Allow user to inject specific Pythons to try as step 1 during discovery

At the moment it's unecessarily hard to specify a specific python
executable to use for a given spec. While mangling with the path
might work for environments for tox environments that require a python
differing the host tox python version; for environments matching the
host tox interpreter it's impossible to use any other interpreter than
what tox uses. Here we add a CLI argument (fallback to some environment
variable for CI usage) that allows the user to inject custom python
interpreters as step 1 of the discovery mechanism.

Signed-off-by: Bernat Gabor <bgabor8@bloomberg.net>

docs/changelog/x.feature.rst

@@ -0,0 +1,2 @@
+Add ``--discover`` (fallback to ``TOX_DISCOVER`` environment variable via path separator) to inject python executables
+to try as first step of a discovery - note the executable still needs to match the environment by :user:`gaborbernat`.

src/tox/config/__init__.py

@@ -475,6 +475,14 @@ def tox_addoption(parser):
         help="write a json file with detailed information "
         "about all commands and results involved.",
     )
+    parser.add_argument(
+        "--discover",
+        dest="discover",
+        nargs="+",
+        metavar="PATH",
+        help="for python discovery first try the python executables under these paths",
+        default=[],
+    )
 
     # We choose 1 to 4294967295 because it is the range of PYTHONHASHSEED.
     parser.add_argument(

src/tox/interpreters/common.py

@@ -0,0 +1,25 @@
+import os
+
+from tox.interpreters.py_spec import CURRENT, PythonSpec
+from tox.interpreters.via_path import exe_spec
+
+
+def base_discover(envconfig):
+    base_python = envconfig.basepython
+    spec = PythonSpec.from_name(base_python)
+
+    # 1. check passed in discover elements
+    discovers = envconfig.config.option.discover
+    if not discovers:
+        discovers = os.environ.get(str("TOX_DISCOVER"), "").split(os.pathsep)
+    for discover in discovers:
+        if os.path.exists(discover):
+            cur_spec = exe_spec(discover, envconfig.basepython)
+            if cur_spec is not None and cur_spec.satisfies(spec):
+                return spec, cur_spec.path
+
+    # 2. check current
+    if spec.name is not None and CURRENT.satisfies(spec):
+        return spec, CURRENT.path
+
+    return spec, None

src/tox/interpreters/unix.py

@@ -2,20 +2,18 @@
 
 import tox
 
-from .py_spec import CURRENT, PythonSpec
+from .common import base_discover
 from .via_path import check_with_path
 
 
 @tox.hookimpl
 def tox_get_python_executable(envconfig):
-    base_python = envconfig.basepython
-    spec = PythonSpec.from_name(base_python)
-    # first, check current
-    if spec.name is not None and CURRENT.satisfies(spec):
-        return CURRENT.path
-    # second check if the literal base python
-    candidates = [base_python]
-    # third check if the un-versioned name is good
-    if spec.name is not None and spec.name != base_python:
+    spec, path = base_discover(envconfig)
+    if path is not None:
+        return path
+    # 3. check if the literal base python
+    candidates = [envconfig.basepython]
+    # 4. check if the un-versioned name is good
+    if spec.name is not None and spec.name != envconfig.basepython:
         candidates.append(spec.name)
     return check_with_path(candidates, spec)

src/tox/interpreters/windows/__init__.py

@@ -4,18 +4,16 @@
 
 import tox
 
-from ..py_spec import CURRENT, PythonSpec
+from ..common import base_discover
+from ..py_spec import CURRENT
 from ..via_path import check_with_path
 
 
 @tox.hookimpl
 def tox_get_python_executable(envconfig):
-    base_python = envconfig.basepython
-    spec = PythonSpec.from_name(base_python)
-    # first, check current
-    if spec.name is not None and CURRENT.satisfies(spec):
-        return CURRENT.path
-
+    spec, path = base_discover(envconfig)
+    if path is not None:
+        return path
     # second check if the py.exe has it (only for non path specs)
     if spec.path is None:
         py_exe = locate_via_pep514(spec)
@@ -25,7 +23,7 @@ def tox_get_python_executable(envconfig):
     # third check if the literal base python is on PATH
     candidates = [envconfig.basepython]
     # fourth check if the name is on PATH
-    if spec.name is not None and spec.name != base_python:
+    if spec.name is not None and spec.name != envconfig.basepython:
         candidates.append(spec.name)
     # or check known locations
     if spec.major is not None and spec.minor is not None: