my_scanner.py

#to know the place of the character you will deal with it next
index = 0

#reading from file (input.txt)
fileHandler = open('input.txt','r')
input = fileHandler.read()

#writing to file (output.txt)
outFile = open('output.txt','w')
#function declaration that we will use it in parser
#def getToken (input):
#global index

#declaring our states , special symbols and reserved words in the DFA
states = ['start', 'inComment', 'inDigit', 'inLetter', 'inAssign', 'done']
specialSymbols = {'+' : 'plus', '-' : 'minus', '*' : 'multiply', '/' : 'division', '=' : 'equal', '<' : 'less than', '(' : 'openning bracket', ')' : 'closing bracket', ';' : 'semi colon'}
reservedWords = ['if','then','else','end','repeat','until','read','write']

while index < len(input):
    #defining my current state and the next one
    currentState = 'start'
    nextState = ''
    change = False

    #variable that will carry the value of the token
    token = ''
    tokenType = ''

    #the DFA process converts to code
    while currentState != 'done':
        if index >= len(input):
            break
        character = input[index]
        change = False
        #case of assign :=
        if currentState == 'start' and character == ':':
            nextState = 'inAssign'
            change = True
        elif currentState == 'inAssign' and character == '=':
            token = ':='
            tokenType = 'assign'
            nextState = 'done'
            change = True
        #case of having letter
        elif currentState == 'start' and character.isalpha():
            nextState = 'inLetter'
            change = True
        elif currentState == 'inLetter' and not(character.isalpha()):
            nextState = 'done'
            change = True
            tokenType = 'letter'
        #case of having numbers
        elif currentState == 'start' and character.isdigit():
            nextState = 'inDigit'
            change = True
        elif currentState == 'inDigit' and not(character.isdigit()):
            nextState = 'done'
            change = True
            tokenType = 'Number'
        #case of special symbols
        elif currentState == 'start' and (character == '+' or character == '-' or character == '/' or character == '*' or character == ';' or character == '(' or character == ')' or character == '=' or character == '<'):
            nextState = 'done'
            change = True
            token = character
            tokenType = 'specialSymbol'
        #handling comments by ignoring them
        elif currentState == 'start' and  character == '{':
            nextState = 'inComment'
            change = True
        elif currentState == 'inComment' and character == '}':
            nextState = 'start'
            change = True
        #handle the delimiter character
        if (nextState == 'done') and (currentState == 'inLetter' or currentState == 'inDigit'):
            index = index -1
        #getting the next character
        index = index + 1
        #if we have transition from one state to another
        if change:
            currentState = nextState
            nextState = ''
        #accumelating the token in case of set of letters and numbers
        if nextState == 'inLetter' or currentState == 'inLetter' or nextState == 'inDigit' or currentState == 'inDigit':
            token = token + character


    #another checks for special symbols , reserved words and identifiers
    if tokenType == 'letter':
        if token in reservedWords:
            tokenType = 'reserved word'
        else:
            tokenType = 'identifier'
    elif tokenType == 'specialSymbol':
        tokenType = specialSymbols[token]
    #handling case of empty token
    if token != '':
        outFile.write(token + ' , ' + tokenType + '\n')