Change how numpy behaves as an optional import (#268)

- [x] Closes #267
- [x] Executed `pre-commit run --all-files` with no errors
- [x] The change is fully covered by automated unit tests
- [x] Documented in docs/ as appropriate
- [x] Added an entry to the CHANGES file

CHANGES.rst

@@ -4,6 +4,15 @@ Change Log
 Unreleased
 ----------
 
+Changes
+
+- Changed how `numpy` is handled as an optional dependency. Previously,
+   importing a `numpy`-dependent function, like `correlated_values`,
+   without `numpy` installed would result in an `ImportError` at import
+   time. Now such a function can be imported but if the user attempts to
+   execute it, a `NotImplementedError` is raised indicating that the
+   function can't be used because `numpy` couldn't be imported.
+
 Fixes:
 
 - fix `readthedocs` configuration so that the build passes (#254)

tests/test_uncertainties.py

@@ -2,9 +2,16 @@
 import math
 import random  # noqa
 
+import pytest
+
 import uncertainties.core as uncert_core
 from uncertainties.core import ufloat, AffineScalarFunc, ufloat_fromstr
-from uncertainties import umath
+from uncertainties import (
+    umath,
+    correlated_values,
+    correlated_values_norm,
+    correlation_matrix,
+)
 from helpers import (
     power_special_cases,
     power_all_cases,
@@ -15,6 +22,12 @@
 )
 
 
+try:
+    import numpy as np
+except ImportError:
+    np = None
+
+
 def test_value_construction():
     """
     Tests the various means of constructing a constant number with
@@ -1313,3 +1326,42 @@ def test_correlated_values_correlation_mat():
             numpy.array(cov_mat),
             numpy.array(uncert_core.covariance_matrix([x2, y2, z2])),
         )
+
+
+@pytest.mark.skipif(
+    np is not None,
+    reason="This test is only run when numpy is not installed.",
+)
+def test_no_numpy():
+    nom_values = [1, 2, 3]
+    std_devs = [0.1, 0.2, 0.3]
+    cov = [
+        [1, 0, 0],
+        [0, 1, 0],
+        [0, 0, 1],
+    ]
+
+    with pytest.raises(
+        NotImplementedError,
+        match="not able to import numpy",
+    ):
+        _ = correlated_values(nom_values, cov)
+
+    with pytest.raises(
+        NotImplementedError,
+        match="not able to import numpy",
+    ):
+        _ = correlated_values_norm(
+            list(zip(nom_values, std_devs)),
+            cov,
+        )
+
+    x = ufloat(1, 0.1)
+    y = ufloat(2, 0.2)
+    z = ufloat(3, 0.3)
+
+    with pytest.raises(
+        NotImplementedError,
+        match="not able to import numpy",
+    ):
+        _ = correlation_matrix([x, y, z])