Question

if city == 'Chennai':
   print "this is Chennai city"
elif city == 'Delhi':
   print "this is Delhi city"
else:
   print "invalid city"

我想在城市无效时提出ValueError。怎么做？

Answer 1

city = 'some city'

if city == 'Pune':
    print "this is pune city"
elif city == 'Delhi':
    print "this is Delhi city"
else:
    raise ValueError("Invalid City")

Traceback (most recent call last):
  File "<pyshell#2>", line 8, in <module>
    raise ValueError('Invalid City')
ValueError: Invalid City

Answer 2

我稍微重构了您的代码，但它显示了如何引发ValueError：

def validate_city(city):
    city = city.title()  # Capitalize properly
    # use set(["Pune", "Delhi"]) for backwards-compatibility
    valid_cities = {"Pune", "Delhi"}
    if city in valid_cities:
        print "This is %s city" % city
    else:
        raise ValueError("Invalid city %s" % repr(city))

示例输出：

>>> validate_city("gotham city")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in validate_city
ValueError: Invalid city 'Gotham City'
>>> validate_city("delhi")
This is Delhi city

注意：

用于查找有效城市的集合，比许多if语句行更容易维护。
一套是faster for lookups than a list。
title()方法用于正确地将城市名称大写。在验证用户输入之前规范化用户输入是一种很好的做法。即将“dehli”变成“Dehli”，将“new york”变成“New York”等等。

Answer 3

这取决于您希望如何处理这种情况。但是，您将使用命令

raise ValueError

在您确定这是一个无效城市的时候

validcities = ('Pune', 'Delhi', 'Any other valid city')

try:
  if city not in validcities:
    raise ValueError
  print 'This is', city, 'city'
except ValueError:
  print city, ' is an invalid city'
  # perform other invalid city here

为多个if-else引发ValueError

3 个答案: