Live coded during my "Virtual Machines and Brainfuck" talk at PyConIL 2017

bf.py

import sys

def parse_parens(code):
    result = {}
    stack = []
    for i, c in enumerate(code):
        if c == '[':
            stack.append(i)
        elif c == ']':
            match = stack.pop()
            result[i] = match
            result[match] = i
    return result
assert parse_parens('') == {}
assert parse_parens('[]') == {0: 1, 1: 0}
assert parse_parens('[+]') == {0: 2, 2: 0}
assert parse_parens('[+[]]') == {0: 4, 4: 0, 2: 3, 3: 2}

def bf(code, input=''):
    def get():
        if get.i >= len(input):
            return ''
        char = input[get.i]
        get.i += 1
        return char
    get.i = 0
    output = []
    def put(char):
        output.append(char)
    brainfuck(code, getchar=get, putchar=put)
    return ''.join(output)

def brainfuck(code, getchar=lambda: sys.stdin.read(1), putchar=sys.stdout.write):
    matched_parens = parse_parens(code)
    memory = [0] * 40000
    cursor = 0
    instruction_pointer = 0
    while instruction_pointer < len(code):
        instruction = code[instruction_pointer]
        if instruction == '+':
            memory[cursor] = (memory[cursor] + 1) % 255
        elif instruction == '-':
            memory[cursor] = (memory[cursor] - 1) % 255
        elif instruction == '<':
            cursor = (cursor - 1) % len(memory)
        elif instruction == '>':
            cursor = (cursor + 1) % len(memory)
        elif instruction == '[':
            if memory[cursor] == 0:
                instruction_pointer = matched_parens[instruction_pointer]
        elif instruction == ']':
            instruction_pointer = matched_parens[instruction_pointer]
            continue
        elif instruction == ',':
            char = getchar()
            if char == '':
                break
            memory[cursor] = ord(char)
        elif instruction == '.':
            putchar(chr(memory[cursor]))
        else:
            # everything else is a comment
            pass
        instruction_pointer += 1
        #print instruction_pointer, memory[:10]

assert bf('') == ''
assert bf('++++++++++.') == '\n'
assert bf('+++++++++++++.---.') == '\r\n'
assert bf('++++++++[>++++++++<-]>+.') == 'A'
assert bf('+++[>,.<-]', 'abcde') == 'abc'
assert bf('+[>,.<]', 'abcde') == 'abcde'
assert bf(bf('+++++++++[<+++++>-],[[-<--.++<+>>]<+.-<[->.<]>>,]', 'hello, world!')) == 'hello, world!'

def main():
    assert len(sys.argv) == 2, 'usage: bf.py <code>'
    _, code = sys.argv
    brainfuck(code)
    return 0

if __name__ == '__main__':
    sys.exit(main())