refactor: pull out env selection to dict

Signed-off-by: Henry Schreiner <[email protected]>

nox/_options.py

@@ -26,6 +26,7 @@
 
 from nox import _option_set
 from nox.tasks import discover_manifest, filter_manifest, load_nox_module
+from nox.virtualenv import ALL_VENVS
 
 if sys.version_info < (3, 8):
     from typing_extensions import Literal
@@ -423,10 +424,9 @@ def _tag_completer(
         merge_func=_default_venv_backend_merge_func,
         help=(
             "Virtual environment backend to use by default for Nox sessions, this is"
-            " ``'virtualenv'`` by default but any of ``('uv, 'virtualenv',"
-            " 'conda', 'mamba', 'venv')`` are accepted."
+            " ``'virtualenv'`` by default but any of ``{list(ALL_VENVS)!r}`` are accepted."
         ),
-        choices=["none", "virtualenv", "conda", "mamba", "venv", "uv"],
+        choices=list(ALL_VENVS),
     ),
     _option_set.Option(
         "force_venv_backend",
@@ -438,10 +438,9 @@ def _tag_completer(
         help=(
             "Virtual environment backend to force-use for all Nox sessions in this run,"
             " overriding any other venv backend declared in the Noxfile and ignoring"
-            " the default backend. Any of ``('uv', 'virtualenv', 'conda', 'mamba',"
-            " 'venv')`` are accepted."
+            " the default backend. Any of ``{list(ALL_VENVS)!r}`` are accepted."
         ),
-        choices=["none", "virtualenv", "conda", "mamba", "venv", "uv"],
+        choices=list(ALL_VENVS),
     ),
     _option_set.Option(
         "no_venv",

nox/sessions.py

@@ -38,6 +38,7 @@
 )
 
 import nox.command
+import nox.virtualenv
 from nox._decorators import Func
 from nox.logger import logger
 from nox.virtualenv import CondaEnv, PassthroughEnv, ProcessEnv, VirtualEnv
@@ -757,39 +758,29 @@ def envdir(self) -> str:
         return _normalize_path(self.global_config.envdir, self.friendly_name)
 
     def _create_venv(self) -> None:
+        reuse_existing = self.reuse_existing_venv()
+
         backend = (
             self.global_config.force_venv_backend
             or self.func.venv_backend
             or self.global_config.default_venv_backend
+            or "virtualenv"
         )
 
+        if backend not in nox.virtualenv.ALL_VENVS:
+            msg = f"Expected venv_backend one of {list(nox.virtualenv.ALL_VENVS)!r}, but got {backend!r}."
+            raise ValueError(msg)
+
         if backend == "none" or self.func.python is False:
-            self.venv = PassthroughEnv()
+            self.venv = nox.virtualenv.ALL_VENVS["none"]()
             return
 
-        reuse_existing = self.reuse_existing_venv()
-
-        if backend is None or backend in {"virtualenv", "venv", "uv"}:
-            self.venv = VirtualEnv(
-                self.envdir,
-                interpreter=self.func.python,  # type: ignore[arg-type]
-                reuse_existing=reuse_existing,
-                venv_backend=backend or "virtualenv",
-                venv_params=self.func.venv_params,
-            )
-        elif backend in {"conda", "mamba"}:
-            self.venv = CondaEnv(
-                self.envdir,
-                interpreter=self.func.python,  # type: ignore[arg-type]
-                reuse_existing=reuse_existing,
-                venv_params=self.func.venv_params,
-                conda_cmd=backend,
-            )
-        else:
-            raise ValueError(
-                "Expected venv_backend one of ('virtualenv', 'conda', 'mamba',"
-                f" 'venv'), but got '{backend}'."
-            )
+        self.venv = nox.virtualenv.ALL_VENVS[backend](
+            self.envdir,
+            interpreter=self.func.python,
+            reuse_existing=reuse_existing,
+            venv_params=self.func.venv_params,
+        )
 
         self.venv.create()
 

nox/virtualenv.py

@@ -15,13 +15,14 @@
 from __future__ import annotations
 
 import contextlib
+import functools
 import os
 import platform
 import re
 import shutil
 import subprocess
 import sys
-from collections.abc import Mapping
+from collections.abc import Callable, Mapping
 from socket import gethostbyname
 from typing import Any, ClassVar
 
@@ -532,3 +533,13 @@ def create(self) -> bool:
         nox.command.run(cmd, silent=True, log=nox.options.verbose or False)
 
         return True
+
+
+ALL_VENVS: dict[str, Callable[..., ProcessEnv]] = {
+    "conda": functools.partial(CondaEnv, conda_cmd="conda"),
+    "mamba": functools.partial(CondaEnv, conda_cmd="mamba"),
+    "virtualenv": functools.partial(VirtualEnv, venv_backend="virtualenv"),
+    "venv": functools.partial(VirtualEnv, venv_backend="venv"),
+    "uv": functools.partial(VirtualEnv, venv_backend="uv"),
+    "none": PassthroughEnv,
+}