do not evaluate unnecessarily

src/sage/data_structures/stream.py

@@ -1853,16 +1853,19 @@ def get_coefficient(self, n):
         fv = self._left._approximate_order
         gv = self._right._approximate_order
         if n < 0:
-            return sum(self._left[i] * self._neg_powers[-i][n]
-                       for i in range(fv, n // gv + 1))
+            return sum(f_coeff_i * self._neg_powers[-i][n]
+                       for i in range(fv, n // gv + 1)
+                       if (f_coeff_i := self._left[i]))
         # n > 0
         while len(self._pos_powers) <= n // gv:
             # TODO: possibly we always want a dense cache here?
             self._pos_powers.append(Stream_cauchy_mul(self._pos_powers[-1], self._right, self._is_sparse))
-        ret = sum(self._left[i] * self._neg_powers[-i][n] for i in range(fv, 0))
+        ret = sum(f_coeff_i * self._neg_powers[-i][n] for i in range(fv, 0)
+                  if (f_coeff_i := self._left[i]))
         if n == 0:
             ret += self._left[0]
-        return ret + sum(self._left[i] * self._pos_powers[i][n] for i in range(1, n // gv+1))
+        return ret + sum(f_coeff_i * self._pos_powers[i][n] for i in range(1, n // gv + 1)
+                         if (f_coeff_i := self._left[i]))
 
 
 class Stream_plethysm(Stream_binary):

src/sage/rings/lazy_series.py

@@ -4507,6 +4507,21 @@ def __call__(self, *g, check=True):
             sage: T(1-x-2*y + x*y^2)(1/(1-a), 3)
             3 + 8*a + 8*a^2 + 8*a^3 + 8*a^4 + 8*a^5 + 8*a^6 + O(a,b)^7
 
+        Check that issue :trac:`35261` is fixed::
+
+            sage: L.<z> = LazyPowerSeriesRing(QQ)
+            sage: fun = lambda n: 1 if ZZ(n).is_power_of(2) else 0
+            sage: f = L(fun)
+            sage: g = L.undefined(valuation=1)
+            sage: g.define((~f.shift(-1)(g)).shift(1))
+            sage: g
+            z - z^2 + 2*z^3 - 6*z^4 + 20*z^5 - 70*z^6 + 256*z^7 + O(z^8)
+
+            sage: f = L(fun)
+            sage: g = L.undefined(valuation=1)
+            sage: g.define((z - (f - z)(g)))
+            sage: g
+            z - z^2 + 2*z^3 - 6*z^4 + 20*z^5 - 70*z^6 + 256*z^7 + O(z^8)
         """
         fP = parent(self)
         if len(g) != fP._arity:
@@ -4712,6 +4727,16 @@ def revert(self):
             sage: f = L([-1], valuation=1, degree=3, constant=-1)
             sage: f.revert()
             (-z) + z^3 + (-z^4) + (-2*z^5) + 6*z^6 + z^7 + O(z^8)
+
+        Check that issue :trac:`35261` is fixed::
+
+            sage: L.<z> = LazyPowerSeriesRing(QQ)
+            sage: f = L(lambda n: 1 if ZZ(n).is_power_of(2) else 0)
+            sage: f
+            z + z^2 + z^4 + O(z^7)
+            sage: f.revert()
+            z - z^2 + 2*z^3 - 6*z^4 + 20*z^5 - 70*z^6 + 256*z^7 + O(z^8)
+
         """
         P = self.parent()
         if P._arity != 1: