Parse Mathematical Expressions With Pyparsing

I'm trying to parse a mathematical expression using pyparsing. I know i could just copy the example calculator from pyparsing site, but i want to understand it so i can add to it l

Solution 1:

The actual name for this parsing problem is "infix notation" (and in recent versions of pyparsing, I am renaming operatorPrecedence to infixNotation). To see the typical implementation of infix notation parsing, look at the fourFn.py example on the pyparsing wiki. There you will see an implementation of this simplified BNF to implement 4-function arithmetic, with precedence of operations:

operand :: integer or real number
factor :: operand | '('expr')'
term :: factor ( ('*' | '/') factor )*
expr :: term ( ('+' | '-') term )*

So an expression is one or more terms separated by addition or subtraction operations.

A term is one or more factors separated by multiplication or division operations.

A factor is either a lowest-level operand (in this case, just integers or reals), OR an expr enclosed in ()'s.

Note that this is a recursive parser, since factor is used indirectly in the definition of expr, but expr is also used to define factor.

In pyparsing, this looks roughly like this (assuming that integer and real have already been defined):

LPAR,RPAR = map(Suppress, '()')
expr = Forward()
operand = real | integerfactor = operand | Group(LPAR + expr + RPAR)
term = factor + ZeroOrMore( oneOf('* /') + factor )
expr <<= term + ZeroOrMore( oneOf('+ -') + term )

Now using expr, you can parse any of these:

3
3+2
3+2*4
(3+2)*4

The infixNotation pyparsing helper method takes care of all the recursive definitions and groupings, and lets you define this as:

expr = infixNotation(operand,
        [
        (oneOf('* /'), 2, opAssoc.LEFT),
        (oneOf('+ -'), 2, opAssoc.LEFT),
        ])

But this obscures all the underlying theory, so if you are trying to understand how this is implemented, look at the raw solution in fourFn.py.