sagemathgh-38635: fixing most ruff PERF4 warnings in combinat
    
this is fixing most suggestions of `ruff check --select=PERF4` in the
combinat folder.

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.
    
URL: sagemath#38635
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe

Author: Release Manager

src/sage/combinat/composition_tableau.py

@@ -211,10 +211,7 @@ def descent_set(self):
             sage: CompositionTableau([[1],[3,2],[4,4]]).descent_set()
             [1, 3]
         """
-        cols = {}
-        for row in self:
-            for col, i in enumerate(row):
-                cols[i] = col
+        cols = {i: col for row in self for col, i in enumerate(row)}
         return sorted(i for i in cols if i + 1 in cols and cols[i + 1] >= cols[i])
 
     def descent_composition(self):

src/sage/combinat/diagram.py

@@ -336,12 +336,9 @@ def _latex_(self):
 
         lr = r'\def\lr#1{\multicolumn{1}{|@{\hspace{.6ex}}c@{\hspace{.6ex}}|}{\raisebox{-.3ex}{$#1$}}}'
 
-        array = []
-        for i in range(self._n_rows):
-            row = []
-            for j in range(self._n_cols):
-                row.append("\\phantom{x}" if (i, j) in self else None)
-            array.append(row)
+        array = [[("\\phantom{x}" if (i, j) in self else None)
+                  for j in range(self._n_cols)]
+                 for i in range(self._n_rows)]
 
         def end_line(r):
             # give the line ending to row ``r``

src/sage/combinat/diagram_algebras.py

@@ -3013,7 +3013,7 @@ def jucys_murphy_element(self, i):
             sage: L = [P.L(i/2) for i in range(1,2*k+1)]
             sage: all(x.dual() == x for x in L)
             True
-            sage: all(x * y == y * x for x in L for y in L)  # long time
+            sage: all(x * y == y * x for x, y in Subsets(L, 2))  # long time
             True
             sage: Lsum = sum(L)
             sage: gens = [P.s(i) for i in range(1,k)]
@@ -3045,13 +3045,13 @@ def jucys_murphy_element(self, i):
 
         The same tests for a half integer partition algebra::
 
-            sage: k = 9/2
+            sage: k = 7/2
             sage: R.<n> = QQ[]
             sage: P = PartitionAlgebra(k, n)
             sage: L = [P.L(i/2) for i in range(1,2*k+1)]
             sage: all(x.dual() == x for x in L)
             True
-            sage: all(x * y == y * x for x in L for y in L)  # long time
+            sage: all(x * y == y * x for x, y in Subsets(L, 2))  # long time
             True
             sage: Lsum = sum(L)
             sage: gens = [P.s(i) for i in range(1,k-1/2)]
@@ -5847,11 +5847,9 @@ def to_Brauer_partition(l, k=None):
         True
     """
     L = to_set_partition(l, k=k)
-    L2 = []
     paired = []
     not_paired = []
-    for i in L:
-        L2.append(list(i))
+    L2 = (list(i) for i in L)
     for i in L2:
         if len(i) > 2:
             raise ValueError("blocks must have size at most 2, but {} has {}".format(i, len(i)))

src/sage/combinat/dlx.py

@@ -484,10 +484,7 @@ def AllExactCovers(M):
     ones = []
     r = 1    # damn 1-indexing
     for R in M.rows():
-        row = []
-        for i in range(len(R)):
-            if R[i]:
-                row.append(i + 1)  # damn 1-indexing
+        row = [i for i, Ri in enumerate(R, start=1) if Ri]
         ones.append([r, row])
         r += 1
     for s in DLXMatrix(ones):

src/sage/combinat/finite_state_machine.py

@@ -9437,9 +9437,9 @@ def graph(self, edge_labels='words_in_out'):
             transitions = state.transitions
             if not transitions:
                 isolated_vertices.append(state.label())
-            for t in transitions:
-                graph_data.append((t.from_state.label(), t.to_state.label(),
-                                   label_fct(t)))
+            graph_data.extend((t.from_state.label(), t.to_state.label(),
+                               label_fct(t))
+                              for t in transitions)
 
