#14034: added the argparse tutorial. Patch by Tshepang Lekhonkhobe.
This commit is contained in:
Ezio Melotti
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************************
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:mod:`argparse` Tutorial
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************************
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:author: Tshepang Lekhonkhobe <tshepang@gmail.com>
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.. _argparse-tutorial:
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This tutorial is intended to be a gentle introduction to :mod:`argparse`, the
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recommended command-line parsing module in the Python standard library.
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.. note::
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There's two other modules that fulfill the same task, namely
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:mod:`getopt` (an equivalent for :c:func:`getopt` from the C
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language) and the deprecated :mod:`optparse`.
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Note also that :mod:`argparse` is based on :mod:`optparse`,
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and therefore very similar in terms of usage.
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Concepts
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========
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Let's show the sort of functionality that we are going to explore in this
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introductory tutorial by making use of the :command:`ls` command:
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.. code-block:: sh
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$ ls
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cpython devguide prog.py pypy rm-unused-function.patch
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$ ls pypy
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ctypes_configure demo dotviewer include lib_pypy lib-python ...
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$ ls -l
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total 20
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drwxr-xr-x 19 wena wena 4096 Feb 18 18:51 cpython
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drwxr-xr-x 4 wena wena 4096 Feb 8 12:04 devguide
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-rwxr-xr-x 1 wena wena 535 Feb 19 00:05 prog.py
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drwxr-xr-x 14 wena wena 4096 Feb 7 00:59 pypy
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-rw-r--r-- 1 wena wena 741 Feb 18 01:01 rm-unused-function.patch
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$ ls --help
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Usage: ls [OPTION]... [FILE]...
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List information about the FILEs (the current directory by default).
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Sort entries alphabetically if none of -cftuvSUX nor --sort is specified.
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...
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A few concepts we can learn from the four commands:
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* The :command:`ls` command is useful when run without any options at all. It defaults
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to displaying the contents of the current directory.
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* If we want beyond what it provides by default, we tell it a bit more. In
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this case, we want it to display a different directory, ``pypy``.
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What we did is specify what is known as a positional argument. It's named so
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because the program should know what to do with the value, solely based on
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where it appears on the command line. This concept is more relevant
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to a command like :command:`cp`, whose most basic usage is ``cp SRC DEST``.
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The first position is *what you want copied,* and the second
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position is *where you want it copied to*.
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* Now, say we want to change behaviour of the program. In our example,
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we display more info for each file instead of just showing the file names.
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The ``-l`` in that case is known as an optional argument.
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* That's a snippet of the help text. It's very useful in that you can
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come across a program you have never used before, and can figure out
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how it works simply by reading it's help text.
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The basics
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==========
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Let us start with a very simple example which does (almost) nothing::
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import argparse
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parser = argparse.ArgumentParser()
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parser.parse_args()
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Following is a result of running the code:
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.. code-block:: sh
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$ python3 prog.py
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$ python3 prog.py --help
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usage: prog.py [-h]
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optional arguments:
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-h, --help show this help message and exit
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$ python3 prog.py --verbose
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usage: prog.py [-h]
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prog.py: error: unrecognized arguments: --verbose
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$ python3 prog.py foo
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usage: prog.py [-h]
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prog.py: error: unrecognized arguments: foo
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Here is what is happening:
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* Running the script without any options results in nothing displayed to
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stdout. Not so useful.
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* The second one starts to display the usefulness of the :mod:`argparse`
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module. We have done almost nothing, but already we get a nice help message.
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* The ``--help`` option, which can also be shortened to ``-h``, is the only
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option we get for free (i.e. no need to specify it). Specifying anything
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else results in an error. But even then, we do get a useful usage message,
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also for free.
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Introducing Positional arguments
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================================
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An example::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("echo")
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args = parser.parse_args()
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print(args.echo)
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And running the code:
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.. code-block:: sh
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$ python3 prog.py
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usage: prog.py [-h] echo
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prog.py: error: the following arguments are required: echo
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$ python3 prog.py --help
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usage: prog.py [-h] echo
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positional arguments:
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echo
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optional arguments:
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-h, --help show this help message and exit
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$ python3 prog.py foo
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foo
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Here is what's happening:
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* We've added the :meth:`add_argument` method, which is what we use to specify
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which command-line options the program is willing to accept. In this case,
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I've named it ``echo`` so that it's in line with its function.
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* Calling our program now requires us to specify an option.
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* The :meth:`parse_args` method actually returns some data from the
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options specified, in this case, ``echo``.
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* The variable is some form of 'magic' that :mod:`argparse` performs for free
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(i.e. no need to specify which variable that value is stored in).
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You will also notice that its name matches the string argument given
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to the method, ``echo``.
