diff --git a/crates/stdlib/src/math.rs b/crates/stdlib/src/math.rs
index 9ea0be522c6..cb50261a3b1 100644
--- a/crates/stdlib/src/math.rs
+++ b/crates/stdlib/src/math.rs
@@ -6,13 +6,11 @@ use crate::vm::{VirtualMachine, builtins::PyBaseExceptionRef};
 #[pymodule]
 mod math {
     use crate::vm::{
-        AsObject, Py, PyObject, PyObjectRef, PyRef, PyResult, VirtualMachine,
+        AsObject, Py, PyObject, PyObjectRef, PyResult, VirtualMachine,
         builtins::{
             PyFloat, PyInt, PyIntRef, PyModule, PyStrInterned, try_bigint_to_f64, try_f64_to_bigint,
         },
-        function::{
-            ArgIndex, ArgIntoFloat, ArgIterable, Either, NameCoordinates, OptionalArg, PosArgs,
-        },
+        function::{ArgIndex, ArgIntoFloat, ArgIterable, NameCoordinates, OptionalArg, PosArgs},
         identifier,
     };
     use malachite_bigint::BigInt;
@@ -457,7 +455,7 @@ mod math {
     #[derive(FromArgs)]
     struct LdexpArgs {
         #[pyarg(positional)]
-        x: Either<PyRef<PyFloat>, PyIntRef>,
+        x: ArgIntoFloat,
         // Refuses anything that is not an `int`, including objects with `__index__`.
         #[pyarg(positional, error_msg = "Expected an int as second argument to ldexp.")]
         i: PyIntRef,
@@ -466,11 +464,8 @@ mod math {
     #[pyfunction]
     fn ldexp(args: LdexpArgs, vm: &VirtualMachine) -> PyResult<f64> {
         let LdexpArgs { x, i } = args;
-        let value = match x {
-            Either::A(f) => f.to_f64(),
-            Either::B(z) => try_bigint_to_f64(z.as_bigint(), vm)?,
-        };
-        pymath::math::ldexp_bigint(value, i.as_bigint()).map_err(|err| pymath_exception(err, vm))
+        pymath::math::ldexp_bigint(x.into_float(), i.as_bigint())
+            .map_err(|err| pymath_exception(err, vm))
     }
 
     #[pyfunction]
diff --git a/crates/vm/src/function/number.rs b/crates/vm/src/function/number.rs
index 9fdb06a9717..53203a683d3 100644
--- a/crates/vm/src/function/number.rs
+++ b/crates/vm/src/function/number.rs
@@ -1,7 +1,7 @@
 use super::argument::OptionalArg;
 use crate::{
     AsObject, Py, PyObjectRef, PyResult, TryFromObject, VirtualMachine,
-    builtins::{PyInt, PyIntRef},
+    builtins::{PyFloat, PyInt, PyIntRef},
 };
 use core::ops::Deref;
 use malachite_bigint::BigInt;
@@ -88,6 +88,12 @@ impl From<ArgIntoFloat> for f64 {
 impl TryFromObject for ArgIntoFloat {
     // Equivalent to PyFloat_AsDouble.
     fn try_from_object(vm: &VirtualMachine, obj: PyObjectRef) -> PyResult<Self> {
+        // A float subclass is read directly, without calling its __float__.
+        if let Some(float) = obj.downcast_ref::<PyFloat>() {
+            return Ok(Self {
+                value: float.to_f64(),
+            });
+        }
         let value = obj.try_float(vm)?.to_f64();
         Ok(Self { value })
     }
diff --git a/extra_tests/snippets/stdlib_math.py b/extra_tests/snippets/stdlib_math.py
index b06bf8446f6..7b41f039e83 100644
--- a/extra_tests/snippets/stdlib_math.py
+++ b/extra_tests/snippets/stdlib_math.py
@@ -541,3 +541,33 @@ def assertAllNotClose(examples, *args, **kwargs):
 assert math.perm(5) == math.perm(5, None) == 120
 assert math.perm(2**64, 0) == 1
 assert math.comb(2**64, 2**64 - 1) == 2**64
+
+
+class LdexpFloat:
+    def __float__(self):
+        return 1.5
+
+
+class LdexpIndex:
+    def __index__(self):
+        return 3
+
+
+class LdexpRaisingFloat:
+    def __float__(self):
+        raise RuntimeError
+
+
+assert math.ldexp(LdexpFloat(), 1) == 3.0
+assert math.ldexp(LdexpIndex(), 1) == 6.0
+assert_raises(RuntimeError, math.ldexp, LdexpRaisingFloat(), 1)
+assert_raises(OverflowError, math.ldexp, 10**400, 1)
+
+
+class FloatSubclass(float):
+    def __float__(self):
+        return 9.0
+
+
+assert math.sqrt(FloatSubclass(4.0)) == 2.0
+assert math.ldexp(FloatSubclass(1.0), 1) == 2.0