标签归档：comma

知识问答

为什么列表中允许尾随逗号？

问题：为什么列表中允许尾随逗号？

我很好奇为什么在Python中列表中的尾部逗号是有效的语法，并且似乎Python只是忽略了它：

>>> ['a','b',]
['a', 'b']

自从('a')和('a',)是两个元组时，它是一个有意义的元组，但是在列表中呢？

点击查看英文原文

I am curious why in Python a trailing comma in a list is valid syntax, and it seems that Python simply ignores it:

>>> ['a','b',]
['a', 'b']

It makes sense when its a tuple since ('a') and ('a',) are two different things, but in lists?

回答 0

主要优点是，它使多行列表更易于编辑，并减少了差异。

变更：

s = ['manny',
     'mo',
     'jack',
]

至：

s = ['manny',
     'mo',
     'jack',
     'roger',
]

仅涉及差异的单行更改：

  s = ['manny',
       'mo',
       'jack',
+      'roger',
  ]

当省略尾部逗号时，这击败了更令人困惑的多行差异：

  s = ['manny',
       'mo',
-      'jack'
+      'jack',
+      'roger'
  ]

后者的差异使得很难看到仅添加了一行，而另一行没有更改内容。

它还降低了这样做的风险：

s = ['manny',
     'mo',
     'jack'
     'roger'  # Added this line, but forgot to add a comma on the previous line
]

并触发隐式字符串文字串联，产生s = ['manny', 'mo', 'jackroger']而不是预期的结果。

点击查看英文原文

The main advantages are that it makes multi-line lists easier to edit and that it reduces clutter in diffs.

Changing:

s = ['manny',
     'mo',
     'jack',
]

to:

s = ['manny',
     'mo',
     'jack',
     'roger',
]

involves only a one-line change in the diff:

  s = ['manny',
       'mo',
       'jack',
+      'roger',
  ]

This beats the more confusing multi-line diff when the trailing comma was omitted:

  s = ['manny',
       'mo',
-      'jack'
+      'jack',
+      'roger'
  ]

The latter diff makes it harder to see that only one line was added and that the other line didn’t change content.

It also reduces the risk of doing this:

s = ['manny',
     'mo',
     'jack'
     'roger'  # Added this line, but forgot to add a comma on the previous line
]

and triggering implicit string literal concatenation, producing s = ['manny', 'mo', 'jackroger'] instead of the intended result.

回答 1

这是一种常见的语法约定，允许在数组中尾随逗号，C和Java之类的语言都允许，并且Python似乎已对其列表数据结构采用了该约定。在生成用于填充列表的代码时，它特别有用：只需生成一系列元素和逗号，而无需将最后一个元素和逗号视为特殊情况，并且不应该在末尾加逗号。

点击查看英文原文

It’s a common syntactical convention to allow trailing commas in an array, languages like C and Java allow it, and Python seems to have adopted this convention for its list data structure. It’s particularly useful when generating code for populating a list: just generate a sequence of elements and commas, no need to consider the last one as a special case that shouldn’t have a comma at the end.

回答 2

它有助于消除某种错误。有时在多行上写列表会更清晰。但是在以后的维护中，您可能需要重新排列项目。

l1 = [
        1,
        2,
        3,
        4,
        5
]

# Now you want to rearrange

l1 = [
        1,
        2,
        3,
        5
        4,
]

# Now you have an error

但是，如果允许使用尾随逗号，则可以轻松地重新排列行而不会引起错误。

点击查看英文原文

It helps to eliminate a certain kind of bug. It’s sometimes clearer to write lists on multiple lines. But in, later maintenace you may want to rearrange the items.

l1 = [
        1,
        2,
        3,
        4,
        5
]

# Now you want to rearrange

l1 = [
        1,
        2,
        3,
        5
        4,
]

# Now you have an error

But if you allow trailing commas, and use them, you can easily rearrange the lines without introducing an error.

回答 3

元组的不同之处在于，('a')它使用隐式连续和()s作为优先运算符进行扩展，而('a',)引用长度为1的元组。

你原来的例子是 tuple('a')

点击查看英文原文

A tuple is different because ('a') is expanded using implicit continuation and ()s as a precendence operator, whereas ('a',) refers to a length 1 tuple.

Your original example would have been tuple('a')

回答 4

主要原因是使diff变得不那么复杂。例如，您有一个列表：

list = [
    'a',
    'b',
    'c'
]

并且您想要向其中添加另一个元素。然后，您将最终执行此操作：

list = [
    'a',
    'b',
    'c',
    'd'
]

因此，diff将显示出两行已更改，首先在’c’处添加’，’，在最后一行添加’d’。

因此，python允许在列表的最后一个元素中尾部加上’，’，以防止可能引起混淆的额外差异。

点击查看英文原文

The main reason is to make diff less complicated. For example you have a list :

list = [
    'a',
    'b',
    'c'
]

and you want to add another element to it. Then you will be end up doing this:

list = [
    'a',
    'b',
    'c',
    'd'
]

thus, diff will show that two lines have been changed, first adding ‘,’ in line with ‘c’ and adding ‘d’ at last line.

So, python allows trailing ‘,’ in last element of list, to prevent extra diff which can cause confusion.