         G = DiGraph(graph_data, multiedges=True, loops=True)
         G.add_vertices(isolated_vertices)

src/sage/combinat/free_dendriform_algebra.py

@@ -939,14 +939,12 @@ def merge(self, other):
                 return self
             ret = list(self.vars)
             cur_vars = set(ret)
-            for v in other.vars:
-                if v not in cur_vars:
-                    ret.append(v)
+            ret.extend(v for v in other.vars if v not in cur_vars)
             return DendriformFunctor(Alphabet(ret))
-        else:
-            return None
 
-    def _repr_(self):
+        return None
+
+    def _repr_(self) -> str:
         """
         TESTS::
 

src/sage/combinat/free_prelie_algebra.py

@@ -1023,14 +1023,12 @@ def merge(self, other):
                 return self
             ret = list(self.vars)
             cur_vars = set(ret)
-            for v in other.vars:
-                if v not in cur_vars:
-                    ret.append(v)
+            ret.extend(v for v in other.vars if v not in cur_vars)
             return PreLieFunctor(Alphabet(ret))
-        else:
-            return None
 
-    def _repr_(self):
+        return None
+
+    def _repr_(self) -> str:
         """
         TESTS::
 

src/sage/combinat/gelfand_tsetlin_patterns.py

@@ -302,11 +302,9 @@ def boxed_entries(self) -> tuple:
             sage: G.boxed_entries()
             ((1, 0),)
         """
-        ret = []
-        for i in range(1, len(self)):
-            for j in range(len(self[i])):
-                if self[i][j] == self[i - 1][j]:
-                    ret.append((i, j))
+        ret = [(i, j) for i in range(1, len(self))
+               for j, selfij in enumerate(self[i])
+               if selfij == self[i - 1][j]]
         return tuple(ret)
 
     @cached_method
@@ -324,11 +322,9 @@ def circled_entries(self) -> tuple:
             sage: G.circled_entries()
             ((1, 1), (2, 0))
         """
-        ret = []
-        for i in range(1, len(self)):
-            for j in range(len(self[i])):
-                if self[i][j] == self[i - 1][j + 1]:
-                    ret.append((i, j))
+        ret = [(i, j) for i in range(1, len(self))
+               for j, selfij in enumerate(self[i])
+               if selfij == self[i - 1][j + 1]]
         return tuple(ret)
 
     @cached_method
@@ -349,11 +345,9 @@ def special_entries(self) -> tuple:
             sage: G.special_entries()
             ((2, 0),)
         """
-        ret = []
-        for i in range(1, len(self)):
-            for j in range(len(self[i])):
-                if self[i-1][j] > self[i][j] and self[i][j] > self[i-1][j+1]:
-                    ret.append((i, j))
+        ret = [(i, j) for i in range(1, len(self))
+               for j, selfij in enumerate(self[i])
+               if self[i - 1][j] > selfij > self[i - 1][j + 1]]
         return tuple(ret)
 
     def number_of_boxes(self) -> int:

src/sage/combinat/graph_path.py

@@ -236,11 +236,7 @@ def paths_from_source_to_target(self, source, target):
             [[2, 3, 4], [2, 4]]
         """
         source_paths = self.outgoing_paths(source)
-        paths = []
-        for path in source_paths:
-            if path[-1] == target:
-                paths.append(path)
-        return paths
+        return [path for path in source_paths if path[-1] == target]
 
     def paths(self):
         """

src/sage/combinat/integer_vector.py

@@ -1174,19 +1174,17 @@ def _list_rec(self, n, k):
         EXAMPLES::
 
             sage: IV = IntegerVectors(2,3)
-            sage: IV._list_rec(2,3)
+            sage: list(IV._list_rec(2,3))
             [(2, 0, 0), (1, 1, 0), (1, 0, 1), (0, 2, 0), (0, 1, 1), (0, 0, 2)]
         """
-        res = []
-
         if k == 1:
-            return [(n, )]
+            yield (n,)
+            return
 
         for nbar in range(n + 1):
             n_diff = n - nbar
             for rest in self._list_rec(nbar, k - 1):
-                res.append((n_diff,) + rest)
-        return res
+                yield (n_diff,) + rest
 
     def __iter__(self):
         """