Add barrier_label_len option to circuit drawers (#15776)

* Add barrier_label_len option to circuit drawers

* Switch to default value of 16

* consistency

* add comment

* move barrier_label_len to end

* add mpl test

* add ref fig

* Fix comment on LaTeX spaces

---------

Co-authored-by: Jake Lishman <jake@binhbar.com>

Author: ihincks

qiskit/circuit/quantumcircuit.py

@@ -3892,6 +3892,7 @@ def draw(
         wire_order: list[int] | None = None,
         expr_len: int = 30,
         measure_arrows: bool | None = None,
+        barrier_label_len: int = 16,
     ):
         r"""Draw the quantum circuit. Use the output parameter to choose the drawing format:
 
@@ -4003,6 +4004,9 @@ def draw(
                 instead place the name of the bit or register in the measure box.
                 Default is ``True`` unless the user config file (usually ``~/.qiskit/settings.conf``)
                 has an alternative value set. For example, ``circuit_measure_arrows = False``.
+            barrier_label_len: The number of characters to display for
+                :class:`.Barrier` labels in the output circuit. If this number is exceeded,
+                the string will be truncated at that number and '...' added to the end.
 
         Returns:
             :class:`.TextDrawing` or :class:`matplotlib.figure` or :class:`PIL.Image` or
@@ -4054,6 +4058,7 @@ def draw(
             cregbundle=cregbundle,
             wire_order=wire_order,
             expr_len=expr_len,
+            barrier_label_len=barrier_label_len,
             measure_arrows=measure_arrows,
         )
 

qiskit/visualization/circuit/circuit_visualization.py

@@ -75,6 +75,7 @@ def circuit_drawer(
     wire_order: list[int] | None = None,
     expr_len: int = 30,
     measure_arrows: bool | None = None,
+    barrier_label_len: int = 16,
 ):
     r"""Draw the quantum circuit. Use the output parameter to choose the drawing format:
 
@@ -187,6 +188,9 @@ def circuit_drawer(
             instead place the name of the bit or register in the measure box.
             Default is ``True`` unless the user config file (usually ``~/.qiskit/settings.conf``)
             has an alternative value set. For example, ``circuit_measure_arrows = False``.
+        barrier_label_len: The number of characters to display for
+            :class:`.Barrier` labels in the output circuit. If this number is exceeded,
+            the string will be truncated at that number and '...' added to the end.
 
     Returns:
         :class:`.TextDrawing` or :class:`matplotlib.figure` or :class:`PIL.Image` or
@@ -219,6 +223,7 @@ def circuit_drawer(
     """
     image = None
     expr_len = max(expr_len, 0)
+    barrier_label_len = max(barrier_label_len, 0)
     config = user_config.get_config()
     # Get default from config file else use text
     default_output = "text"
@@ -324,6 +329,7 @@ def check_clbit_in_inst(circuit, cregbundle):
             cregbundle=cregbundle,
             wire_order=complete_wire_order,
             expr_len=expr_len,
+            barrier_label_len=barrier_label_len,
             measure_arrows=measure_arrows,
         )
     elif output == "latex":
@@ -340,6 +346,7 @@ def check_clbit_in_inst(circuit, cregbundle):
             initial_state=initial_state,
             cregbundle=cregbundle,
             wire_order=complete_wire_order,
+            barrier_label_len=barrier_label_len,
         )
     elif output == "latex_source":
         return _generate_latex_source(
@@ -355,6 +362,7 @@ def check_clbit_in_inst(circuit, cregbundle):
             initial_state=initial_state,
             cregbundle=cregbundle,
             wire_order=complete_wire_order,
+            barrier_label_len=barrier_label_len,
         )
     elif output == "mpl":
         image = _matplotlib_circuit_drawer(
@@ -406,6 +414,7 @@ def _text_circuit_drawer(
     wire_order=None,
     expr_len=30,
     measure_arrows=True,
+    barrier_label_len=16,
 ):
     """Draws a circuit using ascii art.
 
@@ -441,6 +450,8 @@ def _text_circuit_drawer(
         measure_arrows: If True, draw an arrow from each measure box down to the classical bit
             or register where the measure value is placed. If False, do not draw arrow, but
             instead place the name of the bit or register in the measure box.
+        barrier_label_len (int): Optional. The number of characters to display for
+            :class:`.Barrier` labels. If this number is exceeded, the string will be truncated.
 
     Returns:
         TextDrawing: An instance that, when printed, draws the circuit in ascii art.
@@ -467,6 +478,7 @@ def _text_circuit_drawer(
         encoding=encoding,
         with_layout=with_layout,
         expr_len=expr_len,
+        barrier_label_len=barrier_label_len,
         measure_arrows=measure_arrows,
     )
     text_drawing.plotbarriers = plot_barriers
@@ -499,6 +511,7 @@ def _latex_circuit_drawer(
     initial_state=False,
     cregbundle=None,
     wire_order=None,
+    barrier_label_len=16,
 ):
     """Draw a quantum circuit based on latex (Qcircuit package)
 
@@ -525,6 +538,8 @@ def _latex_circuit_drawer(
         wire_order (list): Optional. A list of integers used to reorder the display
             of the bits. The list must have an entry for every bit with the bits
             in the range 0 to (num_qubits + num_clbits).
+        barrier_label_len (int): Optional. The number of characters to display for
+            :class:`.Barrier` labels. If this number is exceeded, the string will be truncated.
 
