gh-40409: minor details in modular/
    
about using enumerate and not using .keys()

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40409
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/modular/abvar/abvar.py

@@ -4452,19 +4452,17 @@ def decomposition(self, simple=True, bound=None):
                 D = []
                 for N in reversed(divisors(M)):
                     if N > 1:
-                        isogeny_number = 0
                         A = amb.modular_symbols_of_level(N).cuspidal_subspace().new_subspace()
                         if bound is None:
                             X = factor_new_space(A)
                         else:
                             X = A.decomposition(bound=bound)
-                        for B in X:
+                        for isogeny_number, B in enumerate(X):
                             D.extend(ModularAbelianVariety_modsym(B.degeneracy_map(M, t).image(),
                                                                   is_simple=True, newform_level=(N, G),
                                                                   isogeny_number=isogeny_number,
                                                                   number=(t, M))
                                      for t in divisors(M // N))
-                            isogeny_number += 1
             elif A == amb.cuspidal_submodule():
                 D = [ModularAbelianVariety_modsym(B)
                      for B in A.decomposition(bound=bound)]

src/sage/modular/btquotients/btquotient.py

@@ -2916,29 +2916,27 @@ def _get_hecke_data(self, l):
                     if prod([self._character(ZZ((v * Matrix(ZZ, 4, 1, g))[0, 0]))
                              / self._character(p ** (nninc // 2))
                              for v in self.get_extra_embedding_matrices()]) == 1]
-        n_iters = 0
 
         def enumerate_words(v, n=None):
             if n is None:
                 n = []
             while True:
                 add_new = True
-                for jj in range(len(n)):
-                    n[jj] += 1
-                    if n[jj] != len(v):
+                for j in range(len(n)):
+                    n[j] += 1
+                    if n[j] != len(v):
                         add_new = False
                         break
-                    else:
-                        n[jj] = 0
+                    n[j] = 0
                 if add_new:
                     n.append(0)
                 yield [v[x] for x in n]
 
-        for wd in enumerate_words([self._conv(x) for x in letters]):
+        conv_list = [self._conv(x) for x in letters]
+        for n_iters, wd in enumerate(enumerate_words(conv_list)):
             if len(T) == l + 1:
                 break
             v = prod(wd)
-            n_iters += 1
             v0 = v * alpha0
             vinv = self.get_quaternion_algebra()(v0 ** (-1))
             new = True

src/sage/modular/modform/element.py

@@ -3810,8 +3810,8 @@ def _mul_(self, other):
         f_other = other._forms_dictionary
         f_mul = defaultdict(int)
 
-        for k_self in f_self.keys():
-            for k_other in f_other.keys():
+        for k_self in f_self:
+            for k_other in f_other:
                 f_mul[k_self + k_other] += f_self[k_self] * f_other[k_other]
 
         return GM(self.parent(), f_mul)

src/sage/modular/modsym/ambient.py

@@ -980,16 +980,14 @@ def _compute_hecke_matrix_prime(self, p, rows=None):
         K = self.base_ring()
         W = R.new_matrix(nrows=len(B), ncols=R.nrows())
         syms = self.manin_symbols()
-        j = 0
-        for i in B:
+        for j, i in enumerate(B):
             for h in H:
                 entries = syms.apply(i, h)
                 for k, x in entries:
                     f, s = mod2term[k]
                     if s:
                         # W[j,f] = W[j,f] + s*K(x)
                         W.add_to_entry(j, f, s * K(x))
-            j += 1
         tm = verbose("start matrix multiply", tm)
         if hasattr(W, '_matrix_times_matrix_dense'):
             Tp = W._matrix_times_matrix_dense(R)
@@ -2937,9 +2935,8 @@ def _compute_hecke_matrix_prime(self, p, rows=None):
         mod2term = self._mod2term
         R = self.manin_gens_to_basis()
         W = R.new_matrix(nrows=len(B), ncols=R.nrows())  # the 0 with given number of rows and cols.
-        j = 0
         tm = verbose("Matrix non-reduced", tm)
-        for i in B:
+        for j, i in enumerate(B):
             # The following step is where most of the time is spent.
             c, d = P1[i]
             v = H.apply(c, d, N)
@@ -2960,7 +2957,6 @@ def _compute_hecke_matrix_prime(self, p, rows=None):
                     f, s = mod2term[k]
                     if s != 0:
                         W[j, f] = W[j, f] + s*m
-            j += 1
         tm = verbose("done making non-reduced matrix", tm)
         verbose("start matrix-matrix (%s x %s) times (%s x %s) multiply to get Tp" % (W.nrows(), W.ncols(),
                                                                                       R.nrows(), R.ncols()))

src/sage/modular/quasimodform/element.py

@@ -617,7 +617,7 @@ def homogeneous_components(self):
         for i, c in enumerate(self._polynomial.coefficients(sparse=False)):
             if c:
                 forms = c._forms_dictionary
-                for k in forms.keys():
+                for k in forms:
                     try:
                         components[ZZ(k + 2*i)] += QM(forms[k]*(E2**i))
                     except KeyError: