Merge pull request #9838 from uranusjr/sysconfig-header-with-none-pro…

…ject

Set project name to UNKNOWN when empty

news/9838.bugfix.rst

@@ -0,0 +1 @@
+Fix compatibility between distutils and sysconfig when the project name is unknown outside of a virtual environment.

src/pip/_internal/locations/_sysconfig.py

@@ -127,14 +127,20 @@ def get_scheme(
 
     paths = sysconfig.get_paths(scheme=scheme_name, vars=variables)
 
-    # Pip historically uses a special header path in virtual environments.
+    # Logic here is very arbitrary, we're doing it for compatibility, don't ask.
+    # 1. Pip historically uses a special header path in virtual environments.
+    # 2. If the distribution name is not known, distutils uses 'UNKNOWN'. We
+    #    only do the same when not running in a virtual environment because
+    #    pip's historical header path logic (see point 1) did not do this.
     if running_under_virtualenv():
         if user:
             base = variables.get("userbase", sys.prefix)
         else:
             base = variables.get("base", sys.prefix)
         python_xy = f"python{get_major_minor_version()}"
         paths["include"] = os.path.join(base, "include", "site", python_xy)
+    elif not dist_name:
+        dist_name = "UNKNOWN"
 
     scheme = Scheme(
         platlib=paths["platlib"],