Ensure we don't pick up Poetry's virtualenv as the system env

poetry/console/commands/plugin/add.py

@@ -72,7 +72,7 @@ You can specify a package in the following forms:
         plugins = self.argument("plugins")
 
         # Plugins should be installed in the system env to be globally available
-        system_env = EnvManager.get_system_env()
+        system_env = EnvManager.get_system_env(naive=True)
 
         env_dir = Path(
             os.getenv("POETRY_HOME") if os.getenv("POETRY_HOME") else system_env.path

poetry/console/commands/plugin/remove.py

@@ -43,7 +43,7 @@ class PluginRemoveCommand(Command):
 
         plugins = self.argument("plugins")
 
-        system_env = EnvManager.get_system_env()
+        system_env = EnvManager.get_system_env(naive=True)
         env_dir = Path(
             os.getenv("POETRY_HOME") if os.getenv("POETRY_HOME") else system_env.path
         )

poetry/console/commands/plugin/show.py

@@ -38,7 +38,7 @@ class PluginShowCommand(Command):
             + PluginManager("plugin").get_plugin_entry_points()
         )
 
-        system_env = EnvManager.get_system_env()
+        system_env = EnvManager.get_system_env(naive=True)
         installed_repository = InstalledRepository.load(
             system_env, with_dependencies=True
         )

poetry/locations.py

+import os
+
 from pathlib import Path
 
 from .utils.appdirs import user_cache_dir
@@ -10,3 +12,10 @@ DATA_DIR = user_data_dir("pypoetry")
 CONFIG_DIR = user_config_dir("pypoetry")
 
 REPOSITORY_CACHE_DIR = Path(CACHE_DIR) / "cache" / "repositories"
+
+
+def data_dir() -> Path:
+    if os.getenv("POETRY_HOME"):
+        return Path(os.getenv("POETRY_HOME")).expanduser()
+
+    return Path(user_data_dir("pypoetry", roaming=True))

poetry/utils/env.py

@@ -996,8 +996,31 @@ class EnvManager:
                 shutil.rmtree(str(file_path))
 
     @classmethod
-    def get_system_env(cls) -> "SystemEnv":
-        return SystemEnv(Path(sys.prefix), cls.get_base_prefix())
+    def get_system_env(cls, naive: bool = False) -> "SystemEnv":
+        """
+        Retrieve the current Python environment.
+
+        This can be the base Python environment or an activated virtual environment.
+
+        This method also workaround the issue that the virtual environment
+        used by Poetry internally (when installed via the custom installer)
+        is incorrectly detected as the system environment. Note that this workaround
+        happens only when `naive` is False since there are times where we actually
+        want to retrieve Poetry's custom virtual environment
+        (e.g. plugin installation or self update).
+        """
+        prefix, base_prefix = Path(sys.prefix), cls.get_base_prefix()
+        if naive is False:
+            from poetry.locations import data_dir
+
+            try:
+                prefix.relative_to(data_dir())
+            except ValueError:
+                pass
+            else:
+                prefix = base_prefix
+
+        return SystemEnv(prefix, base_prefix)
 
     @classmethod
     def get_base_prefix(cls) -> Path: