Make pylatexenc and pillow optional (Qiskit#2451)

* Make pylatexenc and pillow optional

The pylatexenc and pillow requirements are only used in the latex and
latex_source circuit drawers. Since this isn't used by everyone and is a
non-default circuit drawer there is no reason we should force our users
to install these by default. This commit updates the requirements list
and setup.py to make these 2 requirements optional and included in the
visualization setuptools extras to ease installation. If the packages
are not installed it will raise an ImportError with a detailed exception
explaining how to install the missing dependency.

Fixes Qiskit#2417

* Fix lint

* Fix lint again

CHANGELOG.rst

@@ -19,6 +19,13 @@ The format is based on `Keep a Changelog`_.
 `UNRELEASED`_
 =============
 
+Changed
+-------
+
+- The ``pylatexenc`` and ``pillow`` requirements are now optional. These are
+  only used by the ``latex`` and ``latex_source`` circuit visualization
+  backends. To continue using them ensure these are installed.
+
 Removed
 -------
 - The previously deprecated functions ``qiksit.visualization.plot_state`` and

qiskit/visualization/circuit_visualization.py

@@ -28,7 +28,11 @@
 import subprocess
 import tempfile
 
-from PIL import Image
+try:
+    from PIL import Image
+    HAS_PIL = True
+except ImportError:
+    HAS_PIL = False
 
 from qiskit import user_config
 from qiskit.visualization import exceptions
@@ -369,6 +373,7 @@ def _latex_circuit_drawer(circuit,
         OSError: usually indicates that ```pdflatex``` or ```pdftocairo``` is
                  missing.
         CalledProcessError: usually points errors during diagram creation.
+        ImportError: if pillow is not installed
     """
     tmpfilename = 'circuit'
     with tempfile.TemporaryDirectory() as tmpdirname:
@@ -399,6 +404,10 @@ def _latex_circuit_drawer(circuit,
                            'be found in latex_error.log')
             raise
         else:
+            if not HAS_PIL:
+                raise ImportError('The latex drawer needs pillow installed. '
+                                  'Run "pip install pillow" before using the '
+                                  'latex drawer.')
             try:
                 base = os.path.join(tmpdirname, tmpfilename)
                 subprocess.run(["pdftocairo", "-singlefile", "-png", "-q",

qiskit/visualization/latex.py

@@ -24,7 +24,12 @@
 import operator
 import re
 
-from pylatexenc.latexencode import utf8tolatex
+try:
+    from pylatexenc.latexencode import utf8tolatex
+    HAS_PYLATEX = True
+except ImportError:
+    HAS_PYLATEX = False
+
 import numpy as np
 from qiskit.visualization import qcstyle as _qcstyle
 from qiskit.visualization import exceptions
@@ -53,7 +58,14 @@ def __init__(self, qregs, cregs, ops, scale, style=None,
                registers for the output visualization.
             plot_barriers (bool): Enable/disable drawing barriers in the output
                circuit. Defaults to True.
+        Raises:
+            ImportError: If pylatexenc is not installed
         """
+        if not HAS_PYLATEX:
+            raise ImportError('The latex and latex_source drawers need '
+                              'pylatexenc installed. Run "pip install '
+                              'pylatexenc" before using the latex or '
+                              'latex_source drawers.')
         # style sheet
         self._style = _qcstyle.BWStyle()
         if style:

requirements-dev.txt

@@ -13,3 +13,5 @@ vcrpy
 PyGithub
 wheel
 cython>=0.27.1
+pillow>=4.2.1
+pylatexenc>=1.4

requirements.txt

@@ -3,8 +3,6 @@ marshmallow>=2.17.0,<3
 marshmallow_polyfield>=3.2,<4
 networkx>=2.2
 numpy>=1.13
-pillow>=4.2.1
-pylatexenc>=1.4
 ply>=3.10
 psutil>=5
 scipy>=1.0

setup.py

@@ -30,10 +30,8 @@
     "marshmallow_polyfield>=3.2,<4",
     "networkx>=2.2",
     "numpy>=1.13",
-    "pillow>=4.2.1",
     "ply>=3.10",
     "psutil>=5",
-    "pylatexenc>=1.4",
     "scipy>=1.0",
     "sympy>=1.3"
 ]
@@ -101,7 +99,7 @@
     python_requires=">=3.5",
     extras_require={
         'visualization': ['matplotlib>=2.1', 'nxpd>=0.2', 'ipywidgets>=7.3.0',
-                          'pydot'],
+                          'pydot', "pillow>=4.2.1", "pylatexenc>=1.4"],
         'full-featured-simulators': ['qiskit-aer>=0.1']
     },
     project_urls={