Skip to content
Switch branches/tags

Name already in use

A tag already exists with the provided branch name. Many Git commands accept both tag and branch names, so creating this branch may cause unexpected behavior. Are you sure you want to create this branch?
Go to file
Cannot retrieve contributors at this time
# Token types
# EOF (end-of-file) token is used to indicate that
# there is no more input left for lexical analysis
class Token(object):
def __init__(self, type, value):
# token type: INTEGER, PLUS, or EOF
self.type = type
# token value: 0, 1, 2. 3, 4, 5, 6, 7, 8, 9, '+', or None
self.value = value
def __str__(self):
"""String representation of the class instance.
Token(INTEGER, 3)
Token(PLUS '+')
return 'Token({type}, {value})'.format(
def __repr__(self):
return self.__str__()
class Interpreter(object):
def __init__(self, text):
# client string input, e.g. "3+5"
self.text = text
# self.pos is an index into self.text
self.pos = 0
# current token instance
self.current_token = None
def error(self):
raise Exception('Error parsing input')
def get_next_token(self):
"""Lexical analyzer (also known as scanner or tokenizer)
This method is responsible for breaking a sentence
apart into tokens. One token at a time.
text = self.text
# is self.pos index past the end of the self.text ?
# if so, then return EOF token because there is no more
# input left to convert into tokens
if self.pos > len(text) - 1:
return Token(EOF, None)
# get a character at the position self.pos and decide
# what token to create based on the single character
current_char = text[self.pos]
# if the character is a digit then convert it to
# integer, create an INTEGER token, increment self.pos
# index to point to the next character after the digit,
# and return the INTEGER token
if current_char.isdigit():
token = Token(INTEGER, int(current_char))
self.pos += 1
return token
if current_char == '+':
token = Token(PLUS, current_char)
self.pos += 1
return token
def eat(self, token_type):
# compare the current token type with the passed token
# type and if they match then "eat" the current token
# and assign the next token to the self.current_token,
# otherwise raise an exception.
if self.current_token.type == token_type:
self.current_token = self.get_next_token()
def expr(self):
# set current token to the first token taken from the input
self.current_token = self.get_next_token()
# we expect the current token to be a single-digit integer
left = self.current_token
# we expect the current token to be a '+' token
op = self.current_token
# we expect the current token to be a single-digit integer
right = self.current_token
# after the above call the self.current_token is set to
# EOF token
# at this point INTEGER PLUS INTEGER sequence of tokens
# has been successfully found and the method can just
# return the result of adding two integers, thus
# effectively interpreting client input
result = left.value + right.value
return result
def main():
while True:
text = input('calc> ')
except EOFError:
if not text:
interpreter = Interpreter(text)
result = interpreter.expr()
if __name__ == '__main__':