Trac #27013: py3: some care for Hadamard matrices

URL: https://trac.sagemath.org/27013
Reported by: chapoton
Ticket author(s): Frédéric Chapoton
Reviewer(s): Travis Scrimshaw

src/sage/combinat/matrices/hadamard_matrix.py

@@ -987,7 +987,7 @@ def GS_skew_hadamard_smallcases(n, existence=False, check=True):
          williamson_goethals_seidel_skew_hadamard_matrix as WGS
 
     def pmtoZ(s):
-       return [1 if x == '+' else -1 for x in s]
+        return [1 if x == '+' else -1 for x in s]
 
     if existence:
         return n in [36, 52, 92]
@@ -1209,6 +1209,7 @@ def symmetric_conference_matrix(n, check=True):
         assert (C==C.T and C**2==(n-1)*I(n))
     return C
 
+
 def szekeres_difference_set_pair(m, check=True):
     r"""
     Construct Szekeres `(2m+1,m,1)`-cyclic difference family
@@ -1248,16 +1249,18 @@ def szekeres_difference_set_pair(m, check=True):
     t = F.multiplicative_generator()**2
     G = F.cyclotomic_cosets(t, cosets=[F.one()])[0]
     sG = set(G)
-    A = filter(lambda a: a-F.one() in sG, G)
-    B = filter(lambda b: b+F.one() in sG, G)
+    A = [a for a in G if a - F.one() in sG]
+    B = [b for b in G if b + F.one() in sG]
     if check:
         from itertools import product, chain
-        assert(len(list(A)) == len(list(B)) == m)
-        if m>1:
-            assert(sG==set([xy[0]/xy[1] for xy in chain(product(A,A), product(B,B))]))
-        assert(all(F.one()/b+F.one() in sG for b in B))
-        assert(not any(F.one()/a-F.one() in sG for a in A))
-    return G,A,B
+        assert(len(A) == len(B) == m)
+        if m > 1:
+            assert(sG == set([xy[0] / xy[1]
+                              for xy in chain(product(A, A), product(B, B))]))
+        assert(all(F.one() / b + F.one() in sG for b in B))
+        assert(not any(F.one() / a - F.one() in sG for a in A))
+    return G, A, B
+
 
 def typeI_matrix_difference_set(G,A):
     r"""