diff --git a/Lib/test/test_iterlen.py b/Lib/test/test_iterlen.py
index 39e72169a..5e4b7cf7e 100644
--- a/Lib/test/test_iterlen.py
+++ b/Lib/test/test_iterlen.py
@@ -252,8 +252,6 @@ class NoneLengthHint(object):
 
 class TestLengthHintExceptions(unittest.TestCase):
 
-    # TODO: RUSTPYTHON
-    @unittest.expectedFailure
     def test_issue1242657(self):
         self.assertRaises(RuntimeError, list, BadLen())
         self.assertRaises(RuntimeError, list, BadLengthHint())
diff --git a/vm/src/iterator.rs b/vm/src/iterator.rs
index 2bf41144e..f99bd2fd0 100644
--- a/vm/src/iterator.rs
+++ b/vm/src/iterator.rs
@@ -6,7 +6,7 @@ use crate::builtins::int::{self, PyInt};
 use crate::builtins::iter::PySequenceIterator;
 use crate::exceptions::PyBaseExceptionRef;
 use crate::vm::VirtualMachine;
-use crate::{IdProtocol, PyObjectRef, PyResult, PyValue, TryFromObject, TypeProtocol};
+use crate::{IdProtocol, PyObjectRef, PyResult, PyValue, TypeProtocol};
 use num_traits::Signed;
 
 /*
@@ -73,16 +73,15 @@ pub fn get_next_object(
     }
 }
 
-/* Retrieve all elements from an iterator */
-pub fn get_all<T: TryFromObject>(vm: &VirtualMachine, iter_obj: &PyObjectRef) -> PyResult<Vec<T>> {
-    try_map(vm, iter_obj, |obj| T::try_from_object(vm, obj))
-}
-
-pub fn try_map<F, R>(vm: &VirtualMachine, iter_obj: &PyObjectRef, mut f: F) -> PyResult<Vec<R>>
+pub fn try_map<F, R>(
+    vm: &VirtualMachine,
+    iter_obj: &PyObjectRef,
+    cap: usize,
+    mut f: F,
+) -> PyResult<Vec<R>>
 where
     F: FnMut(PyObjectRef) -> PyResult<R>,
 {
-    let cap = length_hint(vm, iter_obj.clone())?.unwrap_or(0);
     // TODO: fix extend to do this check (?), see test_extend in Lib/test/list_tests.py,
     // https://github.com/python/cpython/blob/v3.9.0/Objects/listobject.c#L922-L928
     if cap >= isize::max_value() as usize {
diff --git a/vm/src/stdlib/collections.rs b/vm/src/stdlib/collections.rs
index 86894c3ed..a4446ed2c 100644
--- a/vm/src/stdlib/collections.rs
+++ b/vm/src/stdlib/collections.rs
@@ -204,7 +204,6 @@ mod _collections {
 
         #[pymethod]
         fn extend(&self, iter: PyObjectRef, vm: &VirtualMachine) -> PyResult<()> {
-            // TODO: use length_hint here and for extendleft
             self.state.fetch_add(1);
             let max_len = self.maxlen;
             let mut elements: Vec<PyObjectRef> = vm.extract_elements(&iter)?;
diff --git a/vm/src/vm.rs b/vm/src/vm.rs
index 8ac21d4c0..3059303e2 100644
--- a/vm/src/vm.rs
+++ b/vm/src/vm.rs
@@ -1261,7 +1261,13 @@ impl VirtualMachine {
                 .collect()
         } else {
             let iter = iterator::get_iter(self, value.clone())?;
-            iterator::try_map(self, &iter, |obj| func(obj))
+            let cap = match iterator::length_hint(self, value.clone()) {
+                Err(e) if e.class().is(&self.ctx.exceptions.runtime_error) => return Err(e),
+                Ok(Some(value)) => value,
+                // Use a power of 2 as a default capacity.
+                _ => 4,
+            };
+            iterator::try_map(self, &iter, cap, |obj| func(obj))
         }
     }
 
@@ -1303,7 +1309,15 @@ impl VirtualMachine {
             // TODO: put internal iterable type
             obj => {
                 let iter = iterator::get_iter(self, obj.clone())?;
-                Ok(iterator::try_map(self, &iter, f))
+                let cap = match iterator::length_hint(self, obj.clone()) {
+                    Err(e) if e.class().is(&self.ctx.exceptions.runtime_error) => {
+                        return Ok(Err(e))
+                    }
+                    Ok(Some(value)) => value,
+                    // Use a power of 2 as a default capacity.
+                    _ => 4,
+                };
+                Ok(iterator::try_map(self, &iter, cap, f))
             }
         })
     }