Code optimization using DAG using python

filename: b4.py

from sys import argv

mp = {}
list_statement = []
replace = {}
not_used = []

class ExternalNode():
    variable_identifier = ''
    def __init__(self, identifier):
        self.variable_identifier = identifier

class InternalNode():
    variable_identifier = ''
    operator_value = ''
    lchild = None
    rchild = None
    def __init__(self, val, l, r, id):
        self.operator_value = val
        self.lchild = l
        self.rchild = r
        self.variable_identifier = id

def read_input(file_name):
    file_var = open(file_name, 'r').read().strip().split("\n")
    for line in file_var:
        if len(line)>=6 and line[:6] == "return":
            break
        line = line.split(":=")
        value = line[0].strip()
        line [1] = line[1].strip().split(" ")
        first = line[1][0]
        operator = line[1][1]
        second = line[1][2]
        if (operator == '=') or (not first in mp):
            temp_var = ExternalNode(first)
            mp[first] = temp_var
            replace[first] = first
        if not second in mp:
            temp_var = ExternalNode(second)
            mp[second] = temp_var
            replace[second] = second

        list_statement.append(value)
        var = InternalNode(operator, mp[first], mp[second], value)
        mp[value] = var
        replace[value] = value

def print_code():
    for st in list_statement:
        if not st in not_used:
            print mp[st].variable_identifier + " = " + replace[mp[st].lchild.variable_identifier] + " " + mp[st].operator_value + " " + replace[mp[st].rchild.variable_identifier]

def optimise():
    for i in range(len(list_statement)):
        for j in range(i, len(list_statement)):
            st = list_statement[i]
            st2 = list_statement[j]
            if mp[st].lchild == mp[st2].lchild and mp[st].rchild == mp[st2].rchild and mp[st].operator_value == mp[st2].operator_value:
                replace[st2] = replace[st]
                if st2 != st:
                    not_used.append(st2)

if __name__ == "__main__":
    file_name = "intermediate_code"
    if len(argv) == 2:
        file_name = argv[1]
    read_input(file_name)
    optimise()
    print_code()

filename: intermediate_code

t1 := a + b
t2 := c * t1
t3 := a + b
t4 := d * t3
t5 := t2 + t4
t6 := r = t5
t7 := t3 + a
t0 := a = t4
t8 := a + b
t9 := a + b
t10 := t9 + a
t11 := a = l
t12 := a + b
t13 := a + b
t14 := j + t13


OUTPUT

ccpvg@ccpvg-HP-Compaq-4000-Pro-SFF-PC:~$python b4.py
t1 = a + b
t2 = c * t1
t4 = d * t1
t5 = t2 + t4
t6 = r = t5
t7 = t1 + a
t0 = a = t4
t8 = a + b
t10 = t8 + a
t11 = a = l
t12 = a + b
t14 = j + t12