Add helpful error when attempting to download with missing system:tim…

…e_start (#54)

test/test_image.py

@@ -6,6 +6,7 @@
 import rasterio
 
 import wxee
+from wxee.exceptions import MissingPropertyError
 
 
 @pytest.mark.ee
@@ -155,3 +156,14 @@ def test_to_tif():
  assert src.descriptions == ("band_name",)
 
  os.remove(file[0])
+
+
+@pytest.mark.ee
+def test_missing_start_time():
+ """
+ Check that a helpful error is thrown when attempting to download an image with no
+ system:time_start property
+ """
+ img = ee.Image.constant(0)
+ with pytest.raises(MissingPropertyError):
+ img.wx.to_xarray()

wxee/exceptions.py

@@ -2,3 +2,9 @@ class DownloadError(Exception):
  """Raised when a download fails."""
 
  pass
+
+
+class MissingPropertyError(Exception):
+ """Raised when an Earth Engine object is missing a required property."""
+
+ pass

wxee/image.py

@@ -9,7 +9,7 @@
 
 from wxee import constants
 from wxee.accessors import wx_accessor
-from wxee.exceptions import DownloadError
+from wxee.exceptions import DownloadError, MissingPropertyError
 from wxee.utils import (
  _dataset_from_files,
  _download_url,
@@ -255,7 +255,21 @@ def _get_download_id(self) -> ee.String:
  the wx:dimension and wx:coordinate have not been set, they will be set to "time" and the formatted system:time_start, respectively.
  """
  img = self._obj
- date = _format_date(ee.Image(img).get("system:time_start"))
+
+ try:
+ date = _format_date(ee.Image(img).get("system:time_start")).getInfo()
+ except ee.EEException as e:
+ if "Parameter 'value' is required" in str(e):
+ raise MissingPropertyError(
+ f"Image is missing a `system:time_start` property which is required for "
+ "downloading.\n\nEarth Engine properties can be lost when modifying images, so make sure to manually "
+ "set or copy properties using the `.set` and `.copyProperties` methods after using methods like "
+ "`.multiply` or `.median`. If you don't need time information, you can set an arbitrary time with "
+ "`img = img.set('system:time_start', 0).`"
+ "\n\nSee https://github.com/aazuspan/wxee/issues/43 for more details."
+ )
+ else:
+ raise e
 
  original_id = _replace_if_null(img.get("system:id"), "null")
  # Replace any invalid file path characters with underscores.