avoid chaining if func/names_glue already exists

Author: samukweku

janitor/functions/utils.py

@@ -671,6 +671,8 @@ def __init__(self, *cols):
         self.names_glue = None
 
     def add_fns(self, func, **kwargs):
+        if self.func:
+            raise ValueError("A function has already been assigned")
         check("Function", func, [str, list, tuple, callable])
         if isinstance(func, (list, tuple)):
             for funcn in func:
@@ -679,6 +681,8 @@ def add_fns(self, func, **kwargs):
         return self
 
     def rename(self, name):
+        if self.names_glue:
+            raise ValueError("A name has already been assigned")
         check("name", name, [str])
         self.names_glue = name
         return self

tests/functions/test_summarize.py

@@ -29,6 +29,28 @@ def test_SD_name_error(dataframe):
         dataframe.summarize(SD("a").add_fns("sum").rename(1))
 
 
+@pytest.mark.functions
+def test_SD_name_dupe_error(dataframe):
+    """Raise if names_glue already exists"""
+    with pytest.raises(
+        ValueError,
+        match=r"A name has already been assigned",
+    ):
+        dataframe.summarize(SD("a").add_fns("sum").rename("1").rename("name"))
+
+
+@pytest.mark.functions
+def test_SD_func_dupe_error(dataframe):
+    """Raise if func already exists"""
+    with pytest.raises(
+        ValueError,
+        match=r"A function has already been assigned",
+    ):
+        dataframe.summarize(
+            SD("a").add_fns("sum").add_fns(np.sum).rename("name")
+        )
+
+
 @pytest.mark.functions
 def test_SD_no_func_error(dataframe):
     """Raise if func is not provided"""