Fix VIRTUALENV_PYTHON environment lookup

docs/changelog/1998.bugfix.rst

@@ -0,0 +1,2 @@
+Fix processing of the ``VIRTUALENV_PYTHON`` environment variable and make it
+multi-value as well (separated by comma) - by :user:`pneff`.

docs/cli_interface.rst

@@ -71,6 +71,12 @@ variable ``VIRTUALENV_PYTHON`` like:
 
    env VIRTUALENV_PYTHON=/opt/python-3.8/bin/python virtualenv
 
+Multiple values can be provided separated with a comma:
+
+.. code-block:: console
+
+   env VIRTUALENV_PYTHON=/opt/python-3.8/bin/python,python3.8 virtualenv
+
 This also works for appending command line options, like :option:`extra-search-dir`, where a literal newline
 is used to separate the values:
 

src/virtualenv/config/convert.py

@@ -46,16 +46,31 @@ def _validate(self):
         """"""
 
     def convert(self, value, flatten=True):
-        if isinstance(value, (str, bytes)):
-            value = filter(None, [x.strip() for x in value.splitlines()])
-        values = list(value)
+        values = self.split_values(value)
         result = []
         for value in values:
             sub_values = value.split(os.pathsep)
             result.extend(sub_values)
         converted = [self.as_type(i) for i in result]
         return converted
 
+    def split_values(self, value):
+        """Split the provided value into a list.
+
+        For strings this is a comma-separated. For more complex types (`Path`
+        for example) this is newline-separated.
+        """
+        if isinstance(value, (str, bytes)):
+            if self.as_type is str:
+                # Simple split
+                value = value.split(",")
+            else:
+                value = value.splitlines()
+
+            value = filter(None, [x.strip() for x in value])
+
+        return list(value)
+
 
 def convert(value, as_type, source):
     """Convert the value as a given type where the value comes from the given source"""

src/virtualenv/discovery/builtin.py

@@ -25,6 +25,7 @@ def add_parser_arguments(cls, parser):
             "--python",
             dest="python",
             metavar="py",
+            type=str,
             action="append",
             default=[],
             help="interpreter based on what to create environment (path/identifier) "

tests/unit/config/test_env_var.py

@@ -4,6 +4,7 @@
 
 import pytest
 
+from virtualenv.config.cli.parser import VirtualEnvOptions
 from virtualenv.config.ini import IniConfig
 from virtualenv.run import session_via_cli
 from virtualenv.util.path import Path
@@ -31,6 +32,20 @@ def test_value_bad(monkeypatch, caplog, empty_conf):
     assert "invalid literal" in caplog.messages[0]
 
 
+def test_python_via_env_var(monkeypatch):
+    options = VirtualEnvOptions()
+    monkeypatch.setenv(str("VIRTUALENV_PYTHON"), str("python3"))
+    session_via_cli(["venv"], options=options)
+    assert options.python == ["python3"]
+
+
+def test_python_multi_value_via_env_var(monkeypatch):
+    options = VirtualEnvOptions()
+    monkeypatch.setenv(str("VIRTUALENV_PYTHON"), str("python3,python2"))
+    session_via_cli(["venv"], options=options)
+    assert options.python == ["python3", "python2"]
+
+
 def test_extra_search_dir_via_env_var(tmp_path, monkeypatch):
     monkeypatch.chdir(tmp_path)
     value = "a{}0{}b{}c".format(os.linesep, os.linesep, os.pathsep)