Allow lightning.qubit to skip C++ compilation and provide a Python-only binary

.github/CHANGELOG.md

@@ -10,6 +10,13 @@
 
 ### Improvements
 
+* PennyLane-Lightning can now be installed without compiling its C++ binaries and will fall back
+  to using the `default.qubit` implementation. Skipping compilation is achieved by setting the
+  `SKIP_COMPILATION` environment variable, e.g., Linux/MacOS: `export SKIP_COMPILATION=True`,
+  Windows: `set SKIP_COMPILATION=True`. This feature is intended for building a pure-Python wheel of
+  PennyLane-Lightning as a backup for platforms without a dedicated wheel.
+  [(129)](https://github.com/PennyLaneAI/pennylane-lightning/pull/129)
+
 * The C++-backed Python bound methods can now be directly called with wires and supplied parameters.
   [(125)](https://github.com/PennyLaneAI/pennylane-lightning/pull/125)
 

.github/workflows/deploy.yml

@@ -1,95 +1,120 @@
-name: Deploy
-on:
-  release:
-    types: [published]
-
-env:
-  CIBW_BUILD: 'cp37-* cp38-* cp39-*'
-
-  # MacOS specific build settings
-
-  CIBW_BEFORE_ALL_MACOS: |
-    brew cask uninstall --force oclint
-    brew install gcc libomp
-
-  # Python build settings
-
-  CIBW_BEFORE_BUILD: |
-    pip install numpy==1.19.5 scipy pybind11
-
-  # Testing of built wheels
-  CIBW_TEST_REQUIRES: numpy scipy pytest pytest-cov pytest-mock flaky
-
-  CIBW_TEST_COMMAND: |
-    pl-device-test --device=lightning.qubit --skip-ops -x --tb=short --no-flaky-report
-
-
-jobs:
-
-  build-wheels:
-    name: ${{ matrix.os }}
-    runs-on: ${{ matrix.os }}
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
-    steps:
-      - name: Cancel Previous Runs
-        uses: styfle/cancel-workflow-action@0.4.1
-        with:
-          access_token: ${{ github.token }}
-
-      - uses: actions/checkout@v2
-
-      - uses: actions/setup-python@v2
-        with:
-          python-version: '3.7'
-
-      - name: Install cibuildwheel
-        run: |
-          python -m pip install --upgrade pip
-          python -m pip install cibuildwheel==1.5.5
-
-      - name: Build wheels
-        run: |
-          python -m cibuildwheel --output-dir wheelhouse
-
-      - uses: actions/upload-artifact@v2
-        with:
-          name: ${{ runner.os }}-wheels.zip
-          path: ./wheelhouse/*.whl
-
-  upload-wheels:
-    runs-on: ubuntu-latest
-    needs: [build-wheels]
-    steps:
-      - uses: actions/download-artifact@v2
-        with:
-          path: dist/
-
-      - name: Publish
-        uses: pypa/gh-action-pypi-publish@master
-        with:
-          user: __token__
-          password: ${{ secrets.PYPI }}
-
-  upload-source:
-    runs-on: ubuntu-latest
-    needs: [build-wheels]
-    steps:
-      - uses: actions/checkout@v2
-
-      - name: Set up Python
-        uses: actions/setup-python@v2
-        with:
-          python-version: 3.7
-
-      - name: Build and install Plugin
-        run: |
-          python -m pip install --upgrade pip wheel
-          python setup.py sdist
-
-      - name: Publish
-        uses: pypa/gh-action-pypi-publish@master
-        with:
-          user: __token__
-          password: ${{ secrets.PYPI }}
+name: Deploy
+on:
+  release:
+    types: [published]
+
+env:
+  CIBW_BUILD: 'cp37-* cp38-* cp39-*'
+
+  # MacOS specific build settings
+
+  CIBW_BEFORE_ALL_MACOS: |
+    brew cask uninstall --force oclint
+    brew install gcc libomp
+
+  # Python build settings
+
+  CIBW_BEFORE_BUILD: |
+    pip install numpy==1.19.5 scipy pybind11
+
+  # Testing of built wheels
+  CIBW_TEST_REQUIRES: numpy scipy pytest pytest-cov pytest-mock flaky
+
+  CIBW_TEST_COMMAND: |
+    pl-device-test --device=lightning.qubit --skip-ops -x --tb=short --no-flaky-report
+
+
+jobs:
+
+  build-wheels:
+    name: ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    steps:
+      - name: Cancel Previous Runs
+        uses: styfle/cancel-workflow-action@0.4.1
+        with:
+          access_token: ${{ github.token }}
+
+      - uses: actions/checkout@v2
+
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.7'
+
+      - name: Install cibuildwheel
+        run: |
+          python -m pip install --upgrade pip
+          python -m pip install cibuildwheel==1.5.5
+
+      - name: Build wheels
+        run: |
+          python -m cibuildwheel --output-dir wheelhouse
+
+      - uses: actions/upload-artifact@v2
+        with:
+          name: ${{ runner.os }}-wheels.zip
+          path: ./wheelhouse/*.whl
+
+  build-pure-python-wheel:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout PennyLane-Lightning
+        uses: actions/checkout@v2
+        with:
+          path: main
+
+      - uses: actions/setup-python@v2
+        with:
+          python-version: '3.7'
+
+      - name: Build wheels
+        run: |
+          python -m pip install --upgrade pip wheel
+          cd main
+          python setup.py bdist_wheel
+        env:
+          SKIP_COMPILATION: True
+
+      - uses: actions/upload-artifact@v2
+        with:
+          name: pure-python-wheels.zip
+          path: main/dist/*.whl
+
+  upload-wheels:
+    runs-on: ubuntu-latest
+    needs: [build-wheels, build-pure-python-wheel]
+    steps:
+      - uses: actions/download-artifact@v2
+        with:
+          path: dist/
+
+      - name: Publish
+        uses: pypa/gh-action-pypi-publish@master
+        with:
+          user: __token__
+          password: ${{ secrets.PYPI }}
+
+  upload-source:
+    runs-on: ubuntu-latest
+    needs: [build-wheels, build-pure-python-wheel]
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Set up Python
+        uses: actions/setup-python@v2
+        with:
+          python-version: 3.7
+
+      - name: Build and install Plugin
+        run: |
+          python -m pip install --upgrade pip wheel
+          python setup.py sdist
+
+      - name: Publish
+        uses: pypa/gh-action-pypi-publish@master
+        with:
+          user: __token__
+          password: ${{ secrets.PYPI }}

