diff --git a/python/ipywidgets/ipywidgets/widgets/tests/test_set_state_noecho.py b/python/ipywidgets/ipywidgets/widgets/tests/test_set_state_noecho.py
new file mode 100644
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+++ b/python/ipywidgets/ipywidgets/widgets/tests/test_set_state_noecho.py
@@ -0,0 +1,279 @@
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+import pytest
+from unittest import mock
+
+from traitlets import Bool, Tuple, List, Instance, CFloat, CInt, Float, Int, TraitError, observe
+
+from .utils import setup, teardown
+
+import ipywidgets
+from ipywidgets import Widget
+
+# Everything in this file assumes echo is false
+@pytest.fixture(autouse=True)
+def echo():
+    oldvalue = ipywidgets.widgets.widget.JUPYTER_WIDGETS_ECHO
+    ipywidgets.widgets.widget.JUPYTER_WIDGETS_ECHO = False
+    yield
+    ipywidgets.widgets.widget.JUPYTER_WIDGETS_ECHO = oldvalue
+
+#
+# First some widgets to test on:
+#
+
+# A widget with simple traits (list + tuple to ensure both are handled)
+class SimpleWidget(Widget):
+    a = Bool().tag(sync=True)
+    b = Tuple(Bool(), Bool(), Bool(), default_value=(False, False, False)).tag(sync=True)
+    c = List(Bool()).tag(sync=True)
+
+
+# A widget with various kinds of number traits
+class NumberWidget(Widget):
+    f = Float().tag(sync=True)
+    cf = CFloat().tag(sync=True)
+    i = Int().tag(sync=True)
+    ci = CInt().tag(sync=True)
+
+
+
+# A widget where the data might be changed on reception:
+def transform_fromjson(data, widget):
+    # Switch the two last elements when setting from json, if the first element is True
+    # and always set first element to False
+    if not data[0]:
+        return data
+    return [False] + data[1:-2] + [data[-1], data[-2]]
+
+class TransformerWidget(Widget):
+    d = List(Bool()).tag(sync=True, from_json=transform_fromjson)
+
+
+
+# A widget that has a buffer:
+class DataInstance():
+    def __init__(self, data=None):
+        self.data = data
+
+def mview_serializer(instance, widget):
+    return { 'data': memoryview(instance.data) if instance.data else None }
+
+def bytes_serializer(instance, widget):
+    return { 'data': bytearray(memoryview(instance.data).tobytes()) if instance.data else None }
+
+def deserializer(json_data, widget):
+    return DataInstance( memoryview(json_data['data']).tobytes() if json_data else None )
+
+class DataWidget(SimpleWidget):
+    d = Instance(DataInstance).tag(sync=True, to_json=mview_serializer, from_json=deserializer)
+
+# A widget that has a buffer that might be changed on reception:
+def truncate_deserializer(json_data, widget):
+    return DataInstance( json_data['data'][:20].tobytes() if json_data else None )
+
+class TruncateDataWidget(SimpleWidget):
+    d = Instance(DataInstance).tag(sync=True, to_json=bytes_serializer, from_json=truncate_deserializer)
+
+
+#
+# Actual tests:
+#
+
+def test_set_state_simple():
+    w = SimpleWidget()
+    w.set_state(dict(
+        a=True,
+        b=[True, False, True],
+        c=[False, True, False],
+    ))
+
+    assert w.comm.messages == []
+
+
+def test_set_state_transformer():
+    w = TransformerWidget()
+    w.set_state(dict(
+        d=[True, False, True]
+    ))
+    # Since the deserialize step changes the state, this should send an update
+    assert w.comm.messages == [((), dict(
+        buffers=[],
+        data=dict(
+            buffer_paths=[],
+            method='update',
+            state=dict(d=[False, True, False])
+        )))]
+
+
+def test_set_state_data():
+    w = DataWidget()
+    data = memoryview(b'x'*30)
+    w.set_state(dict(
+        a=True,
+        d={'data': data},
+    ))
+    assert w.comm.messages == []
+
+
+def test_set_state_data_truncate():
+    w = TruncateDataWidget()
+    data = memoryview(b'x'*30)
+    w.set_state(dict(
+        a=True,
+        d={'data': data},
+    ))
+    # Get message for checking
+    assert len(w.comm.messages) == 1   # ensure we didn't get more than expected
+    msg = w.comm.messages[0]
+    # Assert that the data update (truncation) sends an update
+    buffers = msg[1].pop('buffers')
+    assert msg == ((), dict(
+        data=dict(
+            buffer_paths=[['d', 'data']],
+            method='update',
+            state=dict(d={})
+        )))
+
+    # Sanity:
+    assert len(buffers) == 1
+    assert buffers[0] == data[:20].tobytes()
+
+
+def test_set_state_numbers_int():
+    # JS does not differentiate between float/int.
