Not being familiar enough with Python, I couldn't tell you exactly what it means.
But given the context, it's likely an "in-joke" about the programming language that requires an understanding of the material.
Or it might be a personal anecdote on the character of the individual. Given that "only" is an absolute term, the individual may be someone who doesn't care for absolutes, and thus it would be a joke on his character.
I suggest reading the book a bit and seeing if it makes some sense with an understanding of the language. If not, then it is probably the latter.
Edit: One thing I do know about Python is that it is (compared to other programming languages) flexible by design. So it is possibly a joke on that very nature - most programming languages are fairly rigid, so the high flexibility by comparision means that Python "only" being able to work in a certain way would be something the author might've frequently been corrected about by Frank Willison.