ENH: warn when export of CRS returns None

docs/history.rst

@@ -7,6 +7,7 @@ Latest
 - BUG: remove CustomConstructorCRS @abstractmethod decorator (pull #1018)
 - BUG: Correct type annotation for AreaofUse.bounds (issue #1012)
 - BUG: :func:`pyproj.datadir.get_data_dir` support for conda Windows (issue #1029)
+- ENH: warn when export of CRS returns None (issue #1036)
 
 3.3.0
 -------

pyproj/_crs.pyi

@@ -35,7 +35,7 @@ class Base:
         self,
         version: Union[WktVersion, str] = WktVersion.WKT2_2019,
         pretty: bool = False,
-    ) -> str: ...
+    ) -> Optional[str]: ...
     def to_json(self, pretty: bool = False, indentation: int = 2) -> str: ...
     def to_json_dict(self) -> dict: ...
     def __str__(self) -> str: ...
@@ -222,7 +222,7 @@ class _CRS(Base):
     def to_epsg(self, min_confidence: int = 70) -> Optional[int]: ...
     def to_authority(
         self, auth_name: Optional[str] = None, min_confidence: int = 70
-    ): ...
+    ) -> Optional[tuple]: ...
     def list_authority(
         self, auth_name: Optional[str] = None, min_confidence: int = 70
     ) -> List[AuthorityMatchInfo]: ...

pyproj/crs/crs.py

@@ -1175,7 +1175,7 @@ def to_wkt(
         self,
         version: Union[WktVersion, str] = WktVersion.WKT2_2019,
         pretty: bool = False,
-    ) -> str:
+    ) -> Optional[str]:
         """
         Convert the projection to a WKT string.
 
@@ -1200,7 +1200,15 @@ def to_wkt(
         -------
         str
         """
-        return self._crs.to_wkt(version=version, pretty=pretty)
+        wkt = self._crs.to_wkt(version=version, pretty=pretty)
+        if wkt is None:
+            warnings.warn(
+                f"CRS cannot be converted to a WKT string of a '{version}' version. "
+                "Select a different version of a WKT string or edit your CRS.",
+                RuntimeWarning,
+                stacklevel=2,
+            )
+        return wkt
 
     def to_json(self, pretty: bool = False, indentation: int = 2) -> str:
         """
@@ -1289,9 +1297,18 @@ def to_epsg(self, min_confidence: int = 70) -> Optional[int]:
         Optional[int]:
             The best matching EPSG code matching the confidence level.
         """
-        return self._crs.to_epsg(min_confidence=min_confidence)
+        epsg = self._crs.to_epsg(min_confidence=min_confidence)
+        if epsg is None:
+            warnings.warn(
+                "CRS cannot be converted to the EPSG code. Match not found.",
+                RuntimeWarning,
+                stacklevel=2,
+            )
+        return epsg
 
-    def to_authority(self, auth_name: Optional[str] = None, min_confidence: int = 70):
+    def to_authority(
+        self, auth_name: Optional[str] = None, min_confidence: int = 70
+    ) -> Optional[tuple]:
         """
         .. versionadded:: 2.2.0
 
@@ -1329,9 +1346,17 @@ def to_authority(self, auth_name: Optional[str] = None, min_confidence: int = 70
         tuple(str, str) or None:
             The best matching (<auth_name>, <code>) for the confidence level.
         """
-        return self._crs.to_authority(
+        authority = self._crs.to_authority(
             auth_name=auth_name, min_confidence=min_confidence
         )
+        if authority is None:
+            warnings.warn(
+                "CRS cannot be converted to the authority name and code. "
+                "Match not found.",
+                RuntimeWarning,
+                stacklevel=2,
+            )
+        return authority
 
     def list_authority(
         self, auth_name: Optional[str] = None, min_confidence: int = 70

test/crs/test_crs.py

@@ -321,6 +321,28 @@ def test_epsg():
     assert CRS.from_user_input("epsg:4326").to_epsg() == 4326
 
 
+def test_epsg_none():
+    wkt_string = (
+        'PROJCRS["unknown",BASEGEOGCRS["unknown",DATUM["unknown",'
+        'ELLIPSOID["WGS 84",6378137,298.257223563,LENGTHUNIT["metre",1,'
+        'ID["EPSG",9001]]]],PRIMEM["Greenwich",0,ANGLEUNIT["degree",0.0174532925199],'
+        'ID["EPSG",8901]]],CONVERSION["unknown",METHOD["Equidistant Cylindrical",'
+        'ID["EPSG",1028]],PARAMETER["Latitude of 1st standard parallel",0,'
+        'ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8823]],'
+        'PARAMETER["Longitude of natural origin",0,ANGLEUNIT["degree",0.0174532925199],'
+        'ID["EPSG",8802]],PARAMETER["False easting",0,'
+        'LENGTHUNIT["unknown",111319.490793274],ID["EPSG",8806]],'
+        'PARAMETER["False northing",0,LENGTHUNIT["unknown",111319.490793274],'
+        'ID["EPSG",8807]]],CS[Cartesian,3],AXIS["(E)",east,ORDER[1],'
+        'LENGTHUNIT["unknown",111319.490793274]],AXIS["(N)",north,ORDER[2],'
+        'LENGTHUNIT["unknown",111319.490793274]],AXIS["ellipsoidal height (h)",up,'
+        'ORDER[3],LENGTHUNIT["metre",1,ID["EPSG",9001]]]]'
+    )
+    crs = CRS.from_wkt(wkt_string)
+    with pytest.warns(RuntimeWarning, match="CRS cannot be converted to the EPSG code"):
+        crs.to_epsg()
+
+
 def test_datum():
     datum = CRS.from_epsg(4326).datum
     assert "\n" in repr(datum)
@@ -889,6 +911,32 @@ def test_to_wkt_enum__invalid():
         crs.to_wkt("WKT_INVALID")
 
