# Argparse 教學

author

  Tshepang Lekhonkhobe

這個教學傾向簡介 Python 官方標準含式庫中推薦的命令列解析模組 `argparse`。

備註

另外兩個具有同樣功能的模組 `getopt` （一個相等於 C 語言中的 `getopt()`）以及被棄用的 `optparse`。而 `argparse` 也是根據 `optparse` 為基礎發展而來，因此有非常近似的使用方式。

## 概念

藉由命令 ls 的使用開始這些功能的介紹：

    $ 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.
    ...

我們可以從四個命令中可以學到的幾個概念：

- 命令 ls 在執行時不用其他參數就可以顯示出當前目錄底下的內容。

- 根據這樣的概念延伸後來舉個例子，如果我們想秀出一個不在目錄的資料夾 `pypy` 的內容。我們可以在命令後加上一個位置參數。會用位置參數這樣的名稱是因為程式會知道輸入的參數該做的事情。這樣的概念很像另一個命令 cp，基本的使用方式是 `cp SRC DEST`。第一個位置參數代表的是想要複製的目標，第二個位置的參數代表的則是想要複製到的地方。

- 現在我們想再增加一些，要顯示除了檔名之外更多的資訊。在這裡就可以選擇加上 `-l` 這個參數。

- 這是 help 文件的片段。對於以前從未使用過的程序來說非常有用，可以透過這些 help 文件來了解這些該怎麼使用。

## 基本用法

我們以一個很簡單的例子開始下面的介紹：

    import argparse
    parser = argparse.ArgumentParser()
    parser.parse_args()

下面是運行這些代碼的結果：

    $ 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

接者是發生的情況：

- 運行這個腳本而沒有給與任何參數時就不會顯示任何東西至標準輸出畫面上。這裡並不是這麼的有用。

- 第二個我們呈現出了 `argparse` 模組的用處。我們幾乎沒有做什麼事情，但已經得到一個很好的幫助信息。

- 這個 `--help` 選項可以簡短的表示成 `-h` , 這是唯一一個選項我們不用去指明的（意即，沒有必要在這個參數後加上任何數值）。如果指定其他參數給他會造成錯誤。也因為這樣，我們得到了一個免費的信息。

## 介紹位置參數

例子：

    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument("echo")
    args = parser.parse_args()
    print(args.echo)

運行這段代碼：

    $ 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

接者是發生的情況：

- 我們增加了 `add_argument()` ，利用這個方法可以指名讓我們的程式接受哪些命令列參數。

- 現在呼叫我們的程序時需要指定一個參數選項。

- 在這個例子中， `parse_args()` 這個方法確實根據了 `echo` 這個選項回傳了資料。

- The variable is some form of 'magic' that `argparse` performs for free (i.e. no need to specify which variable that value is stored in). You will also notice that its name matches the string argument given to the method, `echo`.

注意, 雖然 help 秀出了看起來不錯的信息, 但現在並沒有給予到實質幫助。像剛剛增加的 `echo` 這個位置參數，除了猜測和讀原始碼之外，我們根本不曉得該怎麼使用他。因此我們來做一點事讓他變得更有用：

    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument("echo", help="echo the string you use here")
    args = parser.parse_args()
    print(args.echo)

然後我們得到：

    $ 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

現在來做一些更有用處的事情：

    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)

下面是運行這些代碼的結果：

    $ 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'

那並沒有如預期這樣。這是因為 `argparse` 將我們給予選項的值當成字串，除然我們告訴他要怎麼做。所以我們來告訴 `argparse` 將這個輸入值當成整數來使用：

    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)

下面是運行這些代碼的結果：

    $ python3 prog.py 4
    16
    $ python3 prog.py four
    usage: prog.py [-h] square
    prog.py: error: argument square: invalid int value: 'four'

這樣很順利。現在程序在開始之前會因為錯誤的輸入而回報有用的訊息並結束掉。

## 介紹選項參數

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")

接者是結果：

    $ 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

接者是發生的情況：

- 這個程式是寫成如果有指名 `--verbosity` 這個參數選項那才顯示些資訊，反之亦然。

- To show that the option is actually optional, there is no error when running the program without it. Note that by default, if an optional argument isn't used, the relevant variable, in this case `args.verbosity`, is given `None` as a value, which is the reason it fails the truth test of the `if` statement.

- Help 訊息稍微有些不一樣。

- 當使用 `--verbosity` 參數選項時必須要指定一個數值。

在上面的例子中 `--verbosity`，接受任意的整數，但對我們的程式來說只接受兩個輸入值， `True` 或 `False`。所以我們來修改一下程式碼使其符合：

    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")

接者是結果：

    $ 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

接者是發生的情況：

- The option is now more of a flag than something that requires a value. We even changed the name of the option to match that idea. Note that we now specify a new keyword, `action`, and give it the value `"store_true"`. This means that, if the option is specified, assign the value `True` to `args.verbose`. Not specifying it implies `False`.

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

- 注意不同的 help 文件。

### Short options

如果你很熟悉命令列的使用的話，你將會發現我還沒講到關於短參數。其實這很簡單：

    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:

    $ 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

注意新的表示對於幫助文件也是一樣的

## 現在結合位置與選項參數

我們的程式成長的越來越複雜：

    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)

然後現在的輸出結果：

    $ 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.

- 注意現在的順序對於程式來說已經不再重要了.

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)

接者是結果：

    $ 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)

接者是結果：

    $ 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)

我們已經介紹過另一個操作 "count" 用來計算指定的選項參數出現的次數。

    $ 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.

- 現在來秀一下 "count" 這個動作會給予什麼。你可能之前就有見過這種用法。

- And if you don't specify the `-v` flag, that flag is considered to have `None` value.

- 應該要如預期那樣，就算給予長選項我們也要獲得一樣的輸出結果。

- 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 `help` keyword argument).

- That last output exposes a bug in our program.

讓我們來解決問題

    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)

而這也正是它給的：

    $ 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: '>=' not supported between instances of 'NoneType' and '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.

- 第三個輸出不是這麼的好。

我們來修復這個錯誤：

    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 `TypeError` exception).

而且

    $ 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 `argparse` module is very powerful, and we'll explore a bit more of it before we end this tutorial.

## Getting a little more advanced

如果我們想要擴展我們的小程式做比範例更多的事：

    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)

結果：

    $ 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)

結果：

    $ 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 `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("{} 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:

    $ 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.

在我們結論之前，你可能想告訴你的用戶這個程式的主要目的，以防萬一他們不知道：

    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:

    $ 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

## 結論

`argparse` 模組提供了比這裡展示更多的功能。它的文件是非常全面詳細且充滿了例子。通過本教學，你應該比較容易消化它們了。