     Returns:
         PIL.Image: an in-memory representation of the circuit diagram
@@ -552,6 +567,7 @@ def _latex_circuit_drawer(
             initial_state=initial_state,
             cregbundle=cregbundle,
             wire_order=wire_order,
+            barrier_label_len=barrier_label_len,
         )
 
         try:
@@ -618,6 +634,7 @@ def _generate_latex_source(
     initial_state=False,
     cregbundle=None,
     wire_order=None,
+    barrier_label_len=16,
 ):
     """Convert QuantumCircuit to LaTeX string.
 
@@ -641,6 +658,8 @@ def _generate_latex_source(
         wire_order (list): Optional. A list of integers used to reorder the display
             of the bits. The list must have an entry for every bit with the bits
             in the range 0 to (num_qubits + num_clbits).
+        barrier_label_len (int): Optional. The number of characters to display for
+            :class:`.Barrier` labels. If this number is exceeded, the string will be truncated.
 
     Returns:
         str: Latex string appropriate for writing to file.
@@ -664,6 +683,7 @@ def _generate_latex_source(
         cregbundle=cregbundle,
         with_layout=with_layout,
         circuit=circuit,
+        barrier_label_len=barrier_label_len,
     )
     latex = qcimg.latex()
     if filename:
@@ -695,6 +715,7 @@ def _matplotlib_circuit_drawer(
     wire_order=None,
     expr_len=30,
     measure_arrows=None,
+    barrier_label_len=16,
 ):
     """Draw a quantum circuit based on matplotlib.
     If `%matplotlib inline` is invoked in a Jupyter notebook, it visualizes a circuit inline.
@@ -732,6 +753,8 @@ def _matplotlib_circuit_drawer(
         measure_arrows: If True, draw an arrow from each measure box down to the classical bit
             or register where the measure value is placed. If False, do not draw arrow, but
             instead place the name of the bit or register in the measure box.
+        barrier_label_len (int): Optional. The number of characters to display for
+            :class:`.Barrier` labels. If this number is exceeded, the string will be truncated.
 
     Returns:
         matplotlib.figure: a matplotlib figure object for the circuit diagram
@@ -764,6 +787,7 @@ def _matplotlib_circuit_drawer(
         cregbundle=cregbundle,
         with_layout=with_layout,
         expr_len=expr_len,
+        barrier_label_len=barrier_label_len,
         measure_arrows=measure_arrows,
     )
     return qcd.draw(filename)

qiskit/visualization/circuit/latex.py

@@ -63,6 +63,7 @@ def __init__(
         cregbundle=None,
         with_layout=False,
         circuit=None,
+        barrier_label_len=16,
     ):
         """QCircuitImage initializer.
 
@@ -78,13 +79,16 @@ def __init__(
             initial_state (bool): Optional. Adds |0> in the beginning of the line. Default: `False`.
             cregbundle (bool): Optional. If set True bundle classical registers.
             circuit (QuantumCircuit): the circuit that's being displayed
+            barrier_label_len (int): Optional. The number of characters to display for
+               barrier labels. If this number is exceeded, the string will be truncated.
         Raises:
             ImportError: If pylatexenc is not installed
         """
 
         self._circuit = circuit
         self._qubits = qubits
         self._clbits = clbits
+        self._barrier_label_len = barrier_label_len
 
         # list of lists corresponding to layers of the circuit
         self._nodes = nodes
@@ -575,7 +579,10 @@ def _build_barrier(self, node, col):
             self._latex[pos][col - 1] += " \\barrier[0em]{" + str(last - first) + "}"
             if node.op.label is not None:
                 pos = indexes[0]
-                label = node.op.label.replace(" ", "\\,")
+                label = node.op.label
+                if len(label) > self._barrier_label_len:
+                    label = label[: self._barrier_label_len] + "..."
+                label = label.replace(" ", "\\,")  # \, is a LaTeX thin space
                 self._latex[pos][col] = f"\\cds{{0}}{{^{{\\mathrm{{{label}}}}}}}"
 
     def _add_controls(self, wire_list, ctrlqargs, ctrl_state, col):

qiskit/visualization/circuit/matplotlib.py

@@ -110,6 +110,7 @@ def __init__(
         with_layout=False,
         expr_len=30,
         measure_arrows=None,
+        barrier_label_len=16,
     ):
         self._circuit = circuit
         self._qubits = qubits
@@ -138,6 +139,7 @@ def __init__(
         self._initial_state = initial_state
         self._global_phase = self._circuit.global_phase
         self._expr_len = expr_len
+        self._barrier_label_len = barrier_label_len
         self._cregbundle = cregbundle
         self._measure_arrows = measure_arrows
 
@@ -1437,11 +1439,14 @@ def _barrier(self, node, node_data, glob_data):
 
             # display the barrier label at the top if there is one
             if i == 0 and node.op.label is not None:
+                label = node.op.label
+                if len(label) > self._barrier_label_len:
+                    label = label[: self._barrier_label_len] + "..."
                 dir_ypos = ypos + 0.65 * HIG
                 self._ax.text(
                     xpos,
                     dir_ypos,
-                    node.op.label,
+                    label,
                     ha="center",
                     va="top",
                     fontsize=self._style["fs"],

qiskit/visualization/circuit/text.py

@@ -713,6 +713,7 @@ def __init__(
         with_layout=False,
         expr_len=30,
         measure_arrows=None,
+        barrier_label_len=16,
     ):
         self.qubits = qubits
         self.clbits = clbits
@@ -732,6 +733,7 @@ def __init__(
         self.reverse_bits = reverse_bits
         self.line_length = line_length
         self.expr_len = expr_len
+        self.barrier_label_len = barrier_label_len
         self.measure_arrows = measure_arrows
         if vertical_compression not in ["high", "medium", "low"]:
             raise ValueError("Vertical compression can only be 'high', 'medium', or 'low'")
@@ -1164,6 +1166,8 @@ def add_connected_gate(node, gates, layer, current_cons, gate_wire_map):
             for qubit in node.qargs:
                 if qubit in self.qubits:
                     label = op.label if qubit == top_qubit else ""
+                    if label and len(label) > self.barrier_label_len:
+                        label = label[: self.barrier_label_len] + "..."
                     layer.set_qubit(qubit, Barrier(label))
 
         elif isinstance(op, SwapGate):

releasenotes/notes/barrier-label-truncation-eaa14709883fbf75.yaml

@@ -0,0 +1,9 @@
+---
+features_visualization:
+  - |
+    Added a new ``barrier_label_len`` parameter (default 16) to
+    :meth:`.QuantumCircuit.draw` and :func:`.circuit_drawer`. When a
+    :class:`.Barrier` label exceeds this length, it is truncated and ``"..."``
+    is appended. This prevents very long barrier labels from making circuit
+    diagrams unreadable. The parameter is supported by all three drawer
+    backends (text, mpl, latex).

test/python/visualization/references/test_latex_barrier_label_truncation.tex

@@ -228,6 +228,17 @@ def test_barrier_label(self):
 
         self.assertEqualToReference(filename)
 
+    def test_barrier_label_truncation(self):
+        """Test that long barrier labels are truncated"""
+        filename = self._get_resource_path("test_latex_barrier_label_truncation.tex")
+        circuit = QuantumCircuit(2)
+        circuit.barrier()
+        circuit.barrier(label="a" * 20)
+
+        circuit_drawer(circuit, filename=filename, output="latex_source", barrier_label_len=9)
+
+        self.assertEqualToReference(filename)
+
     def test_big_gates(self):
         """Test large gates with params"""
         filename = self._get_resource_path("test_latex_big_gates.tex")

test/python/visualization/test_circuit_latex.py

@@ -1274,6 +1274,42 @@ def test_text_barrier_label(self):
         circuit.barrier(label="End Y/X")
         self.assertEqual(str(circuit_drawer(circuit, output="text", initial_state=True)), expected)
 
+    def test_text_barrier_label_truncation(self):
+        """Barrier labels longer than barrier_label_len are truncated"""
+        expected_truncated = "\n".join(
+            [
+                "      ░  aaaaaaaaa... ",
+                "q_0: ─░───────░───────",
+                "      ░       ░       ",
+                "q_1: ─░───────░───────",
+                "      ░       ░       ",
+            ]
+        )
+        circuit = QuantumCircuit(2)
+        circuit.barrier()
+        circuit.barrier(label="a" * 20)
+        self.assertEqual(
+            str(circuit_drawer(circuit, output="text", barrier_label_len=9)), expected_truncated
+        )
+
+    def test_text_barrier_label_no_truncation(self):
+        """Barrier labels shorter than barrier_label_len are unchanged"""
+        expected = "\n".join(
+            [
+                "      ░  short ",
+                "q_0: ─░────░───",
+                "      ░    ░   ",
+                "q_1: ─░────░───",
+                "      ░    ░   ",
+            ]
+        )
+        circuit = QuantumCircuit(2)
+        circuit.barrier()
+        circuit.barrier(label="short")
+        self.assertEqual(
+            str(circuit_drawer(circuit, output="text", barrier_label_len=30)), expected
+        )
+
     def test_text_barrier_label_reversed_bits(self):
         """Show barrier label with reversed bits"""
         expected = "\n".join(