A command-line calculator with some handy and/or neat features.

calc.py

#!/usr/bin/python

"""
A command line calculator with the following features:
1) Accepts numbers with any decimal and thousand separators.
2) Uses Decimal when possible, which favors "user-friendliness" of results
   over speed of computation.
"""

from __future__ import division, print_function
try:
    input = raw_input
except NameError:
    pass

from math import *
import decimal
from decimal import Decimal
from functools import partial
import sys
import re
import traceback
import readline


num_regex = re.compile(r'\b[0-9.,]+(?: [0-9.,]+)*\b')
replacements = [
    # (re.compile(r'(?<=[\d\s\)])x(?=[\s\d\(])'), '*'),  # Multiplication as 'a x b'
    # (re.compile(r'\^'), '**'),  # Power as 'a^b'
]


def normalize_number(number: str):
    separators = re.sub(r'\d', '', number, flags=re.U)
    for sep in separators[:-1]:
        number = number.replace(sep, '')
    if separators:
        sep = separators[-1]
        number = number.replace(sep, '.' if sep!=' ' else '')
    return number


def preprocess_number(match: re.Match, use_decimal):
    number: str = match.group()
    number = normalize_number(number)
    if use_decimal:
        decimal_number_str = f'Decimal("{number}")'
        return decimal_number_str
    else:
        return number


def compute_expression(expr, use_decimal):
    expr = re.sub(num_regex, partial(preprocess_number, use_decimal=use_decimal), expr)
    for regex, repl in replacements:
        expr = re.sub(regex, repl, expr)
    print(eval(expr))


def enter_calc_prompt():
    decimal_context = decimal.getcontext()
    decimal_context.rounding = decimal.ROUND_HALF_UP
    while True:
        try:
            expr = input("> ")
            if expr in ["q", "quit", "exit"]:
                break
            if not expr:
                continue
            try:
                compute_expression(expr, use_decimal=True)
            except TypeError:
                compute_expression(expr, use_decimal=False)
        except (EOFError, SystemExit):
            print()
            break
        except:
            traceback.print_exc()


if __name__ == "__main__":
    if len(sys.argv) == 1:
        enter_calc_prompt()
    else:
        compute_expression(' '.join(sys.argv[1:]))