One characteristic design decision here is how a Strategy object should report "I cannot solve this". (Actually this design decision covers a huge range of patterns where a function might return just about anything...)

Choices are:

1. Setting a flag on the Result object, which is perhaps the most Javaish or OOPish thing to do.

2. Returning a special value, such as null or None, that means "cannot solve".

3. Raising a certain exception.

#2 is the most common in practice; #3 is probably the most Pythonic thing. I would argue that using #1 makes the sample code longer than it should be.