Maximum recursion depth error for formulas with more than 485 terms

I am working with a dataframe which has 7000 columns and it turns out that once you go beyond 485 terms, patsy throws a recursion error when going from a formula to a design matrix. Is there a better way of doing this?

Thanks!

In [282]: df = pd.DataFrame(dict(('a' + str(i), np.random.randn(5)) for i in xrange(500)))

In [283]: formula = " + ".join(df.columns)

In [284]: dmatrices(formula, df)

....

/Users/xxx/lib/python2.7/site-packages/patsy-0.1.0_dev-py2.7.egg/patsy/desc.pyc in eval(self, tree, require_evalexpr)
    452                                 "'%s' operator" % (tree.type,),
    453                                 tree.token)
--> 454         result = self._evaluators[key](self, tree)
    455         if require_evalexpr and not isinstance(result, IntermediateExpr):
    456             if isinstance(result, ModelDesc):

/Users/xxx/lib/python2.7/site-packages/patsy-0.1.0_dev-py2.7.egg/patsy/desc.pyc in _eval_binary_plus(evaluator, tree)
    283
    284 def _eval_binary_plus(evaluator, tree):
--> 285     left_expr = evaluator.eval(tree.args[0])
    286     if tree.args[1].type == "ZERO":
    287         return IntermediateExpr(False, None, True, left_expr.terms)

/Users/xxx/lib/python2.7/site-packages/patsy-0.1.0_dev-py2.7.egg/patsy/desc.pyc in eval(self, tree, require_evalexpr)
    452                                 "'%s' operator" % (tree.type,),
    453                                 tree.token)
--> 454         result = self._evaluators[key](self, tree)
    455         if require_evalexpr and not isinstance(result, IntermediateExpr):
    456             if isinstance(result, ModelDesc):

/Users/xxx/lib/python2.7/site-packages/patsy-0.1.0_dev-py2.7.egg/patsy/desc.pyc in _eval_binary_plus(evaluator, tree)
    283
    284 def _eval_binary_plus(evaluator, tree):
--> 285     left_expr = evaluator.eval(tree.args[0])
    286     if tree.args[1].type == "ZERO":
    287         return IntermediateExpr(False, None, True,
    left_expr.terms)

/Users/xxx/lib/python2.7/site-packages/patsy-0.1.0_dev-py2.7.egg/patsy/desc.pyc in eval(self, tree, require_evalexpr)
    448         assert isinstance(tree, ParseNode)
    449         key = (tree.type, len(tree.args))
--> 450         if key not in self._evaluators:
    451             raise PatsyError("I don't know how to evaluate this "
    452                                 "'%s' operator" % (tree.type,),

RuntimeError: maximum recursion depth exceeded in cmp

Start by reproducing the failure with the 500-term formula shown in the issue, then inspect the formula evaluation path in patsy/desc.py, especially eval and _eval_binary_plus. Compare behavior below and above the recursion threshold and add a regression test showing that formulas with more than 485 terms no longer raise the recursion error.