Allow CUPED/PrePost to iterate on covariates.

PiperOrigin-RevId: 772163965

Author: tcya

meterstick_demo.ipynb

@@ -13820,6 +13820,17 @@
         "Jackknife('cookie', MetricList((pct, prepost))).compute_on(df_prepost)"
       ]
     },
+    {
+      "cell_type": "markdown",
+      "metadata": {
+        "id": "cD4H8FPkXoDs"
+      },
+      "source": [
+        "Note that\n",
+        "- When you pass multiple base `Metric`s to `CUPED` or `PrePostChange`, they adjust them one by one. Namely, `CUPED(column, baseline, [post1, post2], pre)` is equivalent to `MetricList([CUPED(column, baseline, post1, pre), CUPED(column, baseline, post2, pre)])`.\n",
+        "- When you pass multiple covariates, by default they are all used to do the adjustment. But if you set `multiple_covariates` to `False`, we'll zip the base `Metric`s and the covariates and create a list of single-covariate `CUPED`. Namely, `CUPED(child=[x1, x2], covariates=[y1, y2], multiple_covariates=False)` is equivalent to `MetricList([CUPED(child=x1, covariates=y1), CUPED(child=x2, covariates=y2)])`."
+      ]
+    },
     {
       "cell_type": "markdown",
       "metadata": {

metrics.py

@@ -271,6 +271,8 @@ def where(self, where):
       self.where_ = tuple(where)
 
   def add_where(self, where):
+    if where is None:
+      return self
     where = [where] if isinstance(where, str) else list(where) or []
     if not self.where_:
       self.where = where
@@ -883,7 +885,7 @@ def get_equivalent(self, *auxiliary_cols):
     res = self.get_equivalent_without_filter(*auxiliary_cols)  # pylint: disable=assignment-from-none
     if res:
       res.name = self.name
-      res.where = self.where_
+      res.add_where(self.where_)
     return res
 
   def get_equivalent_without_filter(self, *auxiliary_cols):