ENH: Add source/target CRS to Transformer

docs/history.rst

@@ -6,8 +6,9 @@ Latest
 - DEP: Minimum supported Python version 3.8 (issue #930)
 - DEP: Minimum PROJ version 8.0 (issue #940)
 - BUG: Prepend "Derived" to CRS type name if CRS is derived (issue #932)
-- BUG: Improved handling of inf values in :attr:`pyproj.transformer.Transformer.transform_bounds` (pull #961)
-- ENH: Add support for transforming bounds at the poles in :attr:`pyproj.transformer.Transformer.transform_bounds` (pull #962)
+- BUG: Improved handling of inf values in :meth:`pyproj.transformer.Transformer.transform_bounds` (pull #961)
+- ENH: Add support for transforming bounds at the poles in :meth:`pyproj.transformer.Transformer.transform_bounds` (pull #962)
+- ENH: Added :attr:`pyproj.transformer.Transformer.source_crs` & :attr:`pyproj.transformer.Transformer.target_crs` (pull #976)
 
 3.2.1
 ------

pyproj/_crs.pxd

@@ -1,5 +1,9 @@
 include "proj.pxi"
 
+
+from pyproj.enums import WktVersion
+
+
 cdef extern from "proj_experimental.h":
     PJ *proj_crs_promote_to_3D(PJ_CONTEXT *ctx,
                                const char* crs_3D_name,
@@ -13,6 +17,12 @@ cdef _to_proj4(
     object version,
     bint pretty,
 )
+cdef _to_wkt(
+    PJ_CONTEXT* context,
+    PJ* projobj,
+    object version,
+    bint pretty,
+)
 
 cdef class Axis:
     cdef readonly str name

pyproj/_crs.pyx

@@ -69,8 +69,8 @@ def is_proj(str proj_string not None):
 cdef _to_wkt(
     PJ_CONTEXT* context,
     PJ* projobj,
-    object version=WktVersion.WKT2_2019,
-    bint pretty=False
+    object version,
+    bint pretty,
 ):
     """
     Convert a PJ object to a wkt string.
@@ -2557,12 +2557,10 @@ cdef class _CRS(Base):
     @property
     def source_crs(self):
         """
-        The base CRS of a BoundCRS or a DerivedCRS/ProjectedCRS,
-        or the source CRS of a CoordinateOperation.
-
         Returns
         -------
-        _CRS
+        _CRS:
+            The base CRS of a BoundCRS or a DerivedCRS/ProjectedCRS.
         """
         if self._source_crs is not None:
             return None if self._source_crs is False else self._source_crs
@@ -2572,7 +2570,12 @@ cdef class _CRS(Base):
             self._source_crs = False
             return None
         try:
-            self._source_crs = _CRS(_to_wkt(self.context, projobj))
+            self._source_crs = _CRS(_to_wkt(
+                self.context,
+                projobj,
+                version=WktVersion.WKT2_2019,
+                pretty=False,
+            ))
         finally:
             proj_destroy(projobj)
         return self._source_crs
@@ -2585,7 +2588,7 @@ cdef class _CRS(Base):
         Returns
         -------
         _CRS:
-            The hub CRS of a BoundCRS or the target CRS of a CoordinateOperation.
+            The hub CRS of a BoundCRS.
         """
         if self._target_crs is not None:
             return None if self._target_crs is False else self._target_crs
@@ -2595,7 +2598,12 @@ cdef class _CRS(Base):
             self._target_crs = False
             return None
         try:
-            self._target_crs = _CRS(_to_wkt(self.context, projobj))
+            self._target_crs = _CRS(_to_wkt(
+                self.context,
+                projobj,
+                version=WktVersion.WKT2_2019,
+                pretty=False,
+            ))
         finally:
             proj_destroy(projobj)
         return self._target_crs
@@ -2624,7 +2632,12 @@ cdef class _CRS(Base):
         self._sub_crs_list = []
         while projobj != NULL:
             try:
-                self._sub_crs_list.append(_CRS(_to_wkt(self.context, projobj)))
+                self._sub_crs_list.append(_CRS(_to_wkt(
+                    self.context,
+                    projobj,
+                    version=WktVersion.WKT2_2019,
+                    pretty=False,
+                )))
             finally:
                 proj_destroy(projobj)  # deallocate temp proj
             iii += 1
@@ -2655,7 +2668,12 @@ cdef class _CRS(Base):
             self._geodetic_crs = False
             return None
         try:
-            self._geodetic_crs = _CRS(_to_wkt(self.context, projobj))
+            self._geodetic_crs = _CRS(_to_wkt(
+                self.context,
+                projobj,
+                version=WktVersion.WKT2_2019,
+                pretty=False,
+            ))
         finally:
             proj_destroy(projobj)  # deallocate temp proj
         return self._geodetic_crs
@@ -2905,7 +2923,12 @@ cdef class _CRS(Base):
         if projobj == NULL:
             return self
         try:
-            crs_3d = _CRS(_to_wkt(self.context, projobj))
+            crs_3d = _CRS(_to_wkt(
+                self.context,
+                projobj,
+                version=WktVersion.WKT2_2019,
+                pretty=False,
+            ))
         finally:
             proj_destroy(projobj)
         return crs_3d

pyproj/_transformer.pxd

@@ -1,6 +1,6 @@
 include "proj.pxi"
 
