List all (12416) expressions that can be writen while using up all of the pieces in Max's number foam pad (digits 0 to 9, plus, minus and equal signs)

numbers.py

_DIGITS = frozenset(xrange(0, 10))

def generate(sequence, available_digits):
    if not available_digits:
        yield sequence
    for digit in available_digits:
        for s in generate(sequence + [digit], available_digits - set((digit, ))):
            yield s

def eval(digits):
    if digits[0] == 0:
        return None
    result = 0
    for i, d in enumerate(digits):
        result += d * pow(10, len(digits) - i - 1)
    return result

for digits in generate([], _DIGITS):
    for r0 in xrange(1, 8):
        t0 = eval(digits[0:r0])
        if t0 is None:
            continue
        for r1 in xrange(r0 + 1, 9):
            t1 = eval(digits[r0:r1])
            if t1 is None:
                continue
            for r2 in xrange(r1 + 1, 10):
                t2 = eval(digits[r1:r2])
                if t2 is None:
                    continue
                t3 = eval(digits[r2:])
                if t3 is None:
                    continue
                if t0 + t1 - t2 == t3:
                    print "%d + %d - %d = %d" % (t0, t1, t2, t3)