From ed0397ce0b65b72aaa1d39796a25286eb5a3167f Mon Sep 17 00:00:00 2001
From: Deepak Cherian <deepak@cherian.net>
Date: Wed, 26 Jun 2024 15:51:44 -0400
Subject: [PATCH] Add .cf.crs attribute

---
 cf_xarray/accessor.py | 20 ++++++++++++++++++++
 doc/api.rst           |  2 ++
 pyproject.toml        |  1 +
 3 files changed, 23 insertions(+)

diff --git a/cf_xarray/accessor.py b/cf_xarray/accessor.py
index a915335b8..541150131 100644
--- a/cf_xarray/accessor.py
+++ b/cf_xarray/accessor.py
@@ -8,6 +8,7 @@
 from collections.abc import Hashable, Iterable, Mapping, MutableMapping, Sequence
 from datetime import datetime
 from typing import (
+    TYPE_CHECKING,
     Any,
     Callable,
     Literal,
@@ -57,6 +58,9 @@
     parse_cf_standard_name_table,
 )
 
+if TYPE_CHECKING:
+    from pyproj import CRS
+
 FlagParam = namedtuple("FlagParam", ["flag_mask", "flag_value"])
 
 #: Classes wrapped by cf_xarray.
@@ -2342,6 +2346,22 @@ def __getitem__(self, key: Hashable | Iterable[Hashable]) -> DataArray | Dataset
         """
         return _getitem(self, key)
 
+    @property
+    def crs(self) -> CRS:
+        """
+        Returns a ``pyproj.CRS`` instance constructed from the grid_mapping variable's attributes.
+        """
+        from pyproj import CRS
+
+        try:
+            gm = self["grid_mapping"]
+        except KeyError as e:
+            raise ValueError(
+                "CRS is constructed from the 'grid_mapping' variable, but that could not be detected."
+            ) from e
+
+        return CRS.from_cf(gm.attrs)
+
     @property
     def formula_terms(self) -> dict[Hashable, dict[str, str]]:  # numpydoc ignore=SS06
         """
diff --git a/doc/api.rst b/doc/api.rst
index 4651f017e..cffd4295b 100644
--- a/doc/api.rst
+++ b/doc/api.rst
@@ -45,6 +45,7 @@ Attributes
     DataArray.cf.cell_measures
     DataArray.cf.cf_roles
     DataArray.cf.coordinates
+    DataArray.cf.crs
     DataArray.cf.formula_terms
     DataArray.cf.grid_mapping_name
     DataArray.cf.is_flag_variable
@@ -103,6 +104,7 @@ Attributes
     Dataset.cf.bounds
     Dataset.cf.cell_measures
     Dataset.cf.cf_roles
+    Dataset.cf.crs
     Dataset.cf.coordinates
     Dataset.cf.formula_terms
     Dataset.cf.grid_mapping_names
diff --git a/pyproject.toml b/pyproject.toml
index 073b97777..f0a86ebbf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -139,6 +139,7 @@ module=[
     "pandas",
     "pooch",
     "pint",
+    "pyproj",
     "matplotlib",
     "pytest",
     "shapely",