use snake case (#178)

Author: MarcoGorelli

spec/API_specification/dataframe_api/__init__.py

@@ -17,7 +17,7 @@
     "column_from_sequence",
     "concat",
     "dataframe_from_dict",
-    "isnull",
+    "is_null",
     "null",
 ]
 
@@ -99,7 +99,7 @@ class null:
         From ``__eq__`` and from ``__bool__``.
 
         For ``__eq__``: a missing value must not be compared for equality
-        directly. Instead, use `DataFrame.isnull` or `Column.isnull` to check
+        directly. Instead, use `DataFrame.is_null` or `Column.is_null` to check
         for presence of missing values.
 
         For ``__bool__``: truthiness of a missing value is ambiguous.
@@ -113,7 +113,7 @@ class null:
     """
     ...
 
-def isnull(value: object, /) -> bool:
+def is_null(value: object, /) -> bool:
     """
     Check if an object is a `null` scalar.
 

spec/API_specification/dataframe_api/column_object.py

@@ -426,7 +426,7 @@ def var(self, *, skip_nulls: bool = True) -> DType:
         dtypes.
         """
 
-    def isnull(self) -> Column:
+    def is_null(self) -> Column:
         """
         Check for 'missing' or 'null' entries.
 
@@ -436,7 +436,7 @@ def isnull(self) -> Column:
 
         See also
         --------
-        isnan
+        is_nan
 
         Notes
         -----
@@ -445,7 +445,7 @@ def isnull(self) -> Column:
         but note that the Standard makes no guarantees about them.
         """
 
-    def isnan(self) -> Column:
+    def is_nan(self) -> Column:
         """
         Check for nan entries.
 
@@ -455,7 +455,7 @@ def isnan(self) -> Column:
 
         See also
         --------
-        isnull
+        is_null
 
         Notes
         -----

spec/API_specification/dataframe_api/dataframe_object.py

@@ -696,7 +696,7 @@ def var(self, *, skip_nulls: bool = True) -> DataFrame:
         """
         ...
 
-    def isnull(self) -> DataFrame:
+    def is_null(self) -> DataFrame:
         """
         Check for 'missing' or 'null' entries.
 
@@ -706,7 +706,7 @@ def isnull(self) -> DataFrame:
 
         See also
         --------
-        isnan
+        is_nan
 
         Notes
         -----
@@ -716,7 +716,7 @@ def isnull(self) -> DataFrame:
         """
         ...
 
-    def isnan(self) -> DataFrame:
+    def is_nan(self) -> DataFrame:
         """
         Check for nan entries.
 
@@ -726,7 +726,7 @@ def isnan(self) -> DataFrame:
 
         See also
         --------
-        isnull
+        is_null
 
         Notes
         -----

spec/API_specification/index.rst

@@ -14,7 +14,7 @@ of objects and functions in the top-level namespace. The latter are:
    :nosignatures:
 
    __dataframe_api_version__
-   isnull
+   is_null
    null
 
 The ``DataFrame``, ``Column`` and ``GroupBy`` objects have the following