{catkin,python} plugin: support cleaning (#2171)

Both the Catkin and Python plugins utilize the Pip class in order to
clean their pull steps (specifically, they tell the Pip class to clean
its packages). However, when cleaning the pull step, Python has already
been removed from the installdir, which means the Pip class cannot be
instantiated, causing an error about being unable to find Python.

It's reasonable to ask Pip to clean any fetched packages, and doing so
doesn't require Python. Fix this issue by lazily looking for Python only
when it's required instead of upon class instantiation.

LP: #1778716

Signed-off-by: Kyle Fazzari <kyrofa@ubuntu.com>

snapcraft/plugins/_python/_pip.py

@@ -120,20 +120,32 @@ def __init__(self, *, python_major_version, part_dir, install_dir,
                                            staging or part's install area.
         """
         self._python_major_version = python_major_version
-        self._part_dir = part_dir
         self._install_dir = install_dir
         self._stage_dir = stage_dir
 
-        self._python_package_dir = os.path.join(
-            self._part_dir, 'python-packages')
+        self._python_package_dir = os.path.join(part_dir, 'python-packages')
         os.makedirs(self._python_package_dir, exist_ok=True)
 
-        self._python_command = get_python_command(
-            self._python_major_version, stage_dir=self._stage_dir,
-            install_dir=self._install_dir)
-        self._python_home = get_python_home(
-            self._python_major_version, stage_dir=self._stage_dir,
-            install_dir=self._install_dir)
+        self.__python_command = None  # type:str
+        self.__python_home = None  # type: str
+
+    @property
+    def _python_command(self):
+        """Lazily determine the python command required."""
+        if not self.__python_command:
+            self.__python_command = get_python_command(
+                self._python_major_version, stage_dir=self._stage_dir,
+                install_dir=self._install_dir)
+        return self.__python_command
+
+    @property
+    def _python_home(self):
+        """Lazily determine the correct python home."""
+        if not self.__python_home:
+            self.__python_home = get_python_home(
+                self._python_major_version, stage_dir=self._stage_dir,
+                install_dir=self._install_dir)
+        return self.__python_home
 
     def setup(self):
         """Install pip and dependencies.
@@ -159,20 +171,20 @@ def _ensure_pip_installed(self):
         if not self._is_pip_installed():
             logger.info('Fetching and installing pip...')
 
-            real_python_home = self._python_home
+            real_python_home = self.__python_home
 
             # Make sure we're using pip from the host. Wrapping this operation
             # in a try/finally to make sure we revert away from the host's
             # python at the end.
             try:
-                self._python_home = os.path.join(os.path.sep, 'usr')
+                self.__python_home = os.path.join(os.path.sep, 'usr')
 
                 # Using the host's pip, install our own pip
                 self.download({'pip'})
                 self.install({'pip'}, ignore_installed=True)
             finally:
                 # Now that we have our own pip, reset the python home
-                self._python_home = real_python_home
+                self.__python_home = real_python_home
 
     def _ensure_wheel_installed(self):
         if not self._is_wheel_installed():

snapcraft/plugins/python.py

@@ -195,7 +195,7 @@ def __init__(self, name, options, project):
         # Pip requires only the major version of python rather than the command
         # name like our option requires.
         match = re.match(
-            'python(?P<major_version>\d).*', self.options.python_version)
+            r'python(?P<major_version>\d).*', self.options.python_version)
         if not match:
             raise UnsupportedPythonVersionError(
                 python_version=self.options.python_version)

tests/integration/plugins_python/test_python_plugin.py

@@ -256,3 +256,25 @@ def test_plugin_plays_nice_with_stage_packages(self):
             self.run_snapcraft('stage')
         except subprocess.CalledProcessError:
             self.fail('These parts should not have conflicted')
+
+    def test_python_part_can_be_cleaned(self):
+        # Regression test for LP: #1778716
+
+        snapcraft_yaml = fixture_setup.SnapcraftYaml(self.path)
+        snapcraft_yaml.update_part('python-part', {
+            'plugin': 'python',
+            'source': '.',
+        })
+        snapcraft_yaml.update_part('dummy-part', {
+            'plugin': 'nil',
+        })
+        self.useFixture(snapcraft_yaml)
+
+        # First stage both parts
+        self.run_snapcraft('stage')
+
+        # Now clean the python one, which should succeed
+        try:
+            self.run_snapcraft(['clean', 'python-part'])
+        except subprocess.CalledProcessError:
+            self.fail('Expected python part to clean successfully')