From 754bbf7a25a8dda49b5d08ef0d0443bbf5af0e36 Mon Sep 17 00:00:00 2001 From: Craig Jennings Date: Sun, 7 Apr 2024 13:41:34 -0500 Subject: new repository --- devdocs/python~3.12/howto%2Fargparse.html | 432 ++++++++++++++++++++++++++++++ 1 file changed, 432 insertions(+) create mode 100644 devdocs/python~3.12/howto%2Fargparse.html (limited to 'devdocs/python~3.12/howto%2Fargparse.html') diff --git a/devdocs/python~3.12/howto%2Fargparse.html b/devdocs/python~3.12/howto%2Fargparse.html new file mode 100644 index 00000000..0f175cbc --- /dev/null +++ b/devdocs/python~3.12/howto%2Fargparse.html @@ -0,0 +1,432 @@ +

Argparse Tutorial

author
+

Tshepang Mbambo

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.

Concepts

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
+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.
+...
+

A few concepts we can learn from the four commands:

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:

$ 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
+

Here is what is happening:

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:

$ 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
+

Here is what’s happening:

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
+
+options:
+  -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.py 4
+Traceback (most recent call last):
+  File "prog.py", line 5, in <module>
+    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.py 4
+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.

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="increase output verbosity")
+args = parser.parse_args()
+if args.verbosity:
+    print("verbosity turned on")
+

And the output:

$ 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
+

Here is what is happening:

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 --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
+

Here is what is happening:

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="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]
+
+options:
+  -h, --help     show this help message and exit
+  -v, --verbose  increase output verbosity
+

Note that the new ability is also reflected in the help text.

Combining 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(f"the square of {args.square} equals {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.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
+

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(f"the square of {args.square} equals {answer}")
+elif args.verbosity == 1:
+    print(f"{args.square}^2 == {answer}")
+else:
+    print(answer)
+

And the output:

$ 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
+

These all look good except the last one, which exposes a bug in our program. Let’s fix 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(f"the square of {args.square} equals {answer}")
+elif args.verbosity == 1:
+    print(f"{args.square}^2 == {answer}")
+else:
+    print(answer)
+

And the output:

$ 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
+

Note that the change also reflects 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(f"the square of {args.square} equals {answer}")
+elif args.verbosity == 1:
+    print(f"{args.square}^2 == {answer}")
+else:
+    print(answer)
+

We have introduced another action, “count”, to count the number of occurrences of specific options.

$ 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
+

Let’s fix:

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(f"the square of {args.square} equals {answer}")
+elif args.verbosity >= 1:
+    print(f"{args.square}^2 == {answer}")
+else:
+    print(answer)
+

And this is what it gives:

$ 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 <module>
+    if args.verbosity >= 2:
+TypeError: '>=' not supported between instances of 'NoneType' and 'int'
+

Let’s fix 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(f"the square of {args.square} equals {answer}")
+elif args.verbosity >= 1:
+    print(f"{args.square}^2 == {answer}")
+else:
+    print(answer)
+

We’ve just introduced yet another keyword, default. We’ve set it to 0 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 None value, and that cannot be compared to an int value (hence the TypeError exception).

And:

$ python prog.py 4
+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.

Getting 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(f"{args.x} to the power {args.y} equals {answer}")
+elif args.verbosity >= 1:
+    print(f"{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
+
+options:
+  -h, --help       show this help message and exit
+  -v, --verbosity
+$ python prog.py 4 2 -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(f"Running '{__file__}'")
+if args.verbosity >= 1:
+    print(f"{args.x}^{args.y} == ", end="")
+print(answer)
+

Output:

$ 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
+

Specifying ambiguous arguments

When there is ambiguity in deciding whether an argument is positional or for an argument, -- can be used to tell parse_args() that everything after that is a positional argument:

>>> parser = argparse.ArgumentParser(prog='PROG')
+>>> parser.add_argument('-n', nargs='+')
+>>> parser.add_argument('args', nargs='*')
+
+>>> # ambiguous, so parse_args assumes it's an option
+>>> parser.parse_args(['-f'])
+usage: PROG [-h] [-n N [N ...]] [args ...]
+PROG: error: unrecognized arguments: -f
+
+>>> parser.parse_args(['--', '-f'])
+Namespace(args=['-f'], n=None)
+
+>>> # ambiguous, so the -n option greedily accepts arguments
+>>> parser.parse_args(['-n', '1', '2', '3'])
+Namespace(args=[], n=['1', '2', '3'])
+
+>>> parser.parse_args(['-n', '1', '--', '2', '3'])
+Namespace(args=['2', '3'], n=['1'])
+

Conflicting 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 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 --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(f"{args.x} to the power {args.y} equals {answer}")
+else:
+    print(f"{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.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
+

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.

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(f"{args.x} to the power {args.y} equals {answer}")
+else:
+    print(f"{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
+
+options:
+  -h, --help     show this help message and exit
+  -v, --verbose
+  -q, --quiet
+

How to translate the argparse output

The output of the argparse module such as its help text and error messages are all made translatable using the gettext module. This allows applications to easily localize messages produced by argparse. See also Internationalizing your programs and modules.

For instance, in this argparse output:

$ 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
+

The strings usage:, positional arguments:, options: and show this help message and exit are all translatable.

In order to translate these strings, they must first be extracted into a .po file. For example, using Babel, run this command:

$ pybabel extract -o messages.po /usr/lib/python3.12/argparse.py
+

This command will extract all translatable strings from the argparse module and output them into a file named messages.po. This command assumes that your Python installation is in /usr/lib.

You can find out the location of the argparse module on your system using this script:

import argparse
+print(argparse.__file__)
+

Once the messages in the .po file are translated and the translations are installed using gettext, argparse will be able to display the translated messages.

To translate your own strings in the argparse output, use gettext.

Conclusion

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.

+

+ © 2001–2023 Python Software Foundation
Licensed under the PSF License.
+ https://docs.python.org/3.12/howto/argparse.html +

+
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