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法律研究如何在 Python 中使用选择选项限制参数值？
简介..
假设你被要求编写一个程序，从用户那里接受网球大满贯赛冠军的数量并进行处理。我们已经知道，费德勒和纳达尔分享着网球界最多的 20 个大满贯赛冠军（截至 2020 年），而最少是 0，许多球员仍在努力争取他们的第一个大满贯赛冠军。
让我们创建一个程序来接受冠军数量。
注意 - 从终端执行程序。
示例
import argparse
def get_args():
""" Function : get_args
parameters used in .add_argument
1. metavar - Provide a hint to the user about the data type.
- By default, all arguments are strings.
2. type - The actual Python data type
- (note the lack of quotes around str)
3. help - A brief description of the parameter for the usage
"""
parser = argparse.ArgumentParser(
description='Example for one positional arguments',
formatter_class=argparse.ArgumentDefaultsHelpFormatter)
# Adding our first argument player titles of type int
parser.add_argument('titles',
metavar='titles',
type=int,
help='GrandSlam Titles')
return parser.parse_args()
# define main
def main(titles):
print(f" *** Player had won {titles} GrandSlam titles.")
if __name__ == '__main__':
args = get_args()
main(args.titles)输出
我们的程序现在已准备好接受冠军数量。因此，让我们将任何数字（而不是浮点数）作为参数传递。
<<< python test.py 20 *** Player had won 20 GrandSlam titles. <<< python test.py 50 *** Player had won 50 GrandSlam titles. <<< python test.py -1 *** Player had won -1 GrandSlam titles. <<< python test.py 30 *** Player had won 30 GrandSlam titles.
虽然代码在技术上没有问题，但肯定存在功能性问题，因为我们的程序接受任意数量的大满贯赛冠军，包括负数冠军。
在这种情况下，如果我们想要限制大满贯赛冠军的选择，可以使用 choices 选项。
在下面的示例中，我们将冠军数量限制在 (0, 20) 的范围内。
示例
import argparse
def get_args():
""" Function : get_args
parameters used in .add_argument
1. metavar - Provide a hint to the user about the data type.
- By default, all arguments are strings.
2. type - The actual Python data type
- (note the lack of quotes around str)
3. help - A brief description of the parameter for the usage
4. choices - pre defined range of choices a user can enter to this program
"""
parser = argparse.ArgumentParser(
description='Example for one positional arguments',
formatter_class=argparse.ArgumentDefaultsHelpFormatter)
# Adding our first argument player titles of type int
parser.add_argument('titles',
metavar='titles',
type=int,
choices=range(0, 20),
help='GrandSlam Titles')
return parser.parse_args()
# define main
def main(titles):
print(f" *** Player had won {titles} GrandSlam titles.")
if __name__ == '__main__':
args = get_args()
main(args.titles)输出
>>> python test.py 30 usage: test.py [-h] titles test.py: error: argument titles: invalid choice: 30 (choose from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19) <<< python test.py 10 *** Player had won 10 GrandSlam titles. <<< python test.py -1 usage: test.py [-h] titles test.py: error: argument titles: invalid choice: -1 (choose from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19) <<< python test.py 0 *** Player had won 0 GrandSlam titles. <<< python test.py 20 usage: test.py [-h] titles test.py: error: argument titles: invalid choice: 20 (choose from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19)
结论
choices 选项接受一个值列表。如果用户未能提供其中一个值，argparse 将停止程序。
用户必须从 0 到 19 的数字中选择，否则 argparse 将停止并报错。
最后，你也可以创建一个接受字符串选择的程序。
示例
import argparse
def get_args():
""" Function : get_args
parameters used in .add_argument
1. metavar - Provide a hint to the user about the data type.
- By default, all arguments are strings.
2. type - The actual Python data type
- (note the lack of quotes around str)
3. help - A brief description of the parameter for the usage
4. choices - pre defined range of choices a user can enter to this program
"""
parser = argparse.ArgumentParser(
description='Example for one positional arguments',
formatter_class=argparse.ArgumentDefaultsHelpFormatter)
# Adding our first argument player names of type str.
parser.add_argument('player',
metavar='player',
type=str,
choices=['federer', 'nadal', 'djokovic'],
help='Tennis Players')
# Adding our second argument player titles of type int
parser.add_argument('titles',
metavar='titles',
type=int,
choices=range(0, 20),
help='GrandSlam Titles')
return parser.parse_args()
# define main
def main(player,titles):
print(f" *** {player} had won {titles} GrandSlam titles.")
if __name__ == '__main__':
args = get_args()
main(args.player,args.titles)输出
<<< python test.py usage: test.py [-h] player titles test.py: error: the following arguments are required: player, titles <<< python test.py "federer" 30 usage: test.py [-h] player titles test.py: error: argument titles: invalid choice: 30 (choose from 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19) <<< python test.py "murray" 5 usage: test.py [-h] player titles test.py: error: argument player: invalid choice: 'murray' (choose from 'federer', 'nadal', 'djokovic') <<< python test.py "djokovic" 17 *** djokovic had won 17 GrandSlam titles.
更新于: 2020-11-10
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