test_download_use_pep517_propagation: use a different approach

The approach it uses now doesn't work anymore due to 452d7da.
The installation of `fake_dep` now succeeds whether or not `setuptools`
is installed in the test environment.

Use a different approach instead: try to import `pip` in the `setup.py`
script. If it succeeds, then we are not running in an isolated environment,
and therefore PEP 517 isn't being used.

To add this custom logic to `setup.py`, add a new argument to
`create_basic_sdist_for_package`. Note that to make this work, I had to
switch from f-strings to `str.format`, since the `dedent` has to happen
before formatting.

Author: SpecLad

tests/functional/test_download.py

@@ -1181,12 +1181,25 @@ def test_download_use_pep517_propagation(
     on the command line, but to their dependencies too.
     """
 
-    # Remove setuptools to ensure that metadata retrieval fails unless PEP 517
-    # is used.
-    script.pip("uninstall", "-y", "setuptools")
-
     create_basic_sdist_for_package(script, "fake_proj", "1.0", depends=["fake_dep"])
-    create_basic_sdist_for_package(script, "fake_dep", "1.0")
+
+    # If --use-pep517 is in effect, then setup.py should be running in an isolated
+    # environment that doesn't have pip in it.
+    create_basic_sdist_for_package(
+        script,
+        "fake_dep",
+        "1.0",
+        setup_py_prelude=textwrap.dedent(
+            """\
+            try:
+                import pip
+            except ImportError:
+                pass
+            else:
+                raise Exception(f"not running in isolation")
+            """
+        ),
+    )
 
     download_dir = tmpdir / "download_dir"
     script.pip(

tests/lib/__init__.py

@@ -1212,12 +1212,16 @@ def create_basic_sdist_for_package(
     fails_egg_info: bool = False,
     fails_bdist_wheel: bool = False,
     depends: Optional[List[str]] = None,
+    setup_py_prelude: str = "",
 ) -> pathlib.Path:
     files = {
-        "setup.py": f"""\
+        "setup.py": textwrap.dedent(
+            """\
             import sys
             from setuptools import find_packages, setup
 
+            {setup_py_prelude}
+
             fails_bdist_wheel = {fails_bdist_wheel!r}
             fails_egg_info = {fails_egg_info!r}
 
@@ -1228,19 +1232,21 @@ def create_basic_sdist_for_package(
                 raise Exception("Simulated failure for building a wheel.")
 
             setup(name={name!r}, version={version!r},
-                install_requires={depends or []!r})
-        """,
+                install_requires={depends!r})
+        """
+        ).format(
+            name=name,
+            version=version,
+            depends=depends or [],
+            setup_py_prelude=setup_py_prelude,
+            fails_bdist_wheel=fails_bdist_wheel,
+            fails_egg_info=fails_egg_info,
+        ),
     }
 
     # Some useful shorthands
     archive_name = f"{name}-{version}.tar.gz"
 
-    # Replace key-values with formatted values
-    for key, value in list(files.items()):
-        del files[key]
-        key = key.format(name=name)
-        files[key] = textwrap.dedent(value)
-
     # Add new files after formatting
     if extra_files:
         files.update(extra_files)