Raise TypeError when a dictionary is passed to MeasurementResult cons…

…tructor (#2523)

* add missing license statement

* raise error when passing dict to MeasurementResult

it's natural to think that you can pass results directly from some
sort of experiment into the MeasurementResult class.

* add instatiation example

* upgrade note to warning

mitiq/tests/test_measurement_result.py

@@ -38,6 +38,11 @@ def test_measurement_result_not_bits():
         MeasurementResult(result=[[0, 0], [0, 1], [-1, 0]])
 
 
+def test_measurement_result_invoked_with_dict():
+    with pytest.raises(TypeError, match="from_counts"):
+        MeasurementResult({"001": 123, "010": 456})
+
+
 def test_filter_qubits():
     result = MeasurementResult([[0, 0, 1], [0, 1, 0], [1, 0, 0]])
     assert np.allclose(result.filter_qubits([0]), np.array([[0], [0], [1]]))

mitiq/tests/test_typing.py

@@ -1,3 +1,8 @@
+# Copyright (C) Unitary Fund
+#
+# This source code is licensed under the GPL license (v3) found in the
+# LICENSE file in the root directory of this source tree.
+
 import os
 
 from mitiq.typing import SUPPORTED_PROGRAM_TYPES

mitiq/typing.py

@@ -112,7 +112,12 @@ class MeasurementResult:
             ``tuple(range(self.nqubits))``, where ``self.nqubits``
             is the bitstring length deduced from ``result``.
 
-    Note:
+    Example:
+        >>> mr = MeasurementResult(["001", "010", "001"])
+        >>> mr.get_counts()
+        {'001': 2, '010': 1}
+
+    Warning:
         Use caution when selecting the default option for ``qubit_indices``,
         especially when estimating an :class:`.Observable`
         acting on a subset of qubits. In this case Mitiq
@@ -125,6 +130,11 @@ class MeasurementResult:
 
     def __post_init__(self) -> None:
         # Validate arguments
+        if isinstance(self.result, dict):
+            raise TypeError(
+                "Use the MeasurementResult.from_counts method to instantiate "
+                "a MeasurementResult object from a dictionary."
+            )
         symbols = set(b for bits in self.result for b in bits)
         if not (symbols.issubset({0, 1}) or symbols.issubset({"0", "1"})):
             raise ValueError("Bitstrings should look like '011' or [0, 1, 1].")