Standardise on using virtualenv package

Until now poetry used the built-in venv module available in Python 3.
This has presented a few concerns. Chief of which has been
inconsistent environment setup. This includes, but is not limited to,
pip version used by the `ensurepip` module which is in turn used by the
built-in `venv.EnvBuilder`. Additionally, the `virtualenv` package
allows for faster environment creation. This can also allow us to, going
forward, drop ad-hoc code required to inspect and manage environments.

poetry/utils/env.py

@@ -15,12 +15,14 @@
 from typing import List
 from typing import Optional
 from typing import Tuple
+from typing import Union
 
 import tomlkit
 
 from clikit.api.io import IO
 
 import packaging.tags
+import virtualenv
 
 from packaging.tags import Tag
 from packaging.tags import interpreter_name
@@ -141,27 +143,6 @@ def _version_nodot(version):
 print(json.dumps(sysconfig.get_paths()))
 """
 
-CREATE_VENV_COMMAND = """\
-path = {!r}
-
-try:
-    from venv import EnvBuilder
-
-    builder = EnvBuilder(with_pip=True)
-    builder.create(path)
-except ImportError:
-    try:
-        # We fallback on virtualenv for Python 2.7
-        from virtualenv import create_environment
-
-        create_environment(path)
-    except ImportError:
-        # since virtualenv>20 we have to use cli_run
-        from virtualenv import cli_run
-
-        cli_run([path])
-"""
-
 
 class EnvError(Exception):
 
@@ -697,43 +678,20 @@ def create_venv(
         return VirtualEnv(venv)
 
     @classmethod
-    def build_venv(cls, path, executable=None):
-        if executable is not None:
-            # Create virtualenv by using an external executable
-            try:
-                p = subprocess.Popen(
-                    list_to_shell_command([executable, "-"]),
-                    stdin=subprocess.PIPE,
-                    shell=True,
-                )
-                p.communicate(encode(CREATE_VENV_COMMAND.format(path)))
-            except CalledProcessError as e:
-                raise EnvCommandError(e)
-
-            return
-
-        try:
-            from venv import EnvBuilder
-
-            # use the same defaults as python -m venv
-            if os.name == "nt":
-                use_symlinks = False
-            else:
-                use_symlinks = True
-
-            builder = EnvBuilder(with_pip=True, symlinks=use_symlinks)
-            builder.create(path)
-        except ImportError:
-            try:
-                # We fallback on virtualenv for Python 2.7
-                from virtualenv import create_environment
-
-                create_environment(path)
-            except ImportError:
-                # since virtualenv>20 we have to use cli_run
-                from virtualenv import cli_run
-
-                cli_run([path])
+    def build_venv(
+        cls, path, executable=None
+    ):  # type: (str, Optional[Union[str, Path]]) -> virtualenv.run.session.Session
+        if isinstance(executable, Path):
+            executable = executable.resolve()
+        return virtualenv.cli_run(
+            [
+                "--no-download",
+                "--no-periodic-update",
+                "--python",
+                executable or sys.executable,
+                path,
+            ]
+        )
 
     def remove_venv(self, path):  # type: (str) -> None
         shutil.rmtree(path)

pyproject.toml

@@ -38,6 +38,7 @@ shellingham = "^1.1"
 tomlkit = "^0.5.11"
 pexpect = "^4.7.0"
 packaging = "^20.4"
+virtualenv = { version = "^20.0.26" }
 
 # The typing module is not in the stdlib in Python 2.7
 typing = { version = "^3.6", python = "~2.7" }
@@ -48,8 +49,6 @@ pathlib2 = { version = "^2.3", python = "~2.7" }
 futures = { version = "^3.3.0", python = "~2.7" }
 # Use glob2 for Python 2.7 and 3.4
 glob2 = { version = "^0.6", python = "~2.7" }
-# Use virtualenv for Python 2.7 since venv does not exist
-virtualenv = { version = "^20.0.18", python = "~2.7" }
 # functools32 is needed for Python 2.7
 functools32 = { version = "^3.2.3", python = "~2.7" }
 keyring = [

tests/utils/test_env.py

@@ -9,6 +9,7 @@
 
 from poetry.core.semver import Version
 from poetry.factory import Factory
+from poetry.utils._compat import PY2
 from poetry.utils._compat import Path
 from poetry.utils.env import EnvCommandError
 from poetry.utils.env import EnvManager
@@ -535,17 +536,11 @@ def test_remove_also_deactivates(tmp_dir, manager, poetry, config, mocker):
     assert venv_name not in envs
 
 
+@pytest.mark.skipif(
+    os.name == "nt" or PY2, reason="Symlinks are not support for Windows"
+)
 def test_env_has_symlinks_on_nix(tmp_dir, tmp_venv):
-    venv_available = False
-    try:
-        from venv import EnvBuilder  # noqa
-
-        venv_available = True
-    except ImportError:
-        pass
-
-    if os.name != "nt" and venv_available:
-        assert os.path.islink(tmp_venv.python)
+    assert os.path.islink(tmp_venv.python)
 
 
 def test_run_with_input(tmp_dir, tmp_venv):