Fixes pypa#4390 by allowing explicitly provided --prefix and --target options to overrule implicitly discovered --user.

Author: zooba

src/pip/_internal/cli/parser.py

@@ -108,6 +108,13 @@ def expand_default(self, option):
         return optparse.IndentedHelpFormatter.expand_default(self, option)
 
 
+class CustomOption(optparse.Option):
+
+    def process(self, opt, value, values, parser):
+        parser.provided_options.add(opt)
+        return super(CustomOption, self).process(opt, value, values, parser)
+
+
 class CustomOptionParser(optparse.OptionParser):
 
     def insert_option_group(self, idx, *args, **kwargs):
@@ -138,6 +145,9 @@ def __init__(self, *args, **kwargs):
 
         isolated = kwargs.pop("isolated", False)
         self.config = Configuration(isolated)
+        self.provided_options = set()
+
+        kwargs.setdefault("option_class", CustomOption)
 
         assert self.name
         optparse.OptionParser.__init__(self, *args, **kwargs)

src/pip/_internal/commands/install.py

@@ -226,18 +226,33 @@ def run(self, options, args):
         options.src_dir = os.path.abspath(options.src_dir)
         install_options = options.install_options or []
         if options.use_user_site:
+            explicit_opt = self.parser.provided_options
+            if "--user" not in explicit_opt:
+                # user option was implicit, so allow it to be disabled
+                # by explicit options
+                if options.prefix_path and "--prefix" in explicit_opt:
+                    options.use_user_site = False
+                if options.target_dir and "--target" in explicit_opt:
+                    options.use_user_site = False
+            if options.use_user_site:
+                if options.prefix_path:
+                    raise CommandError(
+                        "Can not combine '--user' and '--prefix' as they imply "
+                        "different installation locations"
+                    )
+                if options.target_dir:
+                    raise CommandError(
+                        "Can not combine '--user' and '--target' as they imply "
+                        "different installation locations"
+                    )
+                if virtualenv_no_global():
+                    raise InstallationError(
+                        "Can not perform a '--user' install. User site-packages "
+                        "are not visible in this virtualenv."
+                    )
+                install_options.append('--user')
             if options.prefix_path:
-                raise CommandError(
-                    "Can not combine '--user' and '--prefix' as they imply "
-                    "different installation locations"
-                )
-            if virtualenv_no_global():
-                raise InstallationError(
-                    "Can not perform a '--user' install. User site-packages "
-                    "are not visible in this virtualenv."
-                )
-            install_options.append('--user')
-            install_options.append('--prefix=')
+                install_options.append('--prefix=')
 
         target_temp_dir = TempDirectory(kind="target")
         if options.target_dir: