Repository: skrub-data/skrub (python)

Issue:
Better error message when an expression is passed to TableVectorizer or to other expressions
I was writing an example with the expressions, and when I tried to combine them with the TableVectorizer I hit an exception with an error message that wasn't very clear: 

```python
from skrub import StringEncoder, TextEncoder
enc = skrub.choose_from({"string": StringEncoder(), "text": TextEncoder()})

TableVectorizer(high_cardinality=enc) # Wrong

import pandas as pd
df = pd.DataFrame({"a": ["a", "b", "c"]})

TableVectorizer(high_cardinality=enc).fit_transform(df)
```

This gives as error message

```

TypeError: Cannot clone object 'choose_from({'string': StringEncoder(), 'text': TextEncoder()})' (type <class 'skrub._expressions._choosing.Choice'>): it does not seem to be a scikit-learn estimator as it does not implement a 'get_params' method.

```

Which makes sense since I'm passing an expression rather than an encoder. I think it may be confusing for users that are learning how to use expressions and the error message is not very clear about waht the error is, so we might want to detect whether the object that is passed is an expression and raise an error accordingly. 


Edit to add:

A similar problem happens if I try to define a skrub variable and use another expression as its value. The code should fail early rather than raising an error at the end when I'm crossvalidating the model

Pull request description:
Provide informative errors when expressions are passed as values to skrub variables
Fixes #1438

Error checking for the value in the variables is already done. I thought to implement both improvements in the same PR, but I'm not sure how to implement the error checking for the encoders. I was thinking about doing it in `_expressions/tests/test_evaluation.py` or somewhere in `_expressions`, but maybe I should add that to the tests for each encoder? 



Patch:
diff --git a/skrub/_dataframe/_common.py b/skrub/_dataframe/_common.py
index 8697b72..540a83a 100644
--- a/skrub/_dataframe/_common.py
+++ b/skrub/_dataframe/_common.py
@@ -325,13 +325,44 @@ def _all_null_like_polars(col, length=None, dtype=None, name=None):
     return pl.Series(name, [None] * length, dtype=dtype)
 
 
+def _check_same_type(objects):
+    is_df = np.array([is_dataframe(obj) for obj in objects])
+    is_col = np.array([is_column(obj) for obj in objects])
+    is_other = ~(is_df | is_col)
+
+    if is_df.all() or is_col.all() or is_other.all():
+        return
+
+    def _indices(array):
+        return np.argwhere(array).ravel()
+
+    prefix = "Mixing types is not allowed, got"
+    hints = []
+    if is_df.any():
+        hints.append(f"dataframes at position {_indices(is_df)}")
+
+    if is_col.any():
+        hints.append(f"series at position {_indices(is_col)}")
+
+    if is_other.any():
+        hints.append(
+            "types that are neither dataframes nor series at position "
+            f"{_indices(is_other)}"
+        )
+
+    msg = prefix + " " + ", ".join(hints) + "."
+
+    raise TypeError(msg)
+
+
 @dispatch
 def concat(*dataframes, axis=0):
     raise NotImplementedError()
 
 
-@concat.specialize("pandas")
+@concat.specialize("pandas", argument_type="DataFrame")
 def _concat_pandas(*dataframes, axis=0):
+    _check_same_type(dataframes)
     kwargs = {"copy": False} if pandas_version < parse_version("3.0") else {}
     if axis == 0:
         return pd.concat(dataframes, axis=0, ignore_index=True, **kwargs)
@@ -344,8 +375,9 @@ def _concat_pandas(*dataframes, axis=0):
         return result
 
 
-@concat.specialize("polars")
+@concat.specialize("polars", argument_type="DataFrame")
 def _concat_polars(*dataframes, axis=0):
+    _check_same_type(dataframes)
     if axis == 0:
         return pl.concat(dataframes, how="diagonal_relaxed")
     else:  # axis == 1
@@ -353,6 +385,18 @@ def _concat_polars(*dataframes, axis=0):
         return pl.concat(dataframes, how="horizontal")
 
 
+@concat.specialize("pandas", argument_type="Column")
+def _concat_pandas_cols(*columns, axis=0):
+    _check_same_type(columns)
+    return _concat_pandas(*[to_frame(col) for col in columns], axis=axis)
+
+
+@concat.specialize("polars", argument_type="Column")
+def _concat_polars_cols(*columns, axis=0):
+    _check_same_type(columns)
+    return _concat_polars(*[to_frame(col) for col in columns], axis=axis)
+
+
 def is_column_list(obj):
     if not isinstance(obj, Sequence):
         return False
diff --git a/skrub/_expressions/_expressions.py b/skrub/_expressions/_expressions.py
index cfa003b..62f12c3 100644
--- a/skrub/_expressions/_expressions.py
+++ b/skrub/_expressions/_expressions.py
@@ -792,8 +792,21 @@ def check_name(name, is_var):
         )
 
