That's correct: class is a reserved word in Python, and since patsy allows arbitrary Python code inside formulas, Python's regular reserved-word rules apply to formulas too.

The workaround is to use patsy's quoting operator, Q. Like:

# doesn't work:
dmatrix("age + class", data=...)

# does work:
dmatrix("y ~ age + Q('class')", data=...)

I suppose we could hack around this by detecting reserved words that can't possibly appear inside an expression (e.g. class, for, def, ...), and "autoquoting" them, but I don't anticipate having time to work on this anytime soon.