test_settings: Prepare for including --pyenv in basepath-related tests

We have some test cases that verify correct interaction between
--code/--deps and basepath. We're about to add --pyenv to --code and
--deps as another option that interacts with basepath, and we'd like the
tests to cover this as well.

At this point the various combinations of passing or not passing these
options on the command line are becoming tedious to hardcode.

Instead, provide a `path_options` collection that list these options, and
a subsets() helper function that automatically enumerates the various
combinations of giving or not giving these options the we need in
test_path_option_overrides_base_path().

tests/test_settings.py

@@ -5,9 +5,21 @@
 import string
 import sys
 from dataclasses import dataclass, field
-from itertools import chain, permutations, product
+from itertools import chain, combinations, permutations, product
 from pathlib import Path
-from typing import Any, Dict, Iterable, List, Optional, Set, Tuple, Type, TypeVar, Union
+from typing import (
+    Any,
+    Dict,
+    Iterable,
+    Iterator,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,
+)
 
 import pytest
 from hypothesis import given, strategies
@@ -82,24 +94,55 @@ def test_code_deps_and_base_unequal__raises_error(code_deps_base):
         run_build_settings(args)
 
 
+path_options = {  # options (-> settings members) that interact with basepath
+    "--code": "code",
+    "--deps": "deps",
+}
+
+Item = TypeVar("Item")
+
+
+def subsets(
+    items: Set[Item],
+    min_size: int = 0,
+    max_size: int = sys.maxsize,
+) -> Iterator[Set[Item]]:
+    """Generate all subsets of the given set within the given size constraints.
+
+    This is equivalent to the "power set" with a size filter applied.
+    """
+    max_size = min(max_size, len(items))
+    for size in range(min_size, max_size + 1):
+        for tpl in combinations(items, size):
+            yield set(tpl)
+
+
 @given(basepaths=nonempty_string_set, fillers=nonempty_string_set)
-@pytest.mark.parametrize(["filled", "unfilled"], [("code", "deps"), ("deps", "code")])
-def test_base_path_respects_path_already_filled_via_cli(
-    basepaths, filled, unfilled, fillers
-):
-    args = list(basepaths) + [f"--{filled}"] + list(fillers)
+@pytest.mark.parametrize(
+    ["filled", "unfilled"],
+    [
+        pytest.param(opts, path_options.keys() - opts, id="Passing " + ", ".join(opts))
+        for opts in subsets(set(path_options.keys()), 1, len(path_options) - 1)
+    ],
+)
+def test_path_option_overrides_base_path(basepaths, filled, unfilled, fillers):
+    assert filled and unfilled and (unfilled | filled) == path_options.keys()
+    args = list(basepaths)
+    for option in filled:
+        args += [option] + list(fillers)
     settings = run_build_settings(args)
-    assert getattr(settings, filled) == to_path_set(fillers)
-    assert getattr(settings, unfilled) == to_path_set(basepaths)
+    for option in filled:
+        assert getattr(settings, path_options[option]) == to_path_set(fillers)
+    for option in unfilled:
+        assert getattr(settings, path_options[option]) == to_path_set(basepaths)
 
 
 @given(basepaths=nonempty_string_set)
-def test_base_path_fills_code_and_deps_when_other_path_settings_are_absent(basepaths):
+def test_base_path_fills_path_options_when_other_path_settings_are_absent(basepaths):
     # Nothing else through CLI nor through config file
     settings = run_build_settings(cmdl=list(basepaths))
     expected = to_path_set(basepaths)
-    assert settings.code == expected
-    assert settings.deps == expected
+    assert all(getattr(settings, memb) == expected for memb in path_options.values())
 
 
 @pytest.mark.parametrize(
@@ -118,7 +161,7 @@ def test_base_path_fills_code_and_deps_when_other_path_settings_are_absent(basep
         ]
     ],
 )
-def test_base_path_overrides_config_file_code_and_deps(
+def test_base_path_overrides_config_file_for_all_path_options(
     config_settings,
     basepaths,
     setup_fawltydeps_config,
@@ -129,8 +172,7 @@ def test_base_path_overrides_config_file_code_and_deps(
 
     settings = run_build_settings(cmdl=list(basepaths), config_file=config_file)
     expected = to_path_set(basepaths)
-    assert settings.code == expected
-    assert settings.deps == expected
+    assert all(getattr(settings, memb) == expected for memb in path_options.values())
 
 
 OPTION_VALUES = {