gh-40439: simplify some nested for loops
    
either using walrus or simpler syntax

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40439
Reported by: Frédéric Chapoton
Reviewer(s): Martin Rubey

Author: Release Manager

src/sage/algebras/quatalg/quaternion_algebra.py

@@ -955,9 +955,10 @@ def maximal_order(self, take_shortcuts=True, order_basis=None):
             Order of Quaternion Algebra (-22, 210) with base ring Rational Field
              with basis (1, i, 1/2*i + 1/2*j, 1/2 + 17/22*i + 1/44*k)
 
-            sage: for d in ( m for m in range(1, 750) if is_squarefree(m) ):        # long time (3s)
-            ....:     A = QuaternionAlgebra(d)
-            ....:     assert A.maximal_order(take_shortcuts=False).is_maximal()
+            sage: Q = QuaternionAlgebra
+            sage: all(Q(d).maximal_order(take_shortcuts=False).is_maximal()
+            ....:     for d in range(1, 350) if is_squarefree(d))
+            True
 
         Specifying an order basis gives an extension of orders::
 

src/sage/categories/finite_semigroups.py

@@ -95,7 +95,7 @@ def j_classes(self):
             return self.cayley_graph(side='twosided', simple=True).strongly_connected_components()
 
         @cached_method
-        def j_classes_of_idempotents(self):
+        def j_classes_of_idempotents(self) -> list[list]:
             r"""
             Return all the idempotents of self, grouped by J-class.
 
@@ -108,7 +108,9 @@ def j_classes_of_idempotents(self):
                 [['a'], ['ab', 'ba'], ['abc', 'acb', 'bac', 'bca', 'cab', 'cba'],
                  ['ac', 'ca'], ['b'], ['bc', 'cb'], ['c']]
             """
-            return [l for l in ([x for x in cl if attrcall('is_idempotent')(x)] for cl in self.j_classes()) if len(l) > 0]
+            it = ([x for x in cl if attrcall('is_idempotent')(x)]
+                  for cl in self.j_classes())
+            return [ell for ell in it if ell]
 
         @cached_method
         def j_transversal_of_idempotents(self):

src/sage/combinat/partition.py

@@ -2043,7 +2043,7 @@ def cell_poset(self, orientation='SE'):
                         covers[(i, j)] = [(i, j - 1)]
         return Poset(covers)
 
-    def frobenius_coordinates(self):
+    def frobenius_coordinates(self) -> tuple[list, list]:
         """
         Return a pair of sequences of Frobenius coordinates aka beta numbers
         of the partition.
@@ -2065,11 +2065,11 @@ def frobenius_coordinates(self):
         mu = self
         muconj = mu.conjugate()     # Naive implementation
         if len(mu) <= len(muconj):
-            a = [x for x in (val-i-1 for i, val in enumerate(mu)) if x >= 0]
-            b = [x for x in (muconj[i]-i-1 for i in range(len(a))) if x >= 0]
+            a = [x for i, val in enumerate(mu) if (x := val - i - 1) >= 0]
+            b = [x for i in range(len(a)) if (x := muconj[i] - i - 1) >= 0]
         else:
-            b = [x for x in (val-i-1 for i, val in enumerate(muconj)) if x >= 0]
-            a = [x for x in (mu[i]-i-1 for i in range(len(b))) if x >= 0]
+            b = [x for i, val in enumerate(muconj) if (x := val - i - 1) >= 0]
+            a = [x for i in range(len(b)) if (x := mu[i] - i - 1) >= 0]
         return (a, b)
 
     def frobenius_rank(self):

src/sage/graphs/generators/smallgraphs.py

@@ -5473,8 +5473,9 @@ def JankoKharaghaniTonchevGraph():
             301, 304, 308, 309, 310, 312, 313, 314, 316, 317, 318)
     Gamma = Graph(multiedges=False, name='Janko-Kharaghani-Tonchev')
     for i, b in ((1, B1), (163, B163)):
-        for j in (x[0] for x in st.OrbitsDomain(b)):
-            Gamma.add_edges(map(tuple, G.Orbit(libgap.Set([i, j]), libgap.OnSets)))
+        for x in st.OrbitsDomain(b):
+            Gamma.add_edges(map(tuple, G.Orbit(libgap.Set([i, x[0]]),
+                                               libgap.OnSets)))
     Gamma.relabel(range(Gamma.order()))
     return Gamma
 

src/sage/graphs/orientations.py

@@ -1361,7 +1361,7 @@ def bounded_outdegree_orientation(G, bound, solver=None, verbose=False,
     d.add_edges(('s', vertices_id[v], b[v]) for v in vertices)
 
     d.add_edges(((vertices_id[u], vertices_id[v]), 't', 1)
-                 for u, v in G.edges(sort=False, labels=None))
+                for u, v in G.edges(sort=False, labels=None))
 
     # each v is linked to its incident edges
 
@@ -1381,7 +1381,7 @@ def bounded_outdegree_orientation(G, bound, solver=None, verbose=False,
     # The flow graph may not contain all the vertices, if they are
     # not part of the flow...
     edges = ((vertices[u], vertices[vv if vv != u else uu])
-             for u in (x for x in range(n) if x in flow)
+             for u in range(n) if u in flow
              for uu, vv in flow.neighbors_out(u))
 
     return _initialize_digraph(G, edges)