.github/workflows/tests_without_binary.yml

@@ -0,0 +1,58 @@
+name: Testing without binary
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+
+env:
+  COVERAGE_FLAGS: "--cov=pennylane_lightning --cov-report=term-missing --cov-report=xml:../coverage.xml --no-flaky-report -p no:warnings --tb=native"
+
+jobs:
+  pythontests:
+    name: Python tests
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-20.04]
+
+    steps:
+      - name: Cancel previous runs
+        uses: styfle/cancel-workflow-action@0.4.1
+        with:
+          access_token: ${{ github.token }}
+
+      - name: Checkout PennyLane-Lightning
+        uses: actions/checkout@v2
+        with:
+          path: main
+
+      - uses: actions/setup-python@v2
+        name: Install Python
+        with:
+          python-version: '3.7'
+
+      - name: Get required Python packages
+        run: |
+          cd main
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+
+      - name: Install lightning.qubit device
+        run: |
+          cd main
+          pip install -e .
+        env:
+          SKIP_COMPILATION: True
+
+      - name: Run PennyLane-Lightning unit tests
+        run: |
+          cd main/
+          pytest tests/ $COVERAGE_FLAGS
+          pl-device-test --device lightning.qubit --skip-ops --shots=20000 $COVERAGE_FLAGS --cov-append
+          pl-device-test --device lightning.qubit --shots=None --skip-ops $COVERAGE_FLAGS --cov-append
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v1.0.12
+        with:
+          file: coverage.xml