+    # Instead, it formats exact floats as ints in JSON (1.0 -> '1').
+
+    w = NumberWidget()
+    # Set everything with ints
+    w.set_state(dict(
+        f = 1,
+        cf = 2,
+        i = 3,
+        ci = 4,
+    ))
+    # Ensure no update message gets produced
+    assert len(w.comm.messages) == 0
+
+
+def test_set_state_numbers_float():
+    w = NumberWidget()
+    # Set floats to int-like floats
+    w.set_state(dict(
+        f = 1.0,
+        cf = 2.0,
+        ci = 4.0
+    ))
+    # Ensure no update message gets produced
+    assert len(w.comm.messages) == 0
+
+
+def test_set_state_float_to_float():
+    w = NumberWidget()
+    # Set floats to float
+    w.set_state(dict(
+        f = 1.2,
+        cf = 2.6,
+    ))
+    # Ensure no update message gets produced
+    assert len(w.comm.messages) == 0
+
+
+def test_set_state_cint_to_float():
+    w = NumberWidget()
+
+    # Set CInt to float
+    w.set_state(dict(
+        ci = 5.6
+    ))
+    # Ensure an update message gets produced
+    assert len(w.comm.messages) == 1
+    msg = w.comm.messages[0]
+    data = msg[1]['data']
+    assert data['method'] == 'update'
+    assert data['state'] == {'ci': 5}
+
+
+# This test is disabled, meaning ipywidgets REQUIRES
+# any JSON received to format int-like numbers as ints
+def _x_test_set_state_int_to_int_like():
+    # Note: Setting i to an int-like float will produce an
+    # error, so if JSON producer were to always create
+    # float formatted numbers, this would fail!
+
+    w = NumberWidget()
+    # Set floats to int-like floats
+    w.set_state(dict(
+        i = 3.0
+    ))
+    # Ensure no update message gets produced
+    assert len(w.comm.messages) == 0
+
+
+def test_set_state_int_to_float():
+    w = NumberWidget()
+
+    # Set Int to float
+    with pytest.raises(TraitError):
+        w.set_state(dict(
+            i = 3.5
+        ))
+
+def test_property_lock():
+    # when this widget's value is set to 42, it sets itself to 2, and then back to 42 again (and then stops)
+    class AnnoyingWidget(Widget):
+        value = Float().tag(sync=True)
+        stop = Bool(False)
+
+        @observe('value')
+        def _propagate_value(self, change):
+            print('_propagate_value', change.new)
+            if self.stop:
+                return
+            if change.new == 42:
+                self.value = 2
+            if change.new == 2:
+                self.stop = True
+                self.value = 42
+
+    widget = AnnoyingWidget(value=1)
+    assert widget.value == 1
+
+    widget._send = mock.MagicMock()
+    # this mimics a value coming from the front end
+    widget.set_state({'value': 42})
+    assert widget.value == 42
+
+    # we expect no new state to be sent
+    calls = []
+    widget._send.assert_has_calls(calls)
+
+def test_hold_sync():
+    # when this widget's value is set to 42, it sets the value to 2, and also sets a different trait value
+    class AnnoyingWidget(Widget):
+        value = Float().tag(sync=True)
+        other = Float().tag(sync=True)
+
+        @observe('value')
+        def _propagate_value(self, change):
+            print('_propagate_value', change.new)
+            if change.new == 42:
+                self.value = 2
+                self.other = 11
+
+    widget = AnnoyingWidget(value=1)
+    assert widget.value == 1
+
+    widget._send = mock.MagicMock()
+    # this mimics a value coming from the front end
+    widget.set_state({'value': 42})
+    assert widget.value == 2
+    assert widget.other == 11
+
+    # we expect only single state to be sent, i.e. the {'value': 42.0} state
+    msg = {'method': 'update', 'state': {'value': 2.0, 'other': 11.0}, 'buffer_paths': []}
+    call42 = mock.call(msg, buffers=[])
+
+    calls = [call42]
+    widget._send.assert_has_calls(calls)