#14034: added the argparse tutorial. Patch by Tshepang Lekhonkhobe.

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+************************
+:mod:`argparse` Tutorial
+************************
+
+:author: Tshepang Lekhonkhobe <tshepang@gmail.com>
+
+.. _argparse-tutorial:
+
+This tutorial is intended to be a gentle introduction to :mod:`argparse`, the
+recommended command-line parsing module in the Python standard library.
+
+.. note::
+
+   There's two other modules that fulfill the same task, namely
+   :mod:`getopt` (an equivalent for :c:func:`getopt` from the C
+   language) and the deprecated :mod:`optparse`.
+   Note also that :mod:`argparse` is based on :mod:`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 :command:`ls` command:
+
+.. code-block:: sh
+
+   $ 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 :command:`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 :command:`cp`, whose most basic usage is ``cp SRC DEST``.
+  The first 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 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 it's help text.
+
+
+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:
+
+.. code-block:: sh
+
+   $ python3 prog.py
+   $ python3 prog.py --help
+   usage: prog.py [-h]
+
+   optional arguments:
+   -h, --help  show this help message and exit
+   $ python3 prog.py --verbose
+   usage: prog.py [-h]
+   prog.py: error: unrecognized arguments: --verbose
+   $ python3 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 :mod:`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.
+
+
+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:
+
+.. code-block:: sh
+
+   $ python3 prog.py
+   usage: prog.py [-h] echo
+   prog.py: error: the following arguments are required: echo
+   $ python3 prog.py --help
+   usage: prog.py [-h] echo
+
+   positional arguments:
+   echo
+
+   optional arguments:
+   -h, --help  show this help message and exit
+   $ python3 prog.py foo
+   foo
+
+Here is what's happening:
+
+* We've added the :meth:`add_argument` 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 ``echo`` so that it's in line with its function.
+
+* Calling our program now requires us to specify an option.
+
+* The :meth:`parse_args` method actually returns some data from the
+  options specified, in this case, ``echo``.
+
+* The variable is some form of 'magic' that :mod:`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:
+
+.. code-block:: sh
+
+   $ python3 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:
+
+.. code-block:: sh
+
+   $ python3 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 :mod:`argparse` treats the options we
+give it as strings, unless we tell it otherwise. So, let's tell
+:mod:`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:
+
+.. code-block:: sh
+
+   $ python3 prog.py 4
+   16
+   $ python3 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:
+
+.. code-block:: sh
+
+   $ python3 prog.py --verbosity 1
+   verbosity turned on
+   $ python3 prog.py
+   $ python3 prog.py --help
+   usage: prog.py [-h] [--verbosity VERBOSITY]
+
+   optional arguments:
+   -h, --help            show this help message and exit
+   --verbosity VERBOSITY
+                           increase output verbosity
+   $ python3 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
+  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 :attr:`args.verbosity`, is
+  given ``None`` as a value, which is the reason it fails the truth
+  test of the :keyword:`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:
+
+.. code-block:: sh
+
+   $ python3 prog.py --verbose
+   verbosity turned on
+   $  python3 prog.py --verbose 1
+   usage: prog.py [-h] [--verbose]
+   prog.py: error: unrecognized arguments: 1
+   $ python3 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 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 :data:`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.
+
+
+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:
+
+.. code-block:: sh
+
+   $ python3 prog.py -v
+   verbosity turned on
+   $ python3 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 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("the square of {} equals {}".format(args.square, answer))
+   else:
+      print(answer)
+
+And now the output:
+
+.. code-block:: sh
+
+   $ python3 prog.py
+   usage: prog.py [-h] [-v] square
+   prog.py: error: the following arguments are required: square
+   $ python3 prog.py 4
+   16
+   $ python3 prog.py 4 --verbose
+   the square of 4 equals 16
+   $ python3 prog.py --verbose 4
+   the square of 4 equals 16
+
+* 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:
+
+.. code-block:: sh
+
+   $ python3 prog.py 4
+   16
+   $ python3 prog.py 4 -v
+   usage: prog.py [-h] [-v VERBOSITY] square
+   prog.py: error: argument -v/--verbosity: expected one argument
+   $ python3 prog.py 4 -v 1
+   4^2 == 16
+   $ python3 prog.py 4 -v 2
+   the square of 4 equals 16
+   $ python3 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("the square of {} equals {}".format(args.square, answer))
+   elif args.verbosity == 1:
+      print("{}^2 == {}".format(args.square, answer))
+   else:
+      print(answer)
+
+And the output:
+
+.. code-block:: sh
+
+   $ python3 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)
+   $ python3 prog.py 4 -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 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("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 occurences of a specific optional arguments:
+
+.. code-block:: sh
+
+   $ python3 prog.py 4
+   16
+   $ python3 prog.py 4 -v
+   4^2 == 16
+   $ python3 prog.py 4 -vv
+   the square of 4 equals 16
+   $ python3 prog.py 4 --verbosity --verbosity
+   the square of 4 equals 16
+   $ python3 prog.py 4 -v 1
+   usage: prog.py [-h] [-v] square
+   prog.py: error: unrecognized arguments: 1
+   $ python3 prog.py 4 -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
+   $ python3 prog.py 4 -vvv
+   16
+
+* Yes, it's now more of a flag (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`` flag,
+  that flag is considered to have ``None`` value.
+
+* As should be expected, specifying the long form of the flag, 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 fixed 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 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("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:
+
+.. code-block:: sh
+
+   $ python3 prog.py 4 -vvv
+   the square of 4 equals 16
+   $ python3 prog.py 4 -vvvv
+   the square of 4 equals 16
+   $ python3 prog.py 4
+   Traceback (most recent call last):
+   File "prog.py", line 11, in <module>
+      if args.verbosity >= 2:
+   TypeError: unorderable types: NoneType() >= int()
+
+* First output went well, and fixes the bug we had before.
+  That is, we want any value >= 2 to be as verbose as possible.
+
+* Third output not so good.
+
+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("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 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 :exc:`TypeError` exception).
+
+And:
+
+.. code-block:: sh
+
+   $ python3 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 :mod:`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("{} 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:
+
+.. code-block:: sh
+
+   $ python3 prog.py
+   usage: prog.py [-h] [-v] x y
+   prog.py: error: the following arguments are required: x, y
+   $ python3 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
+   $ python3 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("Running '{}'".format(__file__))
+   if args.verbosity >= 1:
+      print("{}^{} == ".format(args.x, args.y), end="")
+   print(answer)
+
+Output:
+
+.. code-block:: sh
+
+   $ python3 prog.py 4 2
+   16
+   $ python3 prog.py 4 2 -v
+   4^2 == 16
+   $ python3 prog.py 4 2 -vv
+   Running 'prog.py'
+   4^2 == 16
+
+
+Conflicting options
+-------------------
+
+So far, we have been working with two methods of an
+:class:`argparse.ArgumentParser` instance. Let's introduce a third one,
+:meth:`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 make 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:
+
+.. code-block:: sh
+
+   $ python3 prog.py 4 2
+   4^2 == 16
+   $ python3 prog.py 4 2 -q
+   16
+   $ python3 prog.py 4 2 -v
+   4 to the power 2 equals 16
+   $ python3 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
+   $ python3 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("{} 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:
+
+.. code-block:: sh
+
+   $ python3 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
+
+
+Conclusion
+==========
+
+The :mod:`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.