Avoid late (non-top-level) imports

Avoiding late imports helps to isolate Blender add-ons bundling BAT from
each other.

There was one late/lazy import to avoid a dependency cycle. This was
solved by simply copying that one tiny function.

CHANGELOG.md

@@ -3,7 +3,12 @@
 This file logs the changes that are actually interesting to users (new features,
 changed functionality, fixed bugs).
 
-# Version 1.11 (in development)
+# Version 1.12 (in development)
+
+- Removed "late imports", to help isolate Blender add-ons bundling BAT from each other.
+
+
+# Version 1.11 (2022-02-18)
 
 - Support UDIM images.
 

README.md

@@ -115,3 +115,19 @@ To understand the naming of the properties, look at Blender's `DNA_xxxx.h` files
 definitions. It is those names that are accessed via `blender_asset_tracer.blendfile`. The
 mapping to the names accessible in Blender's Python interface can be found in the `rna_yyyy.c`
 files.
+
+
+## Code Guidelines
+
+This section documents some guidelines for the code in BAT.
+
+### Avoiding Late Imports
+
+All imports should be done at the top level of the file, and not inside
+functions. The goal is to ensure that, once imported, a (sub)module of BAT can
+be used without having to import more parts of BAT.
+
+This requirement helps to keep Blender add-ons separated, as an add-on can
+import the modules of BAT it needs, then remove them from `sys.modules` and
+`sys.path` so that other add-ons don't see them. This should reduce problems
+with various add-ons shipping different versions of BAT.

blender_asset_tracer/pack/__init__.py

@@ -28,6 +28,7 @@ import typing
 
 from blender_asset_tracer import trace, bpathlib, blendfile
 from blender_asset_tracer.trace import file_sequence, result
+
 from . import filesystem, transfer, progress
 
 log = logging.getLogger(__name__)
@@ -121,9 +122,7 @@ class Packer:
 
         self._exclude_globs = set()  # type: typing.Set[str]
 
-        from blender_asset_tracer.cli import common
-
-        self._shorten = functools.partial(common.shorten, self.project)
+        self._shorten = functools.partial(shorten_path, self.project)
 
         if noop:
             log.warning("Running in no-op mode, only showing what will be done.")
@@ -618,3 +617,11 @@ class Packer:
             )
 
         self._file_transferer.queue_move(infopath, self._target_path / infoname)
+
+
+def shorten_path(cwd: pathlib.Path, somepath: pathlib.Path) -> pathlib.Path:
+    """Return 'somepath' relative to CWD if possible."""
+    try:
+        return somepath.relative_to(cwd)
+    except ValueError:
+        return somepath