ENH: Added Transformer.get_last_used_operation

.pylintrc

@@ -23,4 +23,4 @@ disable=cyclic-import,
 
 
 [FORMAT]
-max-module-lines=1300
+max-module-lines=1350

docs/history.rst

@@ -8,6 +8,7 @@ Latest
 - BUG: Fix transformer list for 3D transformations in :class:`.TransformerGroup` (discussion #1072)
 - ENH: Added authority, accuracy, and allow_ballpark kwargs to :class:`.TransformerGroup` (pull #1076)
 - ENH: Added ``force_over`` kwarg to :meth:`.Transformer.from_crs` (issue #997)
+- ENH: Added :meth:`.Transformer.get_last_used_operation` (issue #1071)
 - CLN: Remove deprecated ``skip_equivalent`` kwarg from transformers and ``errcheck`` kwarg from :meth:`.CRS.from_cf` (pull #1077)
 - REF: use regex to process PROJ strings in :meth:`.CRS.to_dict` (pull #1086)
 - BUG: :class:`.MercatorAConversion` defined only for lat_0 = 0 (issue #1089)

pyproj/_transformer.pyi

@@ -65,6 +65,7 @@ class _Transformer(Base):
     def target_crs(self) -> Optional[_CRS]: ...
     @property
     def operations(self) -> Union[Tuple[CoordinateOperation], None]: ...
+    def get_last_used_operation(self) -> _Transformer: ...
     @property
     def is_network_enabled(self) -> bool: ...
     def to_proj4(

pyproj/_transformer.pyx

@@ -30,6 +30,10 @@ proj_version_str = f"{PROJ_VERSION_MAJOR}.{PROJ_VERSION_MINOR}.{PROJ_VERSION_PAT
 PROJ_VERSION = (PROJ_VERSION_MAJOR, PROJ_VERSION_MINOR, PROJ_VERSION_PATCH)
 _AUTH_CODE_RE = re.compile(r"(?P<authority>\w+)\:(?P<code>\w+)")
 
+IF (CTE_PROJ_VERSION_MAJOR, CTE_PROJ_VERSION_MINOR) >= (9, 1):
+    cdef extern from "proj.h" nogil:
+        PJ* proj_trans_get_last_used_operation(PJ *P)
+
 
 cdef dict _PJ_DIRECTION_MAP = {
     TransformDirection.FORWARD: PJ_FWD,
@@ -425,6 +429,29 @@ cdef class _Transformer(Base):
         self._operations = _get_concatenated_operations(self.context, self.projobj)
         return self._operations
 
+    def get_last_used_operation(self):
+        IF (CTE_PROJ_VERSION_MAJOR, CTE_PROJ_VERSION_MINOR) >= (9, 1):
+            cdef PJ* last_used_operation = proj_trans_get_last_used_operation(self.projobj)
+            if last_used_operation == NULL:
+                raise ProjError(
+                    "Last used operation not found. "
+                    "This is likely due to not initiating a transform."
+                )
+            cdef PJ_CONTEXT* context = NULL
+            try:
+                context = pyproj_context_create()
+            except:
+                proj_destroy(last_used_operation)
+                raise
+            proj_assign_context(last_used_operation, context)
+            return _Transformer._from_pj(
+                context,
+                last_used_operation,
+                False,
+            )
+        ELSE:
+            raise NotImplementedError("PROJ 9.1+ required to get last used operation.")
+
     @property
     def is_network_enabled(self):
         """

pyproj/proj.pxi

@@ -18,6 +18,7 @@ cdef extern from "proj.h" nogil:
     PJ_CONTEXT *proj_context_create ()
     PJ_CONTEXT *proj_context_clone (PJ_CONTEXT *ctx)
     PJ_CONTEXT *proj_context_destroy (PJ_CONTEXT *ctx)
+    void proj_assign_context(PJ* pj, PJ_CONTEXT* ctx)
 
     ctypedef enum PJ_LOG_LEVEL:
         PJ_LOG_NONE  = 0

pyproj/transformer.py

@@ -430,6 +430,23 @@ def operations(self) -> Optional[Tuple[CoordinateOperation]]:
         """
         return self._transformer.operations
 
+    def get_last_used_operation(self) -> "Transformer":
+        """
+        .. versionadded:: 3.4.0
+
+        .. note:: Requires PROJ 9.1+
+
+        See: :c:func:`proj_trans_get_last_used_operation`
+
+        Returns
+        -------
+        Transformer:
+            The operation used in the transform call.
+        """
+        return Transformer(
+            TransformerUnsafe(self._transformer.get_last_used_operation())
+        )
+
     @property
     def is_network_enabled(self) -> bool:
         """

test/test_transformer.py

@@ -1598,3 +1598,26 @@ def test_transformer_force_over():
     else:
         with pytest.raises(NotImplementedError, match="force_over requires PROJ 9"):
             Transformer.from_crs("EPSG:4326", "EPSG:3857", force_over=True)
+
+
+def test_transformer__get_last_used_operation():
+    transformer = Transformer.from_crs("EPSG:4326", "EPSG:3857")
+    if PROJ_GTE_91:
+        with pytest.raises(
+            ProjError,
+            match=(
+                r"Last used operation not found\. "
+                r"This is likely due to not initiating a transform\."
+            ),
+        ):
+            transformer.get_last_used_operation()
+        xxx, yyy = transformer.transform(1, 2)
+        operation = transformer.get_last_used_operation()
+        assert isinstance(operation, Transformer)
+        assert xxx, yyy == operation.transform(1, 2)
+    else:
+        with pytest.raises(
+            NotImplementedError,
+            match=r"PROJ 9\.1\+ required to get last used operation\.",
+        ):
+            transformer.get_last_used_operation()