* detect .venv folder in case in-project is true or None

* update docs
* add tests

Author: finswimmer

Diff for: docs/docs/configuration.md

@@ -112,6 +112,10 @@ Defaults to `true`.
 Create the virtualenv inside the project's root directory.
 Defaults to `false`.
 
+If set to `true`, the virtualenv wil be created and expected in a folder named `.venv` within the root directory of the project.
+
+If set to `None`, `poetry` will use the virtualenv from the `.venv` directory only if available. 
+
 ### `virtualenvs.path`: string
 
 Directory where virtual environments will be created.

Diff for: poetry/utils/env.py

@@ -347,7 +347,7 @@ def get(self, reload=False):  # type: (bool) -> Env
         if not in_venv or env is not None:
             # Checking if a local virtualenv exists
             if (cwd / ".venv").exists() and (cwd / ".venv").is_dir():
-                if self._poetry.config.get("virtualenvs.in-project"):
+                if self._poetry.config.get("virtualenvs.in-project") is not False:
                     venv = cwd / ".venv"
 
                     return VirtualEnv(venv)

Diff for: tests/utils/test_env.py

@@ -74,17 +74,35 @@ def test_virtualenvs_with_spaces_in_their_path_work_as_expected(tmp_dir, manager
     assert venv.run("python", "-V", shell=True).startswith("Python")
 
 
-def test_env_get_in_project_venv(manager, poetry):
+@pytest.mark.parametrize("in_project", [True, None])
+def test_env_get_in_project_venv_with_venv_folder_present(manager, poetry, in_project):
     if "VIRTUAL_ENV" in os.environ:
         del os.environ["VIRTUAL_ENV"]
 
-    (poetry.file.parent / ".venv").mkdir()
+    venv_folder = Path(poetry.file.parent) / ".venv"
+    venv_folder.mkdir()
+    poetry.config.config["virtualenvs"]["in-project"] = in_project
 
     venv = manager.get()
 
-    assert venv.path == poetry.file.parent / ".venv"
+    assert venv.path == venv_folder
 
-    shutil.rmtree(str(venv.path))
+    shutil.rmtree(str(venv_folder))
+
+
+def test_env_get_venv_with_venv_folder_present(manager, poetry):
+    if "VIRTUAL_ENV" in os.environ:
+        del os.environ["VIRTUAL_ENV"]
+
+    venv_folder = Path(poetry.file.parent) / ".venv"
+    venv_folder.mkdir()
+    poetry.config.config["virtualenvs"]["in-project"] = False
+
+    venv = manager.get()
+
+    assert venv.path != venv_folder
+
+    shutil.rmtree(str(venv_folder))
 
 
 def build_venv(path, executable=None):  # type: (Union[Path,str], Optional[str]) -> ()