IPython displays a Hamiltonian's str representation instead of `r…

…epr` (#2648)

* hamiltonian ipython_display

* Update doc/releases/changelog-dev.md

* Update pennylane/ops/qubit/hamiltonian.py

* black and docstring

doc/releases/changelog-dev.md

@@ -75,6 +75,10 @@
 
 <h3>Improvements</h3>
 
+* IPython displays the `str` representation of a `Hamiltonian`, rather than the `repr`. This displays
+  more information about the object.
+  [(#2648)](https://github.com/PennyLaneAI/pennylane/pull/2648)
+
 * The qchem openfermion-dependent tests are localized and collected in `tests.qchem.of_tests`. The
   new module `test_structure` is created to collect the tests of the `qchem.structure` module in
   one place and remove their dependency to openfermion.

pennylane/ops/qubit/hamiltonian.py

@@ -426,6 +426,12 @@ def __repr__(self):
         # Constructor-call-like representation
         return f"<Hamiltonian: terms={qml.math.shape(self.coeffs)[0]}, wires={self.wires.tolist()}>"
 
+    def _ipython_display_(self):  # pragma: no-cover
+        """Displays __str__ in ipython instead of __repr__
+        See https://ipython.readthedocs.io/en/stable/config/integrating.html
+        """
+        print(self.__str__())
+
     def _obs_data(self):
         r"""Extracts the data from a Hamiltonian and serializes it in an order-independent fashion.
 

tests/ops/qubit/test_hamiltonian.py

@@ -17,6 +17,8 @@
 import numpy as np
 import pytest
 
+from unittest.mock import patch
+
 import pennylane as qml
 from pennylane import numpy as pnp
 
@@ -642,6 +644,13 @@ def test_hamiltonian_str(self, terms, string):
         H = qml.Hamiltonian(*terms)
         assert H.__str__() == string
 
+    @patch("builtins.print")
+    def test_hamiltonian_ipython_display(self, mock_print):
+        """Test that the ipython_dipslay method prints __str__."""
+        H = 1.0 * qml.PauliX(0)
+        H._ipython_display_()
+        mock_print.assert_called_with(str(H))
+
     @pytest.mark.parametrize("terms, string", zip(valid_hamiltonians, valid_hamiltonians_repr))
     def test_hamiltonian_repr(self, terms, string):
         """Tests that the __repr__ function for printing is correct"""