Fix test function for empty ICGEM gdf file (#345)

The test for checking if proper errors and warnings were raised when reading an empty ICGEM gdf file was failing in my local system due to a change in the Numpy warning for the empty file. Took the opportunity to refactor the test using pytest.raises and pytest.warns to capture the errors and warnings.

harmonica/tests/test_icgem.py

@@ -11,7 +11,8 @@
 
 import numpy as np
 import numpy.testing as npt
-from pytest import raises
+import pytest
+from pytest import raises, warns
 
 from .. import load_icgem_gdf
 
@@ -280,14 +281,20 @@ def test_corrupt_area(tmpdir):
  load_icgem_gdf(corrupt)
 
 
-def test_empty_file(tmpdir, recwarn):
- "Empty ICGEM file"
+@pytest.fixture(name="empty_fname")
+def fixture_empty_fname(tmpdir):
+ """
+ Return the path to a temporary empty file
+ """
  empty_fname = str(tmpdir.join("empty.gdf"))
  with open(empty_fname, "w") as gdf_file:
  gdf_file.write("")
- with raises(IOError):
+ return empty_fname
+
+
+def test_empty_file(empty_fname):
+ "Empty ICGEM file"
+ error = raises(IOError, match=r"Couldn't read \w+ field from gdf file header")
+ warn = warns(UserWarning, match=r"loadtxt: input contained no data")
+ with error, warn:
  load_icgem_gdf(empty_fname)
- assert len(recwarn) == 1
- warning = recwarn.pop()
- assert warning.category == UserWarning
- assert "Empty input file" in str(warning.message)