如何在Django中立即向ManyToMany关系添加多个对象？(How to add multiple objects to ManyToMany relationship at once in Django ?)

 首先，语法用于从表达式构造解析树。 所以如果你已经有一个解析树，你不需要语法。 
 
 根据您的解析器的工作量，通过解析表达式形成的结果树可能已经是抽象语法树。 或者它可以是一个简单的解析树，需要二次构建ast。 
 
 要从语法和表达式构造解析树，您首先必须将您的语法转换为工作代码。 通常，您将工作分解为将代表表达式的输入流分解为令牌列表的令牌程序，以及一个解析器，它从令牌中获取令牌列表并构建解析树\ ast。 
 
 所以表达式1 + 2*(3+4)可能被分成一个这样的令牌列表： 
 
1 - int
+ - add_operator
2 - int
* - mul_operator
( - lparen
3 - int
+ - add_operator
4 - int
) - rparen
 
 第一列是实际的文本值。 第二个代表令牌类型。 这些令牌被输入到从你的语法构建的解析器中，并识别令牌并构建解析树。 
 
 那么，如何编写词法分析器和实际的解析器呢？ 你可以手工滚动自己。 或者更常见的是使用像Coc或antlr或lex / yacc这样的解析器生成器。 这些工具对您的语法进行了描述，并生成了一个tokenzier和解析器的代码。 （代码生成器存在于大多数流行语言和一些不受欢迎的语言中。） 
 
 构建解析器的方式在很大程度上取决于您使用的语言。 在Haskell中写一个解析器的方法与你在C中的说法完全不同 
 
 
 
 这是一个教程，向您展示如何构建自己的递归下降解析器 。 
 
 
 Coco是用于各种语言的解析器生成器，它还附带了有关如何开始的文档。 
 
 
 如果你的Python是你的东西，那么你可能会想到 Python。 
 

Well, first off, the grammar is used to construct a parse tree from an expression. So if you already have a parse tree, you don't need the grammar.
 
Depending on how much work your parser does, the resulting tree that is formed from parsing an expression could already be an abstract syntax tree. Or it could be a simple parse tree which requires a second pass to construct the ast.
 
To construct the parse tree from a grammar and an expression, you would first have to convert your grammar into working code. Typically, you would split the work into a tokenizer which splits the input stream representing the expression into a list of tokens, and a parser which takes the list of tokens and constructs a parse tree\ast from it.
 
So the expression 1 + 2*(3+4) might be split into a list of tokens like this:
 
1 - int
+ - add_operator
2 - int
* - mul_operator
( - lparen
3 - int
+ - add_operator
4 - int
) - rparen
 
The first column is the actual text value. The second represents the token type. These tokens are fed into the parser, which is built from your grammar and recognizes the tokens and builds the parse tree.
 
So, how does one write the lexical tokenizer and the actual parser? You could roll your own by hand. Or, more commonly, use a parser generator like coco or antlr or lex/yacc. These tools take a description of your grammar and generate the code for a tokenzier and parser. (Code generators exist for most popular languages and some unpopular ones as well.)
 
How you construct your parser depends heavily on what language you use. How you would write a parser in Haskell is entirely different from how you would in, say, C.
 
 
 
Here's a tutorial that shows you how to build your own recursive descent parser.
 
 
Coco is a parser generator for various languages, which also comes with documentation on how to get started.
 
 
If Python's your thing, then pyparsing maybe for you.