From 948e51d6f133fbf6a10d016639dd5a08c7beec02 Mon Sep 17 00:00:00 2001
From: Gabe Joseph <gjoseph92@gmail.com>
Date: Thu, 20 Jan 2022 16:11:15 -0700
Subject: [PATCH] Catch some mosaic usage errors with `nodata=nan` (#123)

If you've specified `dtype=int` to `stackstac.stack`, this is an easy way to catch that you need to use a custom nodata value.
---
 stackstac/ops.py | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/stackstac/ops.py b/stackstac/ops.py
index 924d585..a8570ed 100644
--- a/stackstac/ops.py
+++ b/stackstac/ops.py
@@ -45,11 +45,21 @@ def mosaic(
     nodata:
         The value to treat as invalid. Default: NaN.
 
+        To catch common mis-use, raises a ``ValueError`` if ``nodata=nan``
+        is used when the array has an integer or boolean dtype. Since NaN
+        cannot exist in those arrays, this indicates a different ``nodata``
+        value needs to be used.
+
     Returns
     -------
     xarray.DataArray:
         The mosaicked `~xarray.DataArray`.
     """
+    if np.isnan(nodata) and arr.dtype.kind in "biu":
+        # Try to catch usage errors forgetting to set `nodata=`
+        raise ValueError(
+            f"Cannot use {nodata=} when mosaicing a {arr.dtype} array, since {nodata} cannot exist in the array."
+        )
     return arr.reduce(
         _mosaic,
         dim=dim,