如何使用逗号有效地加入列表并添加“和".在最后一个元素之前

问题描述

我一直在经历自动化服装，并且遇到了一个名为逗号代码"的挑战(第4章).您必须编写一个函数，该函数需要一个列表并打印出一个字符串，用逗号将元素连接在一起，并在最后一个元素之前添加"and".

I've been going through Automatetheboringstuff and came across a challenge called Comma Code (end of chapter 4). You have to write a function that takes a list and print out a string, joining the elements with a comma and adding "and" before the last element.

请记住，我对python还是一个新手，或者对它进行编程，它仍然是可管理的任务，但是输出在插入的和"之前带有逗号.因此，我修改了代码以进行清理.这是我的代码:

Keeping in mind that I am fairly new to python, or programing for that matter, it was still a manageable task, but the output had a comma before the inserted "and". So I revised the code to clean that up. This is my code:

def comma_separator(someList):
    """The function comma_separator takes a list and joins it
       into a string with (", ") and adds " and " before the last value."""

    if type(someList) is list and bool(someList) is True:
        return ", ".join(someList[:-1]) + " and " + someList[-1]
    else:
        print("Pass a non-empty list as the argument.")

有更好的方法吗?是否有可以做到这一点的模块?

Is there a better way to do it? Is there a module that can do this?

推荐答案

您必须考虑只有一个元素的情况:

You'll have to account for the case where you have just the one element:

def comma_separator(sequence):
    if not sequence:
        return ''
    if len(sequence) == 1:
        return sequence[0]
    return '{} and {}'.format(', '.join(sequence[:-1]), sequence[-1])

请注意，bool(sequence) is True是测试非空列表的一种非常复杂的方法.只需使用if sequence:就足够了，因为if语句已查找布尔真值 .

Note that bool(sequence) is True is a very elaborate way of testing for a non-empty list; simply using if sequence: is enough as the if statement looks for boolean truth already.

可以说，用序列以外的任何东西(可以被索引并且具有长度的东西)调用该函数只会导致异常.通常，您不测试此类函数中的类型.如果您必须测试类型，请使用isinstance(sequence, list)至少允许有子类.

Arguably, calling the function with anything other than a sequence (something that can be indexed and has a length) should just result in an exception. You generally do not test for types in functions like these. If you did have to test for a type, use isinstance(sequence, list) to at least allow for subclasses.

我也将传递空白列表设为错误.您可以将该异常转换为ValueError:

I'd also make it an error to pass in an empty list. You could turn that exception into a ValueError:

def comma_separator(sequence):
    if len(sequence) > 1:
        return '{} and {}'.format(', '.join(sequence[:-1]), sequence[-1])
    try:
        return sequence[0]
    except IndexError:
        raise ValueError('Must pass in at least one element')

后者的演示

>>> def comma_separator(sequence):
...     if len(sequence) > 1:
...         return '{} and {}'.format(', '.join(sequence[:-1]), sequence[-1])
...     try:
...         return sequence[0]
...     except IndexError:
...         raise ValueError('Must pass in at least one element')
... 
>>> comma_separator(['foo', 'bar', 'baz'])
'foo, bar and baz'
>>> comma_separator(['foo', 'bar'])
'foo and bar'
>>> comma_separator(['foo'])
'foo'
>>> comma_separator([])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 7, in comma_separator
ValueError: Must pass in at least one element

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