Add option to prune snap path when building wheelhouse

When running charm-tools from a snap, the default is to use Python
and associated tools from the snap when building a charm's
wheelhouse etc.

This is normally not what you would want as the Python version in
the snap does not necessarily match the Python version of the
target system.

The wheelhouse needs to be built with a Python version compatible
with the Python version of the target system on which the built
charm is to be run on.

Add `--use-python-from-snap` option where the default is 'False'.
This is a change of behavior and mandates a major version bump for
this tool.

Fixes #623

.github/workflows/tox.yaml

@@ -7,14 +7,17 @@ jobs:
   tox:
     name: Tests
     runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ['3.8', '3.9', '3.10']
     steps:
     - name: Check out code
       uses: actions/checkout@v2
     - name: Setup Python
       uses: actions/setup-python@v2
       with:
-        python-version: 3.7
+        python-version: ${{ matrix.python-version }}
     - name: Install Tox
       run: pip install tox
     - name: Run tests
-      run: tox
+      run: tox -e py

charmtools/build/builder.py

@@ -1185,6 +1185,10 @@ def main(args=None):
                         help='Same as --binary-wheels but build all the '
                              'wheels from source code, even if a binary '
                              'distribution is available on PyPi.')
+    parser.add_argument('--use-python-from-snap', action='store_true',
+                        default=False,
+                        help='Use Python and associated tools from the snap '
+                             'when building the wheelhouse.')
     parser.add_argument('charm', nargs="?", default=".", type=path,
                         help='Source directory for charm layer to build '
                              '(default: .)')
@@ -1205,6 +1209,7 @@ def main(args=None):
     WheelhouseTactic.per_layer = build.wheelhouse_per_layer
     WheelhouseTactic.binary_build = build.binary_wheels
     WheelhouseTactic.binary_build_from_source = build.binary_wheels_from_source
+    WheelhouseTactic.use_python_from_snap = build.use_python_from_snap
 
     configLogging(build)
 

charmtools/build/tactics.py

@@ -1046,6 +1046,7 @@ class WheelhouseTactic(ExactMatch, Tactic):
     per_layer = False
     binary_build = False
     binary_build_from_source = False
+    use_python_from_snap = False
 
     def __init__(self, *args, **kwargs):
         super(WheelhouseTactic, self).__init__(*args, **kwargs)
@@ -1062,6 +1063,25 @@ def __str__(self):
         directory = self.target.directory / 'wheelhouse'
         return "Building wheelhouse in {}".format(directory)
 
+    def _get_env(self):
+        """Get environment appropriate for executing external commands.
+
+        :returns: Dictionary with environment variables
+        :rtype: Dict[str,str]
+        """
+        if self.use_python_from_snap:
+            return os.environ.copy()
+
+        env = os.environ.copy()
+        for key in ('PREFIX', 'PYTHONHOME', 'PYTHONPATH'):
+            if key in env:
+                del(env[key])
+        env['PATH'] = ':'.join([
+            element
+            for element in env['PATH'].split(':')
+            if not element.startswith('/snap/charm/')])
+        return env
+
     def combine(self, existing):
         ""  # suppress inherited doc
         self.previous = existing.previous + [existing]
@@ -1213,7 +1233,7 @@ def _run_in_venv(self, *args):
         # have to use bash to activate the venv properly first
         res = utils.Process(('bash', '-c', ' '.join(
             ('.', self._venv / 'bin' / 'activate', ';') + args
-        )))()
+        )), env=self._get_env())()
         if res.exit_code != 0:
             raise BuildError(res.output)
         return res
@@ -1228,7 +1248,8 @@ def __call__(self):
         wheelhouse.mkdir_p()
         if create_venv:
             utils.Process(
-                ('virtualenv', '--python', 'python3', self._venv)
+                ('virtualenv', '--python', 'python3', self._venv),
+                env=self._get_env()
             ).exit_on_error()()
         if self.per_layer:
             self._process_per_layer(wheelhouse)

tests/test_build.py

@@ -452,7 +452,7 @@ def test_wheelhouse(self, Process, mkdtemp, rmtree_p, ph, pi, pv, sign):
         bu.report = False
         bu.wheelhouse_overrides = self.dirname / 'wh-over.txt'
 
-        def _store_wheelhouses(args):
+        def _store_wheelhouses(args, **kwargs):
             filename = args[-1].split()[-1]
             if filename.endswith('.txt'):
                 Process._wheelhouses.append(path(filename).lines(retain=False))