Fix test suite failure without numpy and improve module init diagnostics

Fixes #357.

tests/conftest.py

@@ -202,3 +202,27 @@ def pytest_namespace():
         'requires_eigen_and_scipy': skipif(not have_eigen or not scipy,
                                            reason="eigen and/or scipy are not installed"),
     }
+
+
+def _test_import_pybind11():
+    """Early diagnostic for test module initialization errors
+
+    When there is an error during initialization, the first import will report the
+    real error while all subsequent imports will report nonsense. This import test
+    is done early (in the pytest configuration file, before any tests) in order to
+    avoid the noise of having all tests fail with identical error messages.
+
+    Any possible exception is caught here and reported manually *without* the stack
+    trace. This further reduces noise since the trace would only show pytest internals
+    which are not useful for debugging pybind11 module issues.
+    """
+    # noinspection PyBroadException
+    try:
+        import pybind11_tests
+    except Exception as e:
+        print("Failed to import pybind11_tests from pytest:")
+        print("  {}: {}".format(type(e).__name__, e))
+        sys.exit(1)
+
+
+_test_import_pybind11()

tests/test_eigen.py

@@ -3,8 +3,7 @@ import pytest
 with pytest.suppress(ImportError):
     import numpy as np
 
-
+    ref = np.array([[ 0,  3,  0,  0,  0, 11],
-ref = np.array([[ 0,  3,  0,  0,  0, 11],
                     [22,  0,  0,  0, 17, 11],
                     [ 7,  5,  0,  1,  0, 11],
                     [ 0,  0,  0,  0,  0, 11],

tests/test_numpy_dtypes.cpp

@@ -268,6 +268,12 @@ py::list test_dtype_methods() {
 }
 
 void init_ex_numpy_dtypes(py::module &m) {
+    try {
+        py::module::import("numpy");
+    } catch (...) {
+        return;
+    }
+
     PYBIND11_NUMPY_DTYPE(SimpleStruct, x, y, z);
     PYBIND11_NUMPY_DTYPE(PackedStruct, x, y, z);
     PYBIND11_NUMPY_DTYPE(NestedStruct, a, b);

tests/test_numpy_dtypes.py

@@ -2,15 +2,15 @@ import pytest
 with pytest.suppress(ImportError):
     import numpy as np
 
+    simple_dtype = np.dtype({'names': ['x', 'y', 'z'],
+                             'formats': ['?', 'u4', 'f4'],
+                             'offsets': [0, 4, 8]})
+    packed_dtype = np.dtype([('x', '?'), ('y', 'u4'), ('z', 'f4')])
+
 
 def assert_equal(actual, expected_data, expected_dtype):
     np.testing.assert_equal(actual, np.array(expected_data, dtype=expected_dtype))
 
-simple_dtype = np.dtype({'names': ['x', 'y', 'z'],
-                         'formats': ['?', 'u4', 'f4'],
-                         'offsets': [0, 4, 8]})
-packed_dtype = np.dtype([('x', '?'), ('y', 'u4'), ('z', 'f4')])
-
 
 @pytest.requires_numpy
 def test_format_descriptors():