-from pyproj._crs cimport Base
+from pyproj._crs cimport _CRS, Base
 
 
 cdef class _TransformerGroup:
@@ -14,6 +14,8 @@ cdef class _Transformer(Base):
     cdef readonly _area_of_use
     cdef readonly str type_name
     cdef readonly tuple _operations
+    cdef readonly _CRS _source_crs
+    cdef readonly _CRS _target_crs
 
     @staticmethod
     cdef _Transformer _from_pj(

pyproj/_transformer.pyi

@@ -56,6 +56,10 @@ class _Transformer(Base):
     @property
     def area_of_use(self) -> AreaOfUse: ...
     @property
+    def source_crs(self) -> Optional[_CRS]: ...
+    @property
+    def target_crs(self) -> Optional[_CRS]: ...
+    @property
     def operations(self) -> Union[Tuple[CoordinateOperation], None]: ...
     @property
     def is_network_enabled(self) -> bool: ...

pyproj/_transformer.pyx

@@ -15,13 +15,14 @@ from pyproj._crs cimport (
     CoordinateOperation,
     _get_concatenated_operations,
     _to_proj4,
+    _to_wkt,
     create_area_of_use,
 )
 from pyproj._datadir cimport pyproj_context_create, pyproj_context_destroy
 
 from pyproj._datadir import _LOGGER
 from pyproj.aoi import AreaOfInterest
-from pyproj.enums import ProjVersion, TransformDirection
+from pyproj.enums import ProjVersion, TransformDirection, WktVersion
 from pyproj.exceptions import ProjError
 
 # version number string for PROJ
@@ -798,6 +799,8 @@ cdef class _Transformer(Base):
         self._area_of_use = None
         self.type_name = "Unknown Transformer"
         self._operations = None
+        self._source_crs = None
+        self._target_crs = None
 
     def _initialize_from_projobj(self):
         self.proj_info = proj_pj_info(self.projobj)
@@ -841,6 +844,62 @@ cdef class _Transformer(Base):
         self._area_of_use = create_area_of_use(self.context, self.projobj)
         return self._area_of_use
 
+    @property
+    def source_crs(self):
+        """
+        .. versionadded:: 3.3.0
+
+        Returns
+        -------
+        Optional[_CRS]:
+            The source CRS of a CoordinateOperation.
+        """
+        if self._source_crs is not None:
+            return None if self._source_crs is False else self._source_crs
+        cdef PJ * projobj = proj_get_source_crs(self.context, self.projobj)
+        ProjError.clear()
+        if projobj == NULL:
+            self._source_crs = False
+            return None
+        try:
+            self._source_crs = _CRS(_to_wkt(
+                self.context,
+                projobj,
+                version=WktVersion.WKT2_2019,
+                pretty=False,
+            ))
+        finally:
+            proj_destroy(projobj)
+        return self._source_crs
+
+    @property
+    def target_crs(self):
+        """
+        .. versionadded:: 3.3.0
+
+        Returns
+        -------
+        Optional[_CRS]:
+            The target CRS of a CoordinateOperation.
+        """
+        if self._target_crs is not None:
+            return None if self._target_crs is False else self._target_crs
+        cdef PJ * projobj = proj_get_target_crs(self.context, self.projobj)
+        ProjError.clear()
+        if projobj == NULL:
+            self._target_crs = False
+            return None
+        try:
+            self._target_crs = _CRS(_to_wkt(
+                self.context,
+                projobj,
+                version=WktVersion.WKT2_2019,
+                pretty=False,
+            ))
+        finally:
+            proj_destroy(projobj)
+        return self._target_crs
+
     @property
     def operations(self):
         """

pyproj/transformer.py

@@ -416,6 +416,38 @@ def is_network_enabled(self) -> bool:
         """
         return self._transformer.is_network_enabled
 
+    @property
+    def source_crs(self) -> Optional[CRS]:
+        """
+        .. versionadded:: 3.3.0
+
+        Returns
+        -------
+        Optional[CRS]:
+            The source CRS of a CoordinateOperation.
+        """
+        return (
+            None
+            if self._transformer.source_crs is None
+            else CRS(self._transformer.source_crs)
+        )
+
+    @property
+    def target_crs(self) -> Optional[CRS]:
+        """
+        .. versionadded:: 3.3.0
+
+        Returns
+        -------
+        Optional[CRS]:
+            The target CRS of a CoordinateOperation.
+        """
+        return (
+            None
+            if self._transformer.target_crs is None
+            else CRS(self._transformer.target_crs)
+        )
+
     @staticmethod
     def from_proj(
         proj_from: Any,

test/test_transformer.py

@@ -1522,3 +1522,15 @@ def test_4d_transform__inplace__numpy__int():
     assert zarr[0] == 5264462
     assert tarr is not t_tarr
     assert tarr[0] == 2019
+
+
+def test_transformer_source_target_crs():
+    transformer = Transformer.from_crs("EPSG:4326", "EPSG:4258")
+    assert transformer.source_crs == "EPSG:4326"
+    assert transformer.target_crs == "EPSG:4258"
+
+
+def test_transformer_source_target_crs__none():
+    transformer = Transformer.from_pipeline("+init=ITRF2008:ITRF2000")
+    assert transformer.source_crs is None
+    assert transformer.target_crs is None