Fallback functionality for interpreter discovery

.pre-commit-config.yaml

@@ -19,18 +19,16 @@ repos:
   rev: 5.6.4
   hooks:
   - id: isort
-- repo: /~https://github.com/ambv/black
+- repo: /~https://github.com/psf/black
   rev: 20.8b1
   hooks:
   - id: black
     args: [--safe]
-    language_version: python3.8
 - repo: /~https://github.com/asottile/blacken-docs
   rev: v1.8.0
   hooks:
   - id: blacken-docs
     additional_dependencies: [black==20.8b1]
-    language_version: python3.8
 - repo: /~https://github.com/pre-commit/pygrep-hooks
   rev: v1.6.0
   hooks:
@@ -50,4 +48,3 @@ repos:
   hooks:
   - id: flake8
     additional_dependencies: ["flake8-bugbear == 20.1.4"]
-    language_version: python3.8

docs/changelog/1995.feature.rst

@@ -0,0 +1,2 @@
+The python specification can now take one or more values, first found is used to create the virtual environment - by
+:user:`gaborbernat`.

setup.py

@@ -6,7 +6,7 @@
 setup(
     use_scm_version={
         "write_to": "src/virtualenv/version.py",
-        "write_to_template": 'from __future__ import  unicode_literals;\n\n__version__ = "{version}"',
+        "write_to_template": 'from __future__ import  unicode_literals\n\n__version__ = "{version}"\n',
     },
     setup_requires=["setuptools_scm >= 2"],
 )

src/virtualenv/discovery/builtin.py

@@ -15,7 +15,7 @@
 class Builtin(Discover):
     def __init__(self, options):
         super(Builtin, self).__init__(options)
-        self.python_spec = options.python
+        self.python_spec = options.python if options.python else [sys.executable]
         self.app_data = options.app_data
 
     @classmethod
@@ -25,18 +25,25 @@ def add_parser_arguments(cls, parser):
             "--python",
             dest="python",
             metavar="py",
-            help="target interpreter for which to create a virtual (either absolute path or identifier string)",
-            default=sys.executable,
+            action="append",
+            default=[],
+            help="interpreter based on what to create environment (path/identifier) "
+            "- by default use the interpreter where the tool is installed - first found wins",
         )
 
     def run(self):
-        return get_interpreter(self.python_spec, self.app_data)
+        for python_spec in self.python_spec:
+            result = get_interpreter(python_spec, self.app_data)
+            if result is not None:
+                return result
+        return None
 
     def __repr__(self):
         return ensure_str(self.__unicode__())
 
     def __unicode__(self):
-        return "{} discover of python_spec={!r}".format(self.__class__.__name__, self.python_spec)
+        spec = self.python_spec[0] if len(self.python_spec) == 1 else self.python_spec
+        return "{} discover of python_spec={!r}".format(self.__class__.__name__, spec)
 
 
 def get_interpreter(key, app_data=None):

tests/unit/discovery/test_discovery.py

@@ -3,11 +3,12 @@
 import logging
 import os
 import sys
+from argparse import Namespace
 from uuid import uuid4
 
 import pytest
 
-from virtualenv.discovery.builtin import get_interpreter
+from virtualenv.discovery.builtin import Builtin, get_interpreter
 from virtualenv.discovery.py_info import PythonInfo
 from virtualenv.info import fs_supports_symlink
 from virtualenv.util.path import Path
@@ -53,3 +54,24 @@ def test_relative_path(tmp_path, session_app_data, monkeypatch):
     relative = str(sys_executable.relative_to(cwd))
     result = get_interpreter(relative, session_app_data)
     assert result is not None
+
+
+def test_discovery_fallback_fail(session_app_data, caplog):
+    caplog.set_level(logging.DEBUG)
+    builtin = Builtin(Namespace(app_data=session_app_data, python=["magic-one", "magic-two"]))
+
+    result = builtin.run()
+    assert result is None
+
+    assert "accepted" not in caplog.text
+
+
+def test_discovery_fallback_ok(session_app_data, caplog):
+    caplog.set_level(logging.DEBUG)
+    builtin = Builtin(Namespace(app_data=session_app_data, python=["magic-one", sys.executable]))
+
+    result = builtin.run()
+    assert result is not None, caplog.text
+    assert result.executable == sys.executable, caplog.text
+
+    assert "accepted" in caplog.text