.readthedocs.yml

@@ -4,7 +4,7 @@ sphinx:
   configuration: doc/conf.py
 
 python:
-  version: 3.9
+  version: 3.8
   install:
     - requirements: doc/requirements.txt
     - requirements: requirements.txt

Makefile

@@ -44,6 +44,7 @@ clean:
 	rm -rf .coverage coverage_html_report/
 	rm -rf tmp
 	rm -rf *.dat
+	rm -rf pennylane_lightning/lightning_qubit_ops*
 
 docs:
 	make -C doc html

README.rst

@@ -51,29 +51,33 @@ PennyLane-Lightning requires Python version 3.7 and above. It can be installed u
 
     $ pip install pennylane-lightning
 
-Alternatively, to build PennyLane-Lightning from source you can run
+To build PennyLane-Lightning from source you can run
 
 .. code-block:: console
 
-    $ git clone https://github.com/XanaduAI/pennylane-lightning.git
-    $ cd pennylane-lightning
-    $ pip install -e .
+    $ pip install pybind11 pennylane-lightning --no-binary :all:
 
-Note that subsequent calls to ``pip install -e .`` will use cached binaries stored in the
-``build`` folder. Run ``make clean`` if you would like to recompile.
+A C++ compiler such as ``g++``, ``clang``, or ``MSVC`` is required. On Debian-based systems, this
+can be installed via ``apt``:
+
+.. code-block:: console
 
-The following dependencies are required to install PennyLane-Lightning:
+    $ sudo apt install g++
 
-* A C++ compiler, such as ``g++``, ``clang``, or ``MSVC``.
-* `pybind11 <https://pybind11.readthedocs.io/en/stable/>`_ a library for binding C++
-  functionality to Python.
+The `pybind11 <https://pybind11.readthedocs.io/en/stable/>`_ library is also used for binding the
+C++ functionality to Python.
 
-On Debian-based systems, these can be installed via ``apt`` and ``pip``:
+Alternatively, for development and testing, you can install by cloning the repository:
 
 .. code-block:: console
 
-    $ sudo apt install g++
-    $ pip install pybind11
+    $ git clone https://github.com/XanaduAI/pennylane-lightning.git
+    $ cd pennylane-lightning
+    $ pip install -r requirements.txt
+    $ pip install -e .
+
+Note that subsequent calls to ``pip install -e .`` will use cached binaries stored in the
+``build`` folder. Run ``make clean`` if you would like to recompile.
 
 Testing
 -------

pennylane_lightning/lightning_qubit.py

@@ -15,10 +15,18 @@
 This module contains the :class:`~.LightningQubit` class, a PennyLane simulator device that
 interfaces with C++ for fast linear algebra calculations.
 """
+from warnings import warn
+
 from pennylane.devices import DefaultQubit
 import numpy as np
 from pennylane import QubitStateVector, BasisState, DeviceError, QubitUnitary
-from .lightning_qubit_ops import apply, StateVectorC64, StateVectorC128
+
+try:
+    from .lightning_qubit_ops import apply, StateVectorC64, StateVectorC128
+
+    CPP_BINARY_AVAILABLE = True
+except ModuleNotFoundError:
+    CPP_BINARY_AVAILABLE = False
 
 from ._version import __version__
 
@@ -122,3 +130,24 @@ def apply_lightning(self, state, operations):
                 method(wires, inv, param)
 
         return np.reshape(state_vector, state.shape)
+
+
+if not CPP_BINARY_AVAILABLE:
+
+    class LightningQubit(DefaultQubit):
+
+        name = "Lightning Qubit PennyLane plugin"
+        short_name = "lightning.qubit"
+        pennylane_requires = ">=0.15"
+        version = __version__
+        author = "Xanadu Inc."
+
+        def __init__(self, *args, **kwargs):
+            warn(
+                "Pre-compiled binaries for lightning.qubit are not available. Falling back to "
+                "using the Python-based default.qubit implementation. To manually compile from "
+                "source, follow the instructions at "
+                "https://pennylane-lightning.readthedocs.io/en/latest/installation.html.",
+                UserWarning,
+            )
+            super().__init__(*args, **kwargs)