} to the power {args.y} equals {answer}")
else:
    print(f"{args.x}^{args.y} == {answer}")
</pre> <p>Note that slight difference in the usage text. Note the <code>[-v | -q]</code>, which tells us that we can either use <code>-v</code> or <code>-q</code>, but not both at the same time:</p> <pre data-language="shell">$ 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

options:
  -h, --help     show this help message and exit
  -v, --verbose
  -q, --quiet
</pre> </section> </section> <section id="how-to-translate-the-argparse-output"> <h2>How to translate the argparse output</h2> <p>The output of the <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> module such as its help text and error messages are all made translatable using the <a class="reference internal" href="../library/gettext#module-gettext" title="gettext: Multilingual internationalization services."><code>gettext</code></a> module. This allows applications to easily localize messages produced by <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a>. See also <a class="reference internal" href="../library/gettext#i18n-howto"><span class="std std-ref">Internationalizing your programs and modules</span></a>.</p> <p>For instance, in this <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> output:</p> <pre data-language="shell">$ 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

options:
  -h, --help     show this help message and exit
  -v, --verbose
  -q, --quiet
</pre> <p>The strings <code>usage:</code>, <code>positional arguments:</code>, <code>options:</code> and <code>show this help message and exit</code> are all translatable.</p> <p>In order to translate these strings, they must first be extracted into a <code>.po</code> file. For example, using <a class="reference external" href="https://babel.pocoo.org/">Babel</a>, run this command:</p> <pre data-language="shell">$ pybabel extract -o messages.po /usr/lib/python3.12/argparse.py
</pre> <p>This command will extract all translatable strings from the <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> module and output them into a file named <code>messages.po</code>. This command assumes that your Python installation is in <code>/usr/lib</code>.</p> <p>You can find out the location of the <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> module on your system using this script:</p> <pre data-language="python">import argparse
print(argparse.__file__)
</pre> <p>Once the messages in the <code>.po</code> file are translated and the translations are installed using <a class="reference internal" href="../library/gettext#module-gettext" title="gettext: Multilingual internationalization services."><code>gettext</code></a>, <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> will be able to display the translated messages.</p> <p>To translate your own strings in the <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> output, use <a class="reference internal" href="../library/gettext#module-gettext" title="gettext: Multilingual internationalization services."><code>gettext</code></a>.</p> </section> <section id="conclusion"> <h2>Conclusion</h2> <p>The <a class="reference internal" href="../library/argparse#module-argparse" title="argparse: Command-line option and argument parsing library."><code>argparse</code></a> 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.</p> </section> <div class="_attribution">
  <p class="_attribution-p">
    &copy; 2001&ndash;2023 Python Software Foundation<br>Licensed under the PSF License.<br>
    <a href="https://docs.python.org/3.12/howto/argparse.html" class="_attribution-link">https://docs.python.org/3.12/howto/argparse.html</a>
  </p>
</div>