My BF interpreter

brain.py

import sys

def interpret(source, memory=1000, max_iterations=100000):
    mem = [0] * memory
    mem_ptr = 0
    loop_stack = []
    src_ptr = 0
    iterations = 0
    while True:
        # safety in brainf?
        iterations += 1
        if iterations > max_iterations:
            raise RuntimeError('Exceeded maximum iterations')
        char = source[src_ptr]
        if char == '>':
            mem_ptr += 1
        elif char == '<':
            mem_ptr -= 1
        elif char == '+':
            mem[mem_ptr] += 1
        elif char == '-':
            mem[mem_ptr] -= 1
        elif char == '.':
            print(chr(mem[mem_ptr]), end='')
        elif char == ',':
            mem[mem_ptr] = ord(input()[0])
        elif char == '[':
            if mem[mem_ptr] != 0:
                loop_stack.append(src_ptr)
            else:
                # find next ] and start after that
                loop_nestedness = 0
                while True:
                    src_ptr += 1
                    if source[src_ptr] == ']':
                        if loop_nestedness == 0:
                            break
                        else:
                            loop_nestedness -= 1
                    if source[src_ptr] == '[':
                        loop_nestedness += 1
        elif char == ']':
            src_ptr = loop_stack.pop() - 1
        # elif char == '#':
        #     # let i be the first non-zero memory cell
        #     for i in range(len(mem) - 1, 10, -1):
        #         if mem[i] != 0:
        #             break
        #     print(mem[:i+1])
        else:
            pass
        src_ptr += 1
        if src_ptr >= len(source):
            return mem

if __name__ == '__main__':
    # copy = '[->+>+<<]>>[-<<+>>]'
    # hello = '++++++++[>++++[>++>+++>+++>+<<<<-]>+>+>->>+[<]<-]>>.>---.+++++++..+++.>>.<-.<.+++.------.--------.>>+.>++.'

    source = ''
    for line in sys.stdin:
        source += line

    interpret(source)