Question

我正在研究一种简单的DSL来转换从MongoDB中提取的数据。我正在使用python和pyparsing，并且在创建一个适用于基本运算符（如+ / - *）的语法方面已经相当远，从提供的示例开始。我目前仍然坚持如何让我的程序来评估Rank[dbRef]形式的函数。我可以通过简单的运算符来评估并对dbRef进行算术运算，但是在评估函数时，我的递归不起作用。我无法弄清楚如何访问函数调用中传递的dbRef参数。

这是语法和相关的setParseActions：

# Define parser, accounting for the fact that some fields contain whitespace
chars = Word(alphanums + "_-/")
expr = Forward()
integer = Word(nums).setParseAction(EvalConstant)
real = Combine(Word(nums) + "." + Word(nums)).setParseAction(EvalConstant)

# Handle database field references that are coming out of Mongo
dbRef = Combine(chars + OneOrMore(":") + chars)

dbRef.setParseAction(EvalDBref)

# Handle function calls
functionCall = (Keyword("Rank") | Keyword("ZS") | Keyword("Ntile")) + "[" + dbRef + "]"
functionCall.setParseAction(EvalFunction)
operand = (real | integer) | functionCall | dbRef 

signop = oneOf('+ -')
multop = oneOf('* /')
plusop = oneOf('+ -')

# Use parse actions to attach Eval constructors to sub-expressions

expr << operatorPrecedence(operand,
    [
     (signop, 1, opAssoc.RIGHT, EvalSignOp),
     (multop, 2, opAssoc.LEFT, EvalMultOp),
     (plusop, 2, opAssoc.LEFT, EvalAddOp),
    ])

formulas = ['Rank[Person:Height]']

for f in formulas:
    ret = expr.parseString(f)[0]
    print p + ": " + line + " --> " + str(ret.eval())

以下是我的评估课的相关代码。类DOES由解析器调用，但是如何访问传递给函数的参数？

# Executes functions contained in expressions
class EvalFunction(object): 
    def __init__(self, tokens): 
        self.value = tokens[0]
    def eval(self):
        func = self.value
        if func == 'Rank':
            # How to evaluate the token that is arg of Function?
            return 'Rank Found';

我想我只需要朝着正确的方向轻推即可进入下一阶段......

Answer 1

我对此进行了整理并希望提供答案。我的函数评估类如下所示：

# Executes functions contained in expressions
class EvalFunction(object):
  pop_ = {}
  def __init__(self, tokens):
    self.func_ = tokens.funcname
    self.field_ = tokens.arg
  def eval(self):
      # Get the name of the requested field and source db
      # Functions can only be called on dbRef so this always done
      v = self.field_.value
      fieldRef = v.split(':')
      source = fieldRef[0]
      field = fieldRef[1]

      # Evaluate the dbRef (get the value from the db)
      val = self.field_.eval()

      if self.func_ == 'Avg':
        rec = db['Stats'].find_one({'_id' : field})   
        return rec['value']['avg']
      elif self.func_ == 'Root':
          return math.sqrt(val)

我的函数语法是：

functionCall = funcNames("funcname") + "[" + dbRef("arg") + "]"
functionCall.setParseAction(EvalFunction)

评估DSL算术语法中的函数

1 个答案: