Python中没有多行Lambda：为什么不呢？

向右看齐 2023-03-03 08:42 3阅读 0赞

本文翻译自：[No Multiline Lambda in Python: Why not?][No Multiline Lambda in Python_ Why not]

*I've heard it said that multiline lambdas can't be added in Python because they would clash syntactically with the other syntax constructs in Python.* **我听说它说不能在Python中添加多行lambda，因为它们会在语法上与Python中的其他语法结构发生冲突。** *I was thinking about this on the bus today and realized I couldn't think of a single Python construct that multiline lambdas clash with.* **我今天在公共汽车上考虑这个问题，意识到我想不出一个与多行lambda冲突的Python结构。** *Given that I know the language pretty well, this surprised me.* **考虑到我非常了解该语言，这令我感到惊讶。**

*Now, I'm sure Guido had a reason for not including multiline lambdas in the language, but out of curiosity: what's a situation where including a multiline lambda would be ambiguous?* **现在，我确定Guido有一个理由不在语言中包含多行lambda，而是出于好奇：在什么情况下包含多行lambda会导致歧义？** *Is what I've heard true, or is there some other reason that Python doesn't allow multiline lambdas?* **我所听到的是对的，还是Python不允许多行lambda的其他原因？**

--------------------

### \#1楼 ###

参考：[https://stackoom.com/question/5aSk/Python中没有多行Lambda-为什么不呢][https_stackoom.com_question_5aSk_Python_Lambda-]

--------------------

### \#2楼 ###

*Look at the following:* **请看以下内容：**

map(multilambda x:
          y=x+1
          return y
       , [1,2,3])

*Is this a lambda returning `(y, [1,2,3])` (thus map only gets one parameter, resulting in an error)?* **这是lambda返回`(y, [1,2,3])` （因此映射仅获取一个参数，从而导致错误）吗？** *Or does it return `y` ?* **还是返回`y` ？** *Or is it a syntax error, because the comma on the new line is misplaced?* **还是语法错误，因为新行上的逗号放置不正确？** *How would Python know what you want?* **Python如何知道您想要什么？**

*Within the parens, indentation doesn't matter to python, so you can't unambiguously work with multilines.* **在括号内，缩进对于python并不重要，因此您不能明确地使用多行。**

*This is just a simple one, there's probably more examples.* **这只是一个简单的例子，可能还有更多示例。**

--------------------

### \#3楼 ###

*Guido van Rossum (the inventor of Python) answers this exact question himself in [an old blog post][] .* **Guido van Rossum（Python的发明者）自己在[一个旧博客文章中][an old blog post]回答了这个确切的问题。**  
*Basically, he admits that it's theoretically possible, but that any proposed solution would be un-Pythonic:* **基本上，他承认这在理论上是可行的，但是任何建议的解决方案都是非Python的：**

> *"But the complexity of any proposed solution for this puzzle is immense, to me: it requires the parser (or more precisely, the lexer) to be able to switch back and forth between indent-sensitive and indent-insensitive modes, keeping a stack of previous modes and indentation level. Technically that can all be solved (there's already a stack of indentation levels that could be generalized). But none of that takes away my gut feeling that it is all an elaborate [Rube Goldberg contraption][] ."* **“但是，对于我来说，任何提议的解决方案的复杂性对我来说都是巨大的：它要求解析器（或更确切地说是词法分析器）能够在缩进敏感模式和缩进不敏感模式之间来回切换，并保持堆栈技术上可以解决所有问题（已经有可以概括的缩进级别堆栈了。）但是，这些都不让我直觉，这完全是精心制作的[Rube Goldberg装置][Rube Goldberg contraption] 。”**

--------------------

### \#4楼 ###

*A couple of relevant links:* **一些相关链接：**

*For a while, I was following the development of Reia, which was initially going to have Python's indentation based syntax with Ruby blocks too, all on top of Erlang.* **一段时间以来，我一直在关注Reia的开发，该开发最初也将基于Python的基于缩进的语法以及Ruby块（全部基于Erlang）。** *But, the designer wound up giving up on indentation sensitivity, and this post he wrote about that decision includes a discussion about problems he ran into with indentation + multi-line blocks, and an increased appreciation he gained for Guido's design issues/decisions:* **但是，设计师最终放弃了缩进敏感性，他写的有关该决定的文章包括有关他在缩进+多行块中遇到的问题的讨论，以及他对Guido的设计问题/决定的越来越多的赞赏：**

