Add custom IOError msg for file not found in loaddata

Author: bobleesj

src/diffpy/utils/parsers/loaddata.py

@@ -13,6 +13,8 @@
 #
 ##############################################################################
 
+import os
+
 import numpy
 
 
@@ -99,6 +101,10 @@ def countcolumnsvalues(line):
             nc = nv = 0
         return nc, nv
 
+    # Check if file exists before trying to open
+    if not os.path.exists(filename):
+        raise IOError(f"File {filename} cannot be found. Please rerun the program specifying a valid filename.")
+
     # make sure fid gets cleaned up
     with open(filename, "rb") as fid:
         # search for the start of datablock

tests/diffpy/utils/parsers/test_loaddata.py

@@ -15,8 +15,11 @@ def test_loadData_default(datafile):
     d2c = np.array([[3, 31], [4, 32], [5, 33]])
 
     with pytest.raises(IOError) as err:
-        loadData("doesnotexist")
-    assert "No such file or directory" in str(err.value)
+        loadData("doesnotexist.txt")
+    assert (
+        str(err.value)
+        == "File doesnotexist.txt cannot be found. Please rerun the program specifying a valid filename."
+    )
 
     # The default minrows=10 makes it read from the third line
     d = loadData(loaddata01)