 
+def _check_var_value(value):
+    """Checking that the value passed to a skrub variable is not an expression
+    or a choice."""
+    from ._evaluation import needs_eval
+
+    if needs_eval(value):
+        raise TypeError(
+            "The `value` of a `skrub.var()` must not contain a skrub "
+            f"expression or skrub choice. Got object {value} of {type(value)}."
+        )
+
+
 class Var(ExprImpl):
     "A `skrub.var()` expression."
+
     _fields = ["name", "value"]
 
     def compute(self, e, mode, environment):
@@ -843,6 +856,11 @@ def var(name, value=NULL):
     -------
     A skrub variable
 
+    Raises
+    ------
+    TypeError
+        If the provided value is a skrub expression or a skrub choose_* function.
+
     See also
     --------
     skrub.X :
@@ -911,6 +929,7 @@ def var(name, value=NULL):
     gallery.
     """
     check_name(name, is_var=True)
+    _check_var_value(value)
     return Expr(Var(name, value=value))
 
 
@@ -936,6 +955,11 @@ def X(value=NULL):
     -------
     A skrub variable
 
+    Raises
+    ------
+    TypeError
+        If the provided value is a skrub expression or a skrub choose_* function.
+
     See also
     --------
     skrub.y :
@@ -966,6 +990,7 @@ def X(value=NULL):
     >>> X.skb.is_X
     True
     """
+    _check_var_value(value)
     return Expr(Var("X", value=value)).skb.mark_as_X()
 
 
@@ -992,6 +1017,11 @@ def y(value=NULL):
     -------
     A skrub variable
 
+    Raises
+    ------
+    TypeError
+        If the provided value is a skrub expression or a skrub choose_* function.
+
     See also
     --------
     skrub.X :
@@ -1022,6 +1052,7 @@ def y(value=NULL):
     >>> y.skb.is_y
     True
     """
+    _check_var_value(value)
     return Expr(Var("y", value=value)).skb.mark_as_y()
 
 


Test patch:
diff --git a/skrub/_dataframe/tests/test_common.py b/skrub/_dataframe/tests/test_common.py
index 8294400..56f4529 100644
--- a/skrub/_dataframe/tests/test_common.py
+++ b/skrub/_dataframe/tests/test_common.py
@@ -232,6 +232,34 @@ def test_concat_vertical(df_module, example_data_dict):
             pass
 
 
+def test_concat_series(df_module):
+    df = df_module.example_dataframe
+    col = df_module.example_column
+
+    # Mixing types is not allowed
+    msg = r"got dataframes at position \[0\], series at position \[1\]"
+    with pytest.raises(TypeError, match=msg):
+        ns.concat(df, col)
+
+    msg = r"got dataframes at position \[1\], series at position \[0\]."
+    with pytest.raises(TypeError, match=msg):
+        ns.concat(col, df)
+
+    msg = (
+        r"got dataframes at position \[2\], series at position \[0\], "
+        r"types that are neither dataframes nor series at position \[1 3\]"
+    )
+    with pytest.raises(TypeError, match=msg):
+        ns.concat(col, 0, df, 1)
+
+    # Cols only is allowed
+    for axis in 0, 1:
+        assert (
+            ns.shape(ns.concat(col, col, axis=axis))[axis]
+            == ns.shape(ns.to_frame(col))[axis] * 2
+        )
+
+
 def test_is_column_list(df_module):
     assert ns.is_column_list([])
     assert ns.is_column_list(())
diff --git a/skrub/_expressions/tests/test_expressions.py b/skrub/_expressions/tests/test_expressions.py
index 654408e..430131f 100644
--- a/skrub/_expressions/tests/test_expressions.py
+++ b/skrub/_expressions/tests/test_expressions.py
@@ -93,6 +93,41 @@ def test_environment_no_values():
     assert e.skb.eval({"d": 2, "c": 3}) == 6
 
 
+def test_environment_wrong_values():
+    a = skrub.var(name="a", value=[1, 2, 3])
+    # Testing expr as value
+    with pytest.raises(
+        TypeError, match=r".*`value` of a `skrub.var\(\)` must not contain a skrub.*"
+    ):
+        skrub.var(name="wrongvar", value=a)
+
+    with pytest.raises(
+        TypeError, match=r".*`value` of a `skrub.var\(\)` must not contain a skrub.*"
+    ):
+        skrub.X(value=a)
+
+    with pytest.raises(
+        TypeError, match=r".*`value` of a `skrub.var\(\)` must not contain a skrub.*"
+    ):
+        skrub.y(value=a)
+
+    # Testing choice as value
+    with pytest.raises(
+        TypeError, match=r".*`value` of a `skrub.var\(\)` must not contain a skrub.*"
+    ):
+        skrub.var("wrongvar", skrub.choose_bool())
+
+    with pytest.raises(
+        TypeError, match=r".*`value` of a `skrub.var\(\)` must not contain a skrub.*"
+    ):
+        skrub.X(skrub.choose_bool())
+
+    with pytest.raises(
+        TypeError, match=r".*`value` of a `skrub.var\(\)` must not contain a skrub.*"
+    ):
+        skrub.y(skrub.choose_bool())
+
+
 def test_choice_in_environment():
     a = skrub.var("a", 100)
     b = skrub.var("b", 10)