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Note however that, although the help display looks nice and all, it currently
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is not as helpful as it can be. For example we see that we got ``echo`` as a
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positional argument, but we don't know what it does, other than by guessing or
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by reading the source code. So, let's make it a bit more useful::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("echo", help="echo the string you use here")
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args = parser.parse_args()
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print(args.echo)
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And we get:
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.. code-block:: sh
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$ python3 prog.py -h
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usage: prog.py [-h] echo
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positional arguments:
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echo echo the string you use here
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optional arguments:
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-h, --help show this help message and exit
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Now, how about doing something even more useful::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("square", help="display a square of a given number")
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args = parser.parse_args()
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print(args.square**2))
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Following is a result of running the code:
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.. code-block:: sh
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$ python3 prog.py 4
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Traceback (most recent call last):
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File "prog.py", line 5, in <module>
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print(args.square**2)
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TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'
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That didn't go so well. That's because :mod:`argparse` treats the options we
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give it as strings, unless we tell it otherwise. So, let's tell
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:mod:`argparse` to treat that input as an integer::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("square", help="display a square of a given number",
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type=int)
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args = parser.parse_args()
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print(args.square**2)
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Following is a result of running the code:
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.. code-block:: sh
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$ python3 prog.py 4
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16
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$ python3 prog.py four
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usage: prog.py [-h] square
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prog.py: error: argument square: invalid int value: 'four'
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That went well. The program now even helpfully quits on bad illegal input
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before proceeding.
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Introducing Optional arguments
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==============================
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So far we, have been playing with positional arguments. Let us
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have a look on how to add optional ones::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("--verbosity", help="increase output verbosity")
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args = parser.parse_args()
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if args.verbosity:
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print("verbosity turned on")
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And the output:
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.. code-block:: sh
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$ python3 prog.py --verbosity 1
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verbosity turned on
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$ python3 prog.py
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$ python3 prog.py --help
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usage: prog.py [-h] [--verbosity VERBOSITY]
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optional arguments:
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-h, --help show this help message and exit
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--verbosity VERBOSITY
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increase output verbosity
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$ python3 prog.py --verbosity
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usage: prog.py [-h] [--verbosity VERBOSITY]
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prog.py: error: argument --verbosity: expected one argument
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Here is what is happening:
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* The program is written so as to display something when ``--verbosity`` is
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specified and display nothing when not.
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* To show that the option is actually optional, there is no error when running
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the program without it. Note that by default, if an optional argument isn't
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used, the relevant variable, in this case :attr:`args.verbosity`, is
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given ``None`` as a value, which is the reason it fails the truth
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test of the :keyword:`if` statement.
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* The help message is a bit different.
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* When using the ``--verbosity`` option, one must also specify some value,
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any value.
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The above example accepts arbitrary integer values for ``--verbosity``, but for
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our simple program, only two values are actually useful, ``True`` or ``False``.
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Let's modify the code accordingly::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("--verbose", help="increase output verbosity",
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action="store_true")
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args = parser.parse_args()
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if args.verbose:
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print("verbosity turned on")
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And the output:
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.. code-block:: sh
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$ python3 prog.py --verbose
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verbosity turned on
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$ python3 prog.py --verbose 1
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usage: prog.py [-h] [--verbose]
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prog.py: error: unrecognized arguments: 1
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$ python3 prog.py --help
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usage: prog.py [-h] [--verbose]
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optional arguments:
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-h, --help show this help message and exit
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--verbose increase output verbosity
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Here is what is happening:
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* The option is now more of a flag than something that requires a value.
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We even changed the name of the option to match that idea.
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Note that we now specify a new keyword, ``action``, and give it the value
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``"store_true"``. This means that, if the option is specified,
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assign the value ``True`` to :data:`args.verbose`.
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Not specifying it implies ``False``.
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* It complains when you specify a value, in true spirit of what flags
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actually are.
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* Notice the different help text.
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Short options
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-------------
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If you are familiar with command line usage,
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you will notice that I haven't yet touched on the topic of short
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versions of the options. It's quite simple::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("-v", "--verbose", help="increase output verbosity",
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action="store_true")
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args = parser.parse_args()
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if args.verbose:
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print("verbosity turned on")
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And here goes:
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.. code-block:: sh
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$ python3 prog.py -v
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verbosity turned on
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$ python3 prog.py --help
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usage: prog.py [-h] [-v]
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optional arguments:
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-h, --help show this help message and exit
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-v, --verbose increase output verbosity
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Note that the new ability is also reflected in the help text.
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Combining Positional and Optional arguments
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===========================================
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Our program keeps growing in complexity::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("square", type=int,
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help="display a square of a given number")
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parser.add_argument("-v", "--verbose", action="store_true",
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help="increase output verbosity")
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args = parser.parse_args()
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answer = args.square**2
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if args.verbose:
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print("the square of {} equals {}".format(args.square, answer))
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else:
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print(answer)
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And now the output:
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.. code-block:: sh
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$ python3 prog.py
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usage: prog.py [-h] [-v] square
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prog.py: error: the following arguments are required: square
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$ python3 prog.py 4
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16
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$ python3 prog.py 4 --verbose
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the square of 4 equals 16
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$ python3 prog.py --verbose 4
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the square of 4 equals 16
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* We've brought back a positional argument, hence the complaint.
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* Note that the order does not matter.