*[http://www.unlimitednovelty.com/2009/03/indentation-sensitivity-post-mortem.html][http_www.unlimitednovelty.com_2009_03_indentation-sensitivity-post-mortem.html]* **[http://www.unlimitednovelty.com/2009/03/indentation-sensitiveivity-post-mortem.html][http_www.unlimitednovelty.com_2009_03_indentation-sensitivity-post-mortem.html]**

*Also, here's an interesting proposal for Ruby-style blocks in Python I ran across where Guido posts a response w/o actually shooting it down (not sure whether there has been any subsequent shoot down, though):* **另外，这是关于Python中Ruby样式块的一个有趣的建议，我遇到了Guido在哪里发布了一个响应，但没有将其实际击落（尽管不确定是否有任何后续击落）：**

*[http://tav.espians.com/ruby-style-blocks-in-python.html][http_tav.espians.com_ruby-style-blocks-in-python.html]* **[http://tav.espians.com/ruby-style-blocks-in-python.html][http_tav.espians.com_ruby-style-blocks-in-python.html]**

--------------------

### \#5楼 ###

*This is generally very ugly (but sometimes the alternatives are even more ugly), so a workaround is to make a braces expression:* **通常这很丑陋（但有时替代方法甚至更丑陋），因此一种解决方法是制作一个大括号表达式：**

lambda: (
        doFoo('abc'),
        doBar(123),
        doBaz())

*It won't accept any assignments though, so you'll have to prepare data beforehand.* **不过，它不会接受任何分配，因此您必须事先准备数据。** *The place I found this useful is the PySide wrapper, where you sometimes have short callbacks.* **我发现这个有用的地方是PySide包装器，有时在其中有简短的回调。** *Writing additional member functions would be even more ugly.* **编写其他成员函数将更加难看。** *Normally you won't need this.* **通常，您不需要这个。**

*Example:* **例：**

pushButtonShowDialog.clicked.connect(
        lambda: (
        field1.clear(),
        spinBox1.setValue(0),
        diag.show())

--------------------

### \#6楼 ###

*Let me try to tackle @balpha parsing problem.* **让我尝试解决@balpha解析问题。** *I would use parentheses around the multiline lamda.* **我会在多行lamda周围使用括号。** *If there is no parentheses, the lambda definition is greedy.* **如果没有括号，则lambda定义为贪婪的。** *So the lambda in* **所以lambda在**

map(lambda x:
          y = x+1
          z = x-1
          y*z,
        [1,2,3]))

*returns a function that returns `(y*z, [1,2,3])`* **返回返回`(y*z, [1,2,3])`的函数**

*But* **但**

map((lambda x:
          y = x+1
          z = x-1
          y*z)
        ,[1,2,3]))

*means* **手段**

map(func, [1,2,3])

*where func is the multiline lambda that return y\*z.* **其中func是返回y \* z的多行lambda。** *Does that work?* **那样有用吗？**

[No Multiline Lambda in Python_ Why not]: https://oldbug.net/q/5aSk/No-Multiline-Lambda-in-Python-Why-not
[https_stackoom.com_question_5aSk_Python_Lambda-]: https://stackoom.com/question/5aSk/Python%E4%B8%AD%E6%B2%A1%E6%9C%89%E5%A4%9A%E8%A1%8CLambda-%E4%B8%BA%E4%BB%80%E4%B9%88%E4%B8%8D%E5%91%A2
[an old blog post]: http://www.artima.com/weblogs/viewpost.jsp?thread=147358
[Rube Goldberg contraption]: http://en.wikipedia.org/wiki/Rube_Goldberg_Machine
[http_www.unlimitednovelty.com_2009_03_indentation-sensitivity-post-mortem.html]: http://www.unlimitednovelty.com/2009/03/indentation-sensitivity-post-mortem.html
[http_tav.espians.com_ruby-style-blocks-in-python.html]: http://tav.espians.com/ruby-style-blocks-in-python.html