reduce decimal for probabilities_dict in Sampler

qiskit/quantum_info/operators/symplectic/pauli.py

@@ -97,7 +97,7 @@ class initialization (``Pauli('-iXYZ')``). A ``Pauli`` object can be
         returned string for large numbers of qubits while :meth:`to_label`
         will return the full string with no truncation. The default
         truncation length is 50 characters. The default value can be
-        changed by setting the class `__truncate__` attribute to an integer
+        changed by setting the class ``__truncate__`` attribute to an integer
         value. If set to ``0`` no truncation will be performed.
 
     **Array Representation**

qiskit/result/distributions/quasi.py

@@ -27,10 +27,18 @@ class QuasiDistribution(dict):
     """A dict-like class for representing quasi-probabilities."""
 
     _bitstring_regex = re.compile(r"^[01]+$")
+    __ndigits__ = 15
 
     def __init__(self, data, shots=None, stddev_upper_bound=None):
         """Builds a quasiprobability distribution object.
 
+        .. note::
+
+            The quasiprobability values might include floating-point errors.
+            ``QuasiDistribution.__repr__`` rounds using :meth:`numpy.round`
+            and the parameter ``ndigits`` can be manipulated with the
+            class attribute ``__ndigits__``. The default is ``15``.
+
         Parameters:
             data (dict): Input quasiprobability data. Where the keys
                 represent a measured classical value and the value is a
@@ -141,3 +149,6 @@ def hex_probabilities(self):
     def stddev_upper_bound(self):
         """Return an upper bound on standard deviation of expval estimator."""
         return self._stddev_upper_bound
+
+    def __repr__(self):
+        return str({key: round(value, ndigits=self.__ndigits__) for key, value in self.items()})

releasenotes/notes/decimals_for_sampler-5aaa56642cce23e0.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    The :class:`~.QuasiDistribution` values might include floating-point errors.
+    ``QuasiDistribution.__repr__`` rounds using :meth:`numpy.round`
+    and the parameter ``ndigits`` can be manipulated with the
+    class attribute ``__ndigits__``. The default is ``15``.