python - Python argparse默认值不起作用

Question

我正在尝试使用 argparse，程序可以工作，但默认值不起作用。这是我的代码：

'''This simple program helps you in understanding how to feed the user input from command line and to show help on passing invalid argument.'''

import argparse
import sys

def calc(args):
    #Enable variables within the function to take on values from 'args' object. 
    operation = args.operation
    x = args.x
    y = args.y

    if (operation == "add"):
        return x + y
    elif (operation == "sub"):
        return x - y

parser = argparse.ArgumentParser(description="This is a summing program")   #parser is an object of the class Argument Parser.
parser.add_argument("x", type=float, default=1.0, help="What is the first number?") #add_argument is a method of the class ArgumentParser.
parser.add_argument("y", type=float, default=1.0, help='What is the second number?')
parser.add_argument("operation", type=str, default="add", help='What operation? Can choose add, sub, mul, or div')
args = parser.parse_args()
print(str(calc(args)))

这个简单的程序工作，但是尝试在没有值的情况下调用它会返回以下错误：

usage: cmdline.py [-h] x y operation
cmdline.py: error: the following arguments are required: x, y, operation

我哪里错了？

标签： pythonargparse

Answer 1

你失踪了nargs='?'。以下作品：

import argparse
import sys 

def calc(args):
    #Enable variables within the function to take on values from 'args' object. 
    operation = args.operation
    x = args.x
    y = args.y

    if (operation == "add"):
        return x + y 
    elif (operation == "sub"):
        return x - y 

parser = argparse.ArgumentParser(description="This is a summing program")   #parser is an object of the class Argument Parser.
parser.add_argument("x", nargs='?', type=float, default=1.0, help="What is the first number?") #add_argument is a method of the class ArgumentParser.
parser.add_argument("y", nargs='?', type=float, default=1.0, help='What is the second number?')
parser.add_argument("operation", nargs='?', type=str, default="add", help='What operation? Can choose add, sub, mul, or div')
args = parser.parse_args()
print(str(calc(args)))