GMTDataArrayAccessor: Fallback to default grid registration and gtype…

… if the grid source file doesn't exist (#2009)

Co-authored-by: Wei Ji <23487320+weiji14@users.noreply.github.com>
Co-authored-by: Michael Grund <23025878+michaelgrund@users.noreply.github.com>

pygmt/accessors.py

@@ -1,6 +1,8 @@
 """
 GMT accessor methods.
 """
+from pathlib import Path
+
 import xarray as xr
 from pygmt.exceptions import GMTInvalidInput
 from pygmt.src.grdinfo import grdinfo
@@ -50,16 +52,22 @@ class GMTDataArrayAccessor:
 
     def __init__(self, xarray_obj):
         self._obj = xarray_obj
-        try:
-            self._source = self._obj.encoding["source"]  # filepath to NetCDF source
-            # Get grid registration and grid type from the last two columns of
-            # the shortened summary information of `grdinfo`.
-            self._registration, self._gtype = map(
-                int, grdinfo(self._source, per_column="n").split()[-2:]
-            )
-        except (KeyError, ValueError):
+
+        self._source = self._obj.encoding.get("source")
+        if self._source is not None and Path(self._source).exists():
+            try:
+                # Get grid registration and grid type from the last two columns
+                # of the shortened summary information of `grdinfo`.
+                self._registration, self._gtype = map(
+                    int, grdinfo(self._source, per_column="n").split()[-2:]
+                )
+            except ValueError:
+                self._registration = 0  # Default to Gridline registration
+                self._gtype = 0  # Default to Cartesian grid type
+        else:
             self._registration = 0  # Default to Gridline registration
             self._gtype = 0  # Default to Cartesian grid type
+        del self._source
 
     @property
     def registration(self):

pygmt/tests/test_accessor.py

@@ -3,11 +3,13 @@
 """
 import os
 import sys
+from pathlib import Path
 
 import pytest
 import xarray as xr
 from packaging.version import Version
 from pygmt import __gmt_version__, which
+from pygmt.datasets import load_earth_relief
 from pygmt.exceptions import GMTInvalidInput
 
 
@@ -101,3 +103,32 @@ def test_accessor_sliced_datacube():
         assert grid.gmt.gtype == 1  # geographic coordinate type
     finally:
         os.remove(fname)
+
+
+def test_accessor_grid_source_file_not_exist():
+    """
+    Check that the accessor fallbacks to the default registration and gtype
+    when the grid source file (i.e., grid.encoding["source"]) doesn't exist.
+    """
+    # Load the 05m earth relief grid, which is stored as tiles
+    grid = load_earth_relief(
+        resolution="05m", region=[0, 5, -5, 5], registration="pixel"
+    )
+    # Registration and gtype are correct
+    assert grid.gmt.registration == 1
+    assert grid.gmt.gtype == 1
+    # The source grid file is defined but doesn't exist
+    assert grid.encoding["source"].endswith(".nc")
+    assert not Path(grid.encoding["source"]).exists()
+
+    # For a sliced grid, fallback to default registration and gtype,
+    # because the source grid file doesn't exist.
+    sliced_grid = grid[1:3, 1:3]
+    assert sliced_grid.gmt.registration == 0
+    assert sliced_grid.gmt.gtype == 0
+
+    # Still possible to manually set registration and gtype
+    sliced_grid.gmt.registration = 1
+    sliced_grid.gmt.gtype = 1
+    assert sliced_grid.gmt.registration == 1
+    assert sliced_grid.gmt.gtype == 1