Rework PEP 518 requirement logic

src/pip/_internal/operations/prepare.py

@@ -95,31 +95,33 @@ class IsSDist(DistAbstraction):
     def prep_for_dist(self, finder, build_isolation):
         # Before calling "setup.py egg_info", we need to set-up the build
         # environment.
-        build_requirements, isolate = self.req.get_pep_518_info()
-        should_isolate = build_isolation and isolate
-
-        minimum_requirements = ('setuptools', 'wheel')
-        missing_requirements = set(minimum_requirements) - set(
-            pkg_resources.Requirement(r).key
-            for r in build_requirements
-        )
-        if missing_requirements:
-            def format_reqs(rs):
-                return ' and '.join(map(repr, sorted(rs)))
-            logger.warning(
-                "Missing build time requirements in pyproject.toml for %s: "
-                "%s.", self.req, format_reqs(missing_requirements)
-            )
-            logger.warning(
-                "This version of pip does not implement PEP 517 so it cannot "
-                "build a wheel without %s.", format_reqs(minimum_requirements)
-            )
+        build_requirements = self.req.get_pep_518_info()
+        should_isolate = build_isolation and build_requirements is not None
 
         if should_isolate:
+            # Haven't implemented PEP 517 yet, so spew a warning about it if
+            # build-requirements don't include setuptools and wheel.
+            missing_requirements = {'setuptools', 'wheel'} - {
+                pkg_resources.Requirement(r).key for r in build_requirements
+            }
+            if missing_requirements:
+                logger.warning(
+                    "Missing build requirements in pyproject.toml for %s.",
+                    self.req,
+                )
+                logger.warning(
+                    "This version of pip does not implement PEP 517 so it "
+                    "cannot build a wheel without %s.",
+                    " and ".join(map(repr, sorted(missing_requirements)))
+                )
+
+            # Isolate in a BuildEnvironment and install the build-time
+            # requirements.
             self.req.build_env = BuildEnvironment()
             self.req.build_env.install_requirements(
                 finder, build_requirements,
-                "Installing build dependencies")
+                "Installing build dependencies"
+            )
 
         self.req.run_egg_info()
         self.req.assert_source_matches_version()

src/pip/_internal/req/req_install.py

@@ -560,42 +560,40 @@ class InstallRequirement(object):
         return pp_toml
 
     def get_pep_518_info(self):
-        """Get a list of the packages required to build the project, if any,
-        and a flag indicating whether pyproject.toml is present, indicating
-        that the build should be isolated.
+        """Get PEP 518 build-time requirements.
 
-        Build requirements can be specified in a pyproject.toml, as described
-        in PEP 518. If this file exists but doesn't specify build
-        requirements, pip will default to installing setuptools and wheel.
+        Returns the list of the packages required to build the project,
+        specified as per PEP 518 within the package. If `pyproject.toml` is not
+        present, returns None to signify not using the same.
         """
-        if os.path.isfile(self.pyproject_toml):
-            with io.open(self.pyproject_toml, encoding="utf-8") as f:
-                pp_toml = pytoml.load(f)
+        if not os.path.isfile(self.pyproject_toml):
+            return None
 
-            build_system = pp_toml.get('build-system', {})
-            if "requires" not in build_system:
-                raise InstallationError(
-                    "{} does not comply with PEP 518 as it since it's "
-                    "pyproject.toml file does not have a '[build-system]' "
-                    "table with a 'requires' key."
-                    .format(self)
-                )
+        with io.open(self.pyproject_toml, encoding="utf-8") as f:
+            pp_toml = pytoml.load(f)
 
-            requires = build_system["requires"]
+        # Extract the build requirements
+        requires = pp_toml.get("build-system", {}).get("requires", None)
+
+        # Error out if it's not valid
+        error_reason = None
+        if requires is None:
+            error_reason = "is missing."
+        else:
             is_list_of_str = isinstance(requires, list) and all(
-                isinstance(req, str) for req in requires
+                isinstance(req, six.string_types) for req in requires
             )
             if not is_list_of_str:
-                raise InstallationError(
-                    "{} does not comply with PEP 518 as it since it's "
-                    "pyproject.toml file contains [build-system].requires "
-                    "which is not a list of strings."
-                    .format(self)
-                )
+                error_reason = "not a list of strings."
+        if error_reason is not None:
+            msg = (
+                "{} does not comply with PEP 518 since pyproject.toml does "
+                "not contain a valid '[build-system].requires' key: {}"
+            ).format(self, error_reason)
+            raise InstallationError(msg)
 
-            return requires, True
-
-        return (['setuptools', 'wheel'], False)
+        # If control flow reaches here, we're good to go.
+        return requires
 
     def run_egg_info(self):
         assert self.source_dir