Trac #27914. py3: fix hash of multivariate Laurent polynomials

src/sage/rings/polynomial/laurent_polynomial.pyx

@@ -560,9 +560,6 @@ cdef class LaurentPolynomial_univariate(LaurentPolynomial):
             sage: hash(R.zero()) == hash(t - t)
             True
         """
-        if self.__n == 0:
-            return hash(self.__u)
-
         # we reimplement below the hash of polynomials to handle negative
         # degrees
         cdef long result = 0
@@ -573,10 +570,11 @@ cdef class LaurentPolynomial_univariate(LaurentPolynomial):
             result_mon = hash(self.__u[i])
             if result_mon:
                 j = i + self.__n
-                result_mon = (1000003 * result_mon) ^ var_hash_name
                 if j > 0:
+                    result_mon = (1000003 * result_mon) ^ var_hash_name
                     result_mon = (1000003 * result_mon) ^ j
                 elif j < 0:
+                    result_mon = (1000003 * result_mon) ^ var_hash_name
                     result_mon = (700005 * result_mon) ^ j
                 result += result_mon
         return result
@@ -1905,7 +1903,7 @@ cdef class LaurentPolynomial_mpair(LaurentPolynomial):
         r"""
         TESTS:
 
-        Test that the hash is non-constant::
+        Test that the hash is non-constant (see also :trac:`27914`)::
 
             sage: L.<w,z> = LaurentPolynomialRing(QQ)
             sage: len({hash(w^i*z^j) for i in [-2..2] for j in [-2..2]})
@@ -1941,6 +1939,18 @@ cdef class LaurentPolynomial_mpair(LaurentPolynomial):
             True
             sage: hash(1 - 7*x0 + x1*x2) == hash(L(1 - 7*x0 + x1*x2))
             True
+
+        Check that :trac:`27914` is fixed::
+
+            sage: L.<w,z> = LaurentPolynomialRing(QQ)
+            sage: Lw = LaurentPolynomialRing(QQ, 'w')
+            sage: Lz = LaurentPolynomialRing(QQ, 'z')
+            sage: all(hash(w^k) == hash(Lw(w^k))
+            ....:     and hash(z^k) == hash(Lz(z^k)) for k in (-5..5))
+            True
+            sage: p = w^-1 + 2 + w
+            sage: hash(p) == hash(Lw(p))
+            True
         """
         # we reimplement the hash from multipolynomial to handle negative exponents
         # (see multi_polynomial.pyx)
@@ -1955,10 +1965,12 @@ cdef class LaurentPolynomial_mpair(LaurentPolynomial):
             if c_hash != 0:
                 for p in range(n):
                     exponent = m[p] + self._mon[p]
-                    if not exponent:
-                        continue
-                    c_hash = (1000003 * c_hash) ^ var_name_hash[p]
-                    c_hash = (1000003 * c_hash) ^ exponent
+                    if exponent > 0:
+                        c_hash = (1000003 * c_hash) ^ var_name_hash[p]
+                        c_hash = (1000003 * c_hash) ^ exponent
+                    elif exponent < 0:
+                        c_hash = (1000003 * c_hash) ^ var_name_hash[p]
+                        c_hash = (700005 * c_hash) ^ exponent
                 result += c_hash
 
         return result