Pratt Parser

This code implements a Python version of the Pratt Parser algorithm.

It is a variation of the code presented in Nathan Vaughn

In this version:

• parser support more math functions (log, pow, sqrt, sin, cos, tan, max, and min)
• resolve arbitrary variables like in a + b / 3 * sin(2 * 3.14), where a and b are assigned in the parser runtime.

Usage

parser = Parser()
print(parser.parse('1+2 * (pow(3,2) * sqrt(9))'))

>>> 81

When resolving variables, initialize the parser with a proper runtime:

runtime = {
    'a': 1,
    'b': 2,
    'c': 3
}

parser = Parser(runtime)
print(parser.parse('a+b * (pow(c,b) * sqrt(9))'))

>>> 81

Extension

To add support to new math functions:

1. add the new function to tokenizer.FUNCTION_LIST
2. edit prattparser.Parser.fn to resolve the new function add

In case the new function has more than 2 arguments, the grammar must be edited accordingly in the function prattparser.Parser.fn_arg_expression

Roadmap

• add support to dynamic variables, e.g., a + 1, where a is an arbitrary function calling a DB:

  def a ():
    db = connect()
    value = db.query(`select count(*) from my_table`)
    return float(value)

• add support to resolve dependent dynamic variables and identify cycles, e.g., a + b + 1:

def a():
    return 3 + b()

def b():
    return 42 + a()

>>> Exception: Cycle identifyed when resolving variable [a]

• generate AST for lazy evaluation