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How about we give this program of ours back the ability to have
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multiple verbosity values, and actually get to use them::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("square", type=int,
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help="display a square of a given number")
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parser.add_argument("-v", "--verbosity", type=int,
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help="increase output verbosity")
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args = parser.parse_args()
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answer = args.square**2
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if args.verbosity == 2:
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print("the square of {} equals {}".format(args.square, answer))
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elif args.verbosity == 1:
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print("{}^2 == {}".format(args.square, answer))
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else:
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print(answer)
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And the output:
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.. code-block:: sh
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$ python3 prog.py 4
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16
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$ python3 prog.py 4 -v
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usage: prog.py [-h] [-v VERBOSITY] square
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prog.py: error: argument -v/--verbosity: expected one argument
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$ python3 prog.py 4 -v 1
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4^2 == 16
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$ python3 prog.py 4 -v 2
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the square of 4 equals 16
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$ python3 prog.py 4 -v 3
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16
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These all look good except the last one, which exposes a bug in our program.
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Let's fix it by restricting the values the ``--verbosity`` option can accept::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("square", type=int,
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help="display a square of a given number")
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parser.add_argument("-v", "--verbosity", type=int, choices=[0, 1, 2],
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help="increase output verbosity")
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args = parser.parse_args()
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answer = args.square**2
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if args.verbosity == 2:
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print("the square of {} equals {}".format(args.square, answer))
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elif args.verbosity == 1:
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print("{}^2 == {}".format(args.square, answer))
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else:
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print(answer)
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And the output:
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.. code-block:: sh
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$ python3 prog.py 4 -v 3
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usage: prog.py [-h] [-v {0,1,2}] square
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prog.py: error: argument -v/--verbosity: invalid choice: 3 (choose from 0, 1, 2)
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$ python3 prog.py 4 -h
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usage: prog.py [-h] [-v {0,1,2}] square
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positional arguments:
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square display a square of a given number
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optional arguments:
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-h, --help show this help message and exit
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-v {0,1,2}, --verbosity {0,1,2}
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increase output verbosity
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Note that the change also reflects both in the error message as well as the
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help string.
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Now, let's use a different approach of playing with verbosity, which is pretty
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common. It also matches the way the CPython executable handles its own
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verbosity argument (check the output of ``python --help``)::
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import argparse
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parser = argparse.ArgumentParser()
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parser.add_argument("square", type=int,
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help="display the square of a given number")
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parser.add_argument("-v", "--verbosity", action="count",
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help="increase output verbosity")
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args = parser.parse_args()
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answer = args.square**2
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if args.verbosity == 2:
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print("the square of {} equals {}".format(args.square, answer))
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elif args.verbosity == 1:
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print("{}^2 == {}".format(args.square, answer))
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else:
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print(answer)
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We have introduced another action, "count",
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to count the number of occurences of a specific optional arguments:
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.. code-block:: sh
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$ python3 prog.py 4
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16
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$ python3 prog.py 4 -v
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4^2 == 16
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$ python3 prog.py 4 -vv
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the square of 4 equals 16
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$ python3 prog.py 4 --verbosity --verbosity
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the square of 4 equals 16
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$ python3 prog.py 4 -v 1
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usage: prog.py [-h] [-v] square
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prog.py: error: unrecognized arguments: 1
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$ python3 prog.py 4 -h
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usage: prog.py [-h] [-v] square
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positional arguments:
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square display a square of a given number
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optional arguments:
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-h, --help show this help message and exit
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-v, --verbosity increase output verbosity
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$ python3 prog.py 4 -vvv
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16
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* Yes, it's now more of a flag (similar to ``action="store_true"``) in the
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previous version of our script. That should explain the complaint.
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* It also behaves similar to "store_true" action.
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* Now here's a demonstration of what the "count" action gives. You've probably
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seen this sort of usage before.
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* And, just like the "store_true" action, if you don't specify the ``-v`` flag,
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that flag is considered to have ``None`` value.
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||||
* 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.
|
||||
|
|
@ -27,4 +27,5 @@ Currently, the HOWTOs are:
|
|||
unicode.rst
|
||||
urllib2.rst
|
||||
webservers.rst
|
||||
argparse.rst
|
||||
|
||||
|
|
|
|||
|
|
@ -12,6 +12,12 @@
|
|||
|
||||
--------------
|
||||
|
||||
.. sidebar:: Tutorial
|
||||
|
||||
This page contains the API reference information. For a more gentle
|
||||
introduction to Python command-line parsing, have a look at the
|
||||
:ref:`argparse tutorial <argparse-tutorial>`.
|
||||
|
||||
The :mod:`argparse` module makes it easy to write user-friendly command-line
|
||||
interfaces. The program defines what arguments it requires, and :mod:`argparse`
|
||||
will figure out how to parse those out of :data:`sys.argv`. The :mod:`argparse`
|
||||
|
|
|
|||
|
|
@ -550,6 +550,7 @@ Joerg Lehmann
|
|||
Robert Lehmann
|
||||
Petri Lehtinen
|
||||
Luke Kenneth Casson Leighton
|
||||
Tshepang Lekhonkhobe
|
||||
Marc-Andre Lemburg
|
||||
John Lenton
|
||||
Christopher Tur Lesniewski-Laas
|
||||
|
|
|
|||
Loading…
Reference in New Issue