Persist packages from original lockfile for platforms not requested f…

…or lock

Addresses #196: for requested platforms replaces lock content without erroneously persisting packages.

Co-authored-by: Arie Knoester <arikstr@gmail.com>
Reviewed-by: Matt Fisher <mfisher87@gmail.com>

conda_lock/conda_lock.py

@@ -361,6 +361,7 @@ def make_lock_files(  # noqa: C901
                     or not check_input_hash
                     or lock_spec.content_hash_for_platform(platform)
                     != lock_content.metadata.content_hash[platform]
+                    # TODO: Modify above line to allow for quick fail on differing environments
                 ):
                     platforms_to_lock.append(platform)
                     if platform in platforms_already_locked:
@@ -378,7 +379,8 @@ def make_lock_files(  # noqa: C901
 
         if platforms_to_lock:
             print(f"Locking dependencies for {platforms_to_lock}...", file=sys.stderr)
-            lock_content = lock_content | create_lockfile_from_spec(
+
+            new_lock_content = create_lockfile_from_spec(
                 conda=conda,
                 spec=lock_spec,
                 platforms=platforms_to_lock,
@@ -389,6 +391,21 @@ def make_lock_files(  # noqa: C901
                 strip_auth=strip_auth,
             )
 
+            if not lock_content:
+                lock_content = new_lock_content
+            else:
+                # Persist packages from original lockfile for platforms not requested for lock
+                packages_not_to_lock = [
+                    dep
+                    for dep in lock_content.package
+                    if dep.platform not in platforms_to_lock
+                ]
+                filtered_lock_content = lock_content.copy(
+                    deep=True,
+                    update={"package": packages_not_to_lock},
+                )
+                lock_content = filtered_lock_content | new_lock_content
+
             if "lock" in kinds:
                 write_conda_lock_file(
                     lock_content,