Use strict optional checking in req_install.py (pypa#11379)

* Use strict optional checking in req_install.py

Suggested by pradyunsg in pypa#11374

Since half of the API of this class depends on self.req not being None,
it seems like we should just prevent users from passing None here.
However, I wasn't able to make that change.

Rather than sprinkle asserts everywhere, I added "checked" properties.
I find this less ad hoc and easier to adapt if e.g. we're able to make
self.req never None in the future.

There are now some code paths where we have asserts that we didn't
before. I relied on other type hints in pip's code base to be accurate.
If that is not the case and we actually relied on some function being
able to accept None when not typed as such, we may hit these asserts.
But hopefully tests would catch such a thing.

* news

* black

* inline asserts

* code review

* fix up merge issue

* fix specifier bug

src/pip/_internal/req/req_install.py

@@ -1,6 +1,3 @@
-# The following comment should be removed at some point in the future.
-# mypy: strict-optional=False
-
 import functools
 import logging
 import os
@@ -244,6 +241,7 @@ def supports_pyproject_editable(self) -> bool:
 
     @property
     def specifier(self) -> SpecifierSet:
+        assert self.req is not None
         return self.req.specifier
 
     @property
@@ -257,7 +255,8 @@ def is_pinned(self) -> bool:
 
         For example, some-package==1.2 is pinned; some-package>1.2 is not.
         """
-        specifiers = self.specifier
+        assert self.req is not None
+        specifiers = self.req.specifier
         return len(specifiers) == 1 and next(iter(specifiers)).operator in {"==", "==="}
 
     def match_markers(self, extras_requested: Optional[Iterable[str]] = None) -> bool:
@@ -305,6 +304,7 @@ def hashes(self, trust_internet: bool = True) -> Hashes:
         else:
             link = None
         if link and link.hash:
+            assert link.hash_name is not None
             good_hashes.setdefault(link.hash_name, []).append(link.hash)
         return Hashes(good_hashes)
 
@@ -314,6 +314,7 @@ def from_path(self) -> Optional[str]:
             return None
         s = str(self.req)
         if self.comes_from:
+            comes_from: Optional[str]
             if isinstance(self.comes_from, str):
                 comes_from = self.comes_from
             else:
@@ -345,7 +346,7 @@ def ensure_build_location(
 
         # When parallel builds are enabled, add a UUID to the build directory
         # name so multiple builds do not interfere with each other.
-        dir_name: str = canonicalize_name(self.name)
+        dir_name: str = canonicalize_name(self.req.name)
         if parallel_builds:
             dir_name = f"{dir_name}_{uuid.uuid4().hex}"
 
@@ -388,6 +389,7 @@ def _set_requirement(self) -> None:
         )
 
     def warn_on_mismatching_name(self) -> None:
+        assert self.req is not None
         metadata_name = canonicalize_name(self.metadata["Name"])
         if canonicalize_name(self.req.name) == metadata_name:
             # Everything is fine.
@@ -457,6 +459,7 @@ def is_wheel_from_cache(self) -> bool:
     # Things valid for sdists
     @property
     def unpacked_source_directory(self) -> str:
+        assert self.source_dir, f"No source dir for {self}"
         return os.path.join(
             self.source_dir, self.link and self.link.subdirectory_fragment or ""
         )
@@ -543,7 +546,7 @@ def prepare_metadata(self) -> None:
         Under PEP 517 and PEP 660, call the backend hook to prepare the metadata.
         Under legacy processing, call setup.py egg-info.
         """
-        assert self.source_dir
+        assert self.source_dir, f"No source dir for {self}"
         details = self.name or f"from {self.link}"
 
         if self.use_pep517:
@@ -592,18 +595,20 @@ def get_dist(self) -> BaseDistribution:
         if self.metadata_directory:
             return get_directory_distribution(self.metadata_directory)
         elif self.local_file_path and self.is_wheel:
+            assert self.req is not None
             return get_wheel_distribution(
-                FilesystemWheel(self.local_file_path), canonicalize_name(self.name)
+                FilesystemWheel(self.local_file_path),
+                canonicalize_name(self.req.name),
             )
         raise AssertionError(
             f"InstallRequirement {self} has no metadata directory and no wheel: "
             f"can't make a distribution."
         )
 
     def assert_source_matches_version(self) -> None:
-        assert self.source_dir
+        assert self.source_dir, f"No source dir for {self}"
         version = self.metadata["version"]
-        if self.req.specifier and version not in self.req.specifier:
+        if self.req and self.req.specifier and version not in self.req.specifier:
             logger.warning(
                 "Requested %s, but installing version %s",
                 self,
@@ -696,9 +701,10 @@ def _clean_zip_name(name: str, prefix: str) -> str:
             name = name.replace(os.path.sep, "/")
             return name
 
+        assert self.req is not None
         path = os.path.join(parentdir, path)
         name = _clean_zip_name(path, rootdir)
-        return self.name + "/" + name
+        return self.req.name + "/" + name
 
     def archive(self, build_dir: Optional[str]) -> None:
         """Saves archive to provided build_dir.
@@ -777,8 +783,9 @@ def install(
         use_user_site: bool = False,
         pycompile: bool = True,
     ) -> None:
+        assert self.req is not None
         scheme = get_scheme(
-            self.name,
+            self.req.name,
             user=use_user_site,
             home=home,
             root=root,
@@ -792,7 +799,7 @@ def install(
                 prefix=prefix,
                 home=home,
                 use_user_site=use_user_site,
-                name=self.name,
+                name=self.req.name,
                 setup_py_path=self.setup_py_path,
                 isolated=self.isolated,
                 build_env=self.build_env,
@@ -805,7 +812,7 @@ def install(
         assert self.local_file_path
 
         install_wheel(
-            self.name,
+            self.req.name,
             self.local_file_path,
             scheme=scheme,
             req_description=str(self.req),