cat_boxes.py

class Box(str):
    def __new__(cls, s):
        lines = s.split("\n")
        width = max(len(line) for line in lines)
        top_bottom = '-' * width
        return super().__new__(cls, box_template.format(
                top_bottom=top_bottom,
                middle="\n".join(f"| {line:{width}} |" for line in lines)))

    def __add__(self, other):
        lines1 = self.split("\n")
        lines2 = other.split("\n")
        width1 = max(len(line) for line in lines1)
        width2 = max(len(line) for line in lines2)
        return "\n".join(
            f'{line1:{width1}} {"+" if i == 2 else " "} '
            f'{line2:{width2}}'
            for i, (line1, line2) in enumerate(zip(lines1, lines2))
        )

    def __radd__(self, other):
        return type(self).__add__(other, self)

box_template = """
+-{top_bottom}-+
{middle}
+-{top_bottom}-+
"""
cat = """
|\---/|
| o_o |
 \_^_/
"""
assert Box(3 * cat + Box(Box(cat) + 2 * cat)) == """
+-------------------------------------+
|                                     |
| |\---/|   +-----------------------+ |
| | o_o | + |                       | |
|  \_^_/    | +---------+   |\---/| | |
|           | |         | + | o_o | | |
| |\---/|   | | |\---/| |    \_^_/  | |
| | o_o |   | | | o_o | |           | |
|  \_^_/    | |  \_^_/  |   |\---/| | |
|           | |         |   | o_o | | |
| |\---/|   | +---------+    \_^_/  | |
| | o_o |   |                       | |
|  \_^_/    +-----------------------+ |
|                                     |
+-------------------------------------+
"""