Fixes to make fades to actually work in a core20 snap.

fades/envbuilder.py

@@ -55,7 +55,14 @@ def __init__(self, env_path=None):
         self.env_bin_path = ''
         logger.debug("Env will be created at: %s", self.env_path)
 
-        if sys.version_info >= (3, 4):
+        if os.environ.get("SNAP"):
+            # running inside a snap: we need to avoid EnvBuilder ending up running ensurepip
+            # because it doesn't work properly (it does a special magic to run the script
+            # and ends up mixing external and internal pips)
+            self.pip_installed = False
+            super().__init__(with_pip=False, symlinks=True)
+
+        elif sys.version_info >= (3, 4):
             # try to install pip using default machinery (which will work in a lot
             # of systems, noticeably it won't in some debians or ubuntus, like
             # Trusty; in that cases mark it to install manually later)

fades/main.py

@@ -208,6 +208,11 @@ def detect_inside_virtualenv(prefix, real_prefix, base_prefix):
         https://github.com/pypa/pip/blob/281eb61b09d87765d7c2b92f6982b3fe76ccb0af/
             pip/locations.py#L39
     """
+    if os.environ.get("SNAP"):
+        # snaps under core20 are really virtualenvs but we have full control of the
+        # system layout, "this is fine" (<insert meme>), skip this control
+        return False
+
     if real_prefix is not None:
         return True
 

pkg/snap/snapcraft.yaml

@@ -25,17 +25,35 @@ adopt-info: fades  # look for 'snapcraftctl set-*' in the fades part
 
 apps:
   fades:
-    command: bin/fades
-    environment:
-      PYTHONPATH: $SNAP/lib/python3.8/site-packages/
+    command: bin/python3 -m fades
 
 parts:
+  # Classic core20 snaps require staged python.
+  python3:
+    plugin: nil
+    build-packages:
+      - python3-dev
+    stage-packages:
+      - libpython3-stdlib
+      - libpython3.8-minimal
+      - libpython3.8-stdlib
+      - python3.8-minimal
+      - python3-distutils
+      - python3-minimal
+      - python3-pkg-resources
+      - python3-pip
+      - python3-setuptools
+      - python3-venv
+      - python3-wheel
+
   fades:
+    after: [python3]
     source: .
     plugin: python
-    requirements:
-      - requirements.txt
     override-pull: |
       snapcraftctl pull
       snapcraftctl set-version "$( python3 -c 'import fades; print(fades._version.__version__)' )"
-
+    override-build: |
+      snapcraftctl build
+      # python3 fixup symlink (snapcraft bug)
+      ln -sf ../usr/bin/python3.8 $SNAPCRAFT_PART_INSTALL/bin/python3