Do not change input inplace

Author: ricardoV94

pymc3/math.py

@@ -244,11 +244,12 @@ def log1mexp_numpy(x):
     https://cran.r-project.org/web/packages/Rmpfr/vignettes/log1mexp-note.pdf
     """
     x = np.asarray(x)
-    mask = x < 0.6931471805599453
-    x[mask] = np.log(-np.expm1(-x[mask]))
+    out = np.empty_like(x)
+    mask = x < 0.6931471805599453  # log(2)
+    out[mask] = np.log(-np.expm1(-x[mask]))
     mask = ~mask
-    x[mask] = np.log1p(-np.exp(-x[mask]))
-    return x
+    out[mask] = np.log1p(-np.exp(-x[mask]))
+    return out
 
 
 def flatten_list(tensors):

pymc3/tests/test_math.py

@@ -133,6 +133,7 @@ def test_log1pexp():
 
 def test_log1mexp():
     vals = np.array([-1, 0, 1e-20, 1e-4, 10, 100, 1e20])
+    vals_ = vals.copy()
     # import mpmath
     # mpmath.mp.dps = 1000
     # [float(mpmath.log(1 - mpmath.exp(-x))) for x in vals]
@@ -151,6 +152,8 @@ def test_log1mexp():
     npt.assert_allclose(actual, expected)
     actual_ = log1mexp_numpy(vals)
     npt.assert_allclose(actual_, expected)
+    # Check that input was not changed in place
+    npt.assert_allclose(vals, vals_)
 
 
 def test_log1mexp_numpy_no_warning():