Merge pull request #27234 from charris/backport-25984

BUG: Allow fitting of degree zero polynomials with Polynomial.fit

numpy/polynomial/_polybase.py

@@ -1041,6 +1041,9 @@ class domain in NumPy 1.4 and ``None`` in later versions.
         """
         if domain is None:
             domain = pu.getdomain(x)
+            if domain[0] == domain[1]:
+                domain[0] -= 1
+                domain[1] += 1   
         elif type(domain) is list and len(domain) == 0:
             domain = cls.domain
 

numpy/polynomial/tests/test_polynomial.py

@@ -5,11 +5,12 @@
 from fractions import Fraction
 import numpy as np
 import numpy.polynomial.polynomial as poly
+import numpy.polynomial.polyutils as pu
 import pickle
 from copy import deepcopy
 from numpy.testing import (
     assert_almost_equal, assert_raises, assert_equal, assert_,
-    assert_array_equal, assert_raises_regex)
+    assert_array_equal, assert_raises_regex, assert_warns)
 
 
 def trim(x):
@@ -628,6 +629,14 @@ def test_polyline(self):
     def test_polyline_zero(self):
         assert_equal(poly.polyline(3, 0), [3])
 
+    def test_fit_degenerate_domain(self):
+        p = poly.Polynomial.fit([1], [2], deg=0)
+        assert_equal(p.coef, [2.])
+        p = poly.Polynomial.fit([1, 1], [2, 2.1], deg=0)
+        assert_almost_equal(p.coef, [2.05])
+        with assert_warns(pu.RankWarning):
+            p = poly.Polynomial.fit([1, 1], [2, 2.1], deg=1)
+
     def test_result_type(self):
         w = np.array([-1, 1], dtype=np.float32)
         p = np.polynomial.Polynomial(w, domain=w, window=w)