 
+@pytest.mark.parametrize(
+    "wkt_version",
+    ["WKT2_2015", "WKT2_2015_SIMPLIFIED", "WKT1_GDAL", "WKT1_ESRI"],
+)
+def test_to_wkt_none_warning(wkt_version):
+    wkt_string = (
+        'PROJCRS["unknown",BASEGEOGCRS["unknown",DATUM["unknown",'
+        'ELLIPSOID["WGS 84",6378137,298.257223563,LENGTHUNIT["metre",1,'
+        'ID["EPSG",9001]]]],PRIMEM["Greenwich",0,ANGLEUNIT["degree",0.0174532925199],'
+        'ID["EPSG",8901]]],CONVERSION["unknown",METHOD["Equidistant Cylindrical",'
+        'ID["EPSG",1028]],PARAMETER["Latitude of 1st standard parallel",0,'
+        'ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8823]],'
+        'PARAMETER["Longitude of natural origin",0,ANGLEUNIT["degree",0.0174532925199],'
+        'ID["EPSG",8802]],PARAMETER["False easting",0,'
+        'LENGTHUNIT["unknown",111319.490793274],ID["EPSG",8806]],'
+        'PARAMETER["False northing",0,LENGTHUNIT["unknown",111319.490793274],'
+        'ID["EPSG",8807]]],CS[Cartesian,3],AXIS["(E)",east,ORDER[1],'
+        'LENGTHUNIT["unknown",111319.490793274]],AXIS["(N)",north,ORDER[2],'
+        'LENGTHUNIT["unknown",111319.490793274]],AXIS["ellipsoidal height (h)",up,'
+        'ORDER[3],LENGTHUNIT["metre",1,ID["EPSG",9001]]]]'
+    )
+    crs = CRS.from_wkt(wkt_string)
+    with pytest.warns(RuntimeWarning, match="CRS cannot be converted to a WKT string"):
+        crs.to_wkt(version=wkt_version)
+
+
 def test_to_proj4_enum():
     crs = CRS.from_epsg(4326)
     with pytest.warns(UserWarning):
@@ -1160,6 +1208,30 @@ def test_from_authority__ignf():
     assert cc.to_epsg() == 25828
 
 
+def test_to_authority_none():
+    wkt_string = (
+        'PROJCRS["unknown",BASEGEOGCRS["unknown",DATUM["unknown",'
+        'ELLIPSOID["WGS 84",6378137,298.257223563,LENGTHUNIT["metre",1,'
+        'ID["EPSG",9001]]]],PRIMEM["Greenwich",0,ANGLEUNIT["degree",0.0174532925199],'
+        'ID["EPSG",8901]]],CONVERSION["unknown",METHOD["Equidistant Cylindrical",'
+        'ID["EPSG",1028]],PARAMETER["Latitude of 1st standard parallel",0,'
+        'ANGLEUNIT["degree",0.0174532925199433],ID["EPSG",8823]],'
+        'PARAMETER["Longitude of natural origin",0,ANGLEUNIT["degree",0.0174532925199],'
+        'ID["EPSG",8802]],PARAMETER["False easting",0,'
+        'LENGTHUNIT["unknown",111319.490793274],ID["EPSG",8806]],'
+        'PARAMETER["False northing",0,LENGTHUNIT["unknown",111319.490793274],'
+        'ID["EPSG",8807]]],CS[Cartesian,3],AXIS["(E)",east,ORDER[1],'
+        'LENGTHUNIT["unknown",111319.490793274]],AXIS["(N)",north,ORDER[2],'
+        'LENGTHUNIT["unknown",111319.490793274]],AXIS["ellipsoidal height (h)",up,'
+        'ORDER[3],LENGTHUNIT["metre",1,ID["EPSG",9001]]]]'
+    )
+    crs = CRS.from_wkt(wkt_string)
+    with pytest.warns(
+        RuntimeWarning, match="CRS cannot be converted to the authority name and code"
+    ):
+        crs.to_authority()
+
+
 def test_ignf_authority_repr():
     assert repr(CRS.from_authority("IGNF", "ETRS89UTM28")).startswith(
         "<Derived Projected CRS: IGNF:ETRS89UTM28>"