Add a --keep-untracked option to the install command.

Author: PetterS

poetry/console/commands/install.py

@@ -19,6 +19,11 @@ class InstallCommand(EnvCommand):
             "Output the operations but do not execute anything "
             "(implicitly enables --verbose).",
         ),
+        option(
+            "keep-untracked",
+            None,
+            "Does not remove packages not present in the lock file.",
+        ),
         option(
             "extras",
             "E",
@@ -58,6 +63,7 @@ def handle(self):
         installer.extras(extras)
         installer.dev_mode(not self.option("no-dev"))
         installer.dry_run(self.option("dry-run"))
+        installer.keep_untracked(self.option("keep-untracked"))
         installer.verbose(self.option("verbose"))
 
         return_code = installer.run()

poetry/installation/installer.py

@@ -39,6 +39,7 @@ def __init__(
         self._pool = pool
 
         self._dry_run = False
+        self._keep_untracked = False
         self._update = False
         self._verbose = False
         self._write_lock = True
@@ -83,6 +84,14 @@ def dry_run(self, dry_run=True):  # type: (bool) -> Installer
     def is_dry_run(self):  # type: () -> bool
         return self._dry_run
 
+    def keep_untracked(self, keep_untracked=True):  # type: (bool) -> Installer
+        self._keep_untracked = keep_untracked
+
+        return self
+
+    def is_keep_untracked(self):  # type: () -> bool
+        return self._keep_untracked
+
     def verbose(self, verbose=True):  # type: (bool) -> Installer
         self._verbose = verbose
 
@@ -424,6 +433,17 @@ def _get_operations_from_lock(
 
             ops.append(op)
 
+        if not self._keep_untracked:
+            for installed in installed_repo.packages:
+                is_in_lock_file = False
+                for locked in locked_repository.packages:
+                    if locked.name == installed.name:
+                        is_in_lock_file = True
+                        break
+
+                if not is_in_lock_file:
+                    ops.append(Uninstall(installed))
+
         return ops
 
     def _filter_operations(