A simpler than pycalc1 calculator written in Python3.

pycalc2.py

#!/usr/bin/env python3
#
# Licensed under the MIT License.
#
# Copyright (c) 2022 Benjamin Stürz <[email protected]>
# 
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# 
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
# 
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
#
# This python3 script implements a basic calculator.
# It is a simpler version of pycalc1.
# which used a combination of lexing and parsing the input before evaluating it.
# This is a much simpler version that evaluates the input directly.

enable_color_output = True

#--------------------------------------------------------------------------------
# Input reading basics.

def read():
    if pos >= len(line):
        return '\0'
    return line[pos]

def peek():
    global pos
    while read().isspace():
        pos += 1
    return read()

def next():
    global pos
    ch = peek()
    pos += 1
    return ch

def matches(x):
    return peek() == x

def match(x):
    global pos
    b = matches(x)
    if b: pos += 1
    return b

def expect(x):
    if not match(x):
        error(f"Expected '{x}', got '{peek()}'")

#--------------------------------------------------------------------------------
# Error handling.

def asColorStr(text, color):
    if enable_color_output:
        return f"\033[{color}m{text}\033[0m"
    else:
        return text

def error(text):
    print((' ' * (pos + 2)) + asColorStr('^: ' + text, 31))
    raise RuntimeError("Error")

#--------------------------------------------------------------------------------
# Parsing & Evaluation of expressions.

# This parses basic integers and sub-expressions.
def factor():
    if match('('):
        val = expression()
        expect(')')
    elif peek().isdigit():
        val = 0
        while peek().isdigit():
            val = val * 10 + (ord(next()) - ord('0'))
    else:
        error("Expected '(' or number.")
    return val

# This parses factors with an optional +/- prefix.
def unary():
    if matches('+') or matches('-'):
        op = next()
        val = unary()
        if op == '-':
            val = -val
        return val
    else:
        return factor()

# This parses multiplications and divisions.
def term():
    left = unary()
    while matches('*') or matches('/'):
        op = next()
        right = unary()

        if op == '*':
            left *= right
        else:
            left /= right
    return left

# This parses additions and subtractions.
def expression():
    left = term()
    while matches('+') or matches('-'):
        op = next()
        right = term()

        if op == '+':
            left += right
        else:
            left -= right
    return left

#--------------------------------------------------------------------------------

if __name__ == "__main__":
    while True:
        try:
            line = input("> ")
        except EOFError:
            break
        except KeyboardInterrupt:
            print()
            continue

        if line == 'exit':
            break
        elif len(line) == 0 or line.isspace():
            continue

        pos = 0

        try:
            e = expression()
        except:
            continue

        print(e)