Respect in-project config even when .venv exists

Before this change, when .venv directory exists  within the project 
root, poetry will use it as the path to the venv regardless whether 
virtualenvs.in-project is set to true or not. This leads to confusion
as described in #1770 and #2756.

With this change poetry will check if virtualenvs.in-project is set to
true if it finds a .venv folder.

This commit also changes the default state of this configuration to be
unset (null), in order to not break any current environments.

Resolves: #1770 #2756

docs/docs/configuration.md

@@ -33,7 +33,7 @@ which will give you something similar to this:
 ```toml
 cache-dir = "/path/to/cache/directory"
 virtualenvs.create = true
-virtualenvs.in-project = false
+virtualenvs.in-project = null
 virtualenvs.path = "{cache-dir}/virtualenvs"  # /path/to/cache/directory/virtualenvs
 ```
 
@@ -110,7 +110,12 @@ Defaults to `true`.
 ### `virtualenvs.in-project`: boolean
 
 Create the virtualenv inside the project's root directory.
-Defaults to `false`.
+Defaults to `None`.
+
+If set to `true`, the virtualenv wil be created and expected in a folder named `.venv` within the root directory of the project.
+
+If not set explicitly (default), `poetry` will use the virtualenv from the `.venv` directory when one is available. If set to `false`, `poetry` will ignore any existing `.venv` directory.
+
 
 ### `virtualenvs.path`: string
 

poetry/config/config.py

@@ -34,7 +34,7 @@ class Config(object):
         "cache-dir": str(CACHE_DIR),
         "virtualenvs": {
             "create": True,
-            "in-project": False,
+            "in-project": None,
             "path": os.path.join("{cache-dir}", "virtualenvs"),
         },
         "experimental": {"new-installer": True},

poetry/utils/env.py

@@ -346,10 +346,11 @@ class EnvManager(object):
 
         if not in_venv or env is not None:
             # Checking if a local virtualenv exists
-            if (cwd / ".venv").exists() and (cwd / ".venv").is_dir():
-                venv = cwd / ".venv"
+            if self._poetry.config.get("virtualenvs.in-project") is not False:
+                if (cwd / ".venv").exists() and (cwd / ".venv").is_dir():
+                    venv = cwd / ".venv"
 
-                return VirtualEnv(venv)
+                    return VirtualEnv(venv)
 
             create_venv = self._poetry.config.get("virtualenvs.create", True)
 

tests/console/commands/test_config.py

@@ -15,7 +15,7 @@ def test_list_displays_default_value_if_not_set(app, config):
     expected = """cache-dir = "/foo"
 experimental.new-installer = true
 virtualenvs.create = true
-virtualenvs.in-project = false
+virtualenvs.in-project = null
 virtualenvs.path = {path}  # /foo{sep}virtualenvs
 """.format(
         path=json.dumps(os.path.join("{cache-dir}", "virtualenvs")), sep=os.path.sep
@@ -35,7 +35,7 @@ def test_list_displays_set_get_setting(app, config):
     expected = """cache-dir = "/foo"
 experimental.new-installer = true
 virtualenvs.create = false
-virtualenvs.in-project = false
+virtualenvs.in-project = null
 virtualenvs.path = {path}  # /foo{sep}virtualenvs
 """.format(
         path=json.dumps(os.path.join("{cache-dir}", "virtualenvs")), sep=os.path.sep
@@ -83,7 +83,7 @@ def test_list_displays_set_get_local_setting(app, config):
     expected = """cache-dir = "/foo"
 experimental.new-installer = true
 virtualenvs.create = false
-virtualenvs.in-project = false
+virtualenvs.in-project = null
 virtualenvs.path = {path}  # /foo{sep}virtualenvs
 """.format(
         path=json.dumps(os.path.join("{cache-dir}", "virtualenvs")), sep=os.path.sep

tests/utils/test_env.py

@@ -74,17 +74,27 @@ 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):
-    if "VIRTUAL_ENV" in os.environ:
-        del os.environ["VIRTUAL_ENV"]
-
-    (poetry.file.parent / ".venv").mkdir()
-
+@pytest.fixture
+def in_project_venv_dir(poetry):
+    os.environ.pop("VIRTUAL_ENV", None)
+    venv_dir = poetry.file.parent.joinpath(".venv")
+    venv_dir.mkdir()
+    try:
+        yield venv_dir
+    finally:
+        venv_dir.rmdir()
+
+
+@pytest.mark.parametrize("in_project", [True, False, None])
+def test_env_get_venv_with_venv_folder_present(
+    manager, poetry, in_project_venv_dir, in_project
+):
+    poetry.config.config["virtualenvs"]["in-project"] = in_project
     venv = manager.get()
-
-    assert venv.path == poetry.file.parent / ".venv"
-
-    shutil.rmtree(str(venv.path))
+    if in_project is False:
+        assert venv.path != in_project_venv_dir
+    else:
+        assert venv.path == in_project_venv_dir
 
 
 def build_venv(path, executable=None):  # type: (Union[Path,str], Optional[str]) -> ()