Mapytex/expression.py

#!/usr/bin/env python
# encoding: utf-8


class Expression(object):
    """A calculus expression. Today it can andle only expression with numbers later it will be able to manipulate unknown"""

    priority = {"*" : 3, "/": 3, "+": 2, "-":2, "(": 1}

    def __init__(self, exp):
        """ Initiate the expression.

        :param exp: the expression. It can be a string or a list of tokens. It can be infix or postfix expression
        """
        self._tokens = {}
        self._strings = {}
        if type(exp) == list:
            fix = self.get_fix(exp)
            self._tokens[fix] = exp
        elif type(exp) == str:
            tokens = self.parseExp(exp)
            fix = self.get_fix(tokens)
            self._tokens[fix] = tokens
            self._strings[fix] = exp
        else:
            raise ValueError("The expression needs to be a string or a list of token")

    # ---------------------
    # String parsing

    # @classmethod ????
    def parseExp(self, exp):
        """ Parse the expression, ie tranform a string into a list of tokens

        :param exp: The expression (a string)
        :returns: list of token

        """
        return exp.split(" ")

    # ---------------------
    # "fix" recognition

    def get_fix(self, tokens):
        """ Give the "fix" of an expression
        infix -> A + B
        prefix -> + A B
        postfix -> A B +

        :param exp: the expression (list of token)
        :returns: the "fix" (infix, postfix, prefix)

        """
        if tokens[0] in  "+-*/":
            return "prefix"
        elif token[0] not in "+-*/" ans token[1] not in "+-*/":
            return "postfix"
        else:
            return "infix"

    # ----------------------
    # Expressions - tokens getters

    def tokens(self, fix = "infix"):
        """Get the list of tokens with the wanted fix

        :param fix: the fix wanted (infix default)
        :returns: list of tokens

        """
        # Si ce fix n'a pas encore été enregistré
        if fix not in self._tokens:
            # Il peut venir de la version string
            if fix in self._string:
                self._tokens[fix] = self.parseExp(self._string[fix])
            # Sinon il faut le calculer à partir d'une autre forme
            else:
                fix_transfo = "to" + fix.capitalize()
                getattr(self,fix_transfo)()

        return self._tokens[fix]

##### On en est là,il faudra faire attention à bien vérifier ce que les "to..." enregistre (string ou tokens)  

    def string(self, fix = "infix"):
        """Get the string with the wanted fix

        :param fix: the fix wanted (infix default)
        :returns: the string representing the expression

        """
        # Si ce fix n'a pas encore été enregistré
        if fix not in self._string:
            # Il peut venir de la version string
            if fix in self._tokens:
                self._string[fix] = self.parseExp(self._string[fix])
            # Sinon il faut le calculer à partir d'une autre forme
            else:
                fix_transfo = "to" + fix.capitalize()
                getattr(self,fix_transfo)()

        return self._string[fix]
    
    # ----------------------
    # "fix" tranformations

    def toPostfix(self):
        """ Transorm the expression into postfix form using the infix form"""
        pass

    def toInfix(self):
        """ Tranform the expression into infix form using postfix form"""
        pass

    # ---------------------
    # Tools for placing parenthesis in infix notation

    def needPar(operande, operator, posi = "after"):
        """ Says whether or not the operande needs parenthesis

        :param operande: the operande
        :param operator: the operator
        :param posi: "after"(default) if the operande will be after the operator, "before" othewise
        :returns: bollean
        """
        if isNumber(operande) and "-" in operande:
            return 1
        elif not isNumber(operande):
            # Si c'est une grande expression ou un chiffre négatif
            stand_alone = get_main_op(operande)
            # Si la priorité de l'operande est plus faible que celle de l'opérateur
            minor_priority = self.priority[get_main_op(operande)] < self.priority[operator]
            # Si l'opérateur est -/ pour after ou juste / pour before
            special = (operator in "-/" and posi == "after") or (operator in "/" and posi == "before")

            return stand_alone and (minor_priority or special)
        else:
            return 0
    
# J'aime pas bien cette endroit faudrait que ce soit une méthode qui s'applique uniquement à l'expression en question (self) pas à n'importe quel string, ça serait plus propre.
    def get_main_op(exp):
        """ Gives the main operation of the expression

        :param exp: the expression
        :returns: the main operation (+, -, * or /) or 0 if the expression is only one element

        """
        parStack = Stack()
        tokenList = exp.split(" ")

        if len(tokenList) == 1:
        # Si l'expression n'est qu'un élément
            return 0

        main_op = []

        for token in tokenList:
            if token == "(":
                parStack.push(token)
            elif token == ")":
                parStack.pop()
            elif token in "+-*/" and parStack.isEmpty():
                main_op.append(token)

        return min(main_op, key = lambda s: priority[s])

    # ---------------------
    # Computing the expression

    def compute(self):
        """ Recursive method for computing as a student the expression
        :returns: list of steps needed to reach the result

        """
        pass
    
    def doMath(op, op1, op2):
        """Compute "op1 op op2"

        :param op: operator
        :param op1: first operande
        :param op2: second operande
        :returns: string representing the result

        """
        return str(eval(op1 + op + op2))

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