Deprecate --egg (#3956)

CHANGES.txt

@@ -1,5 +1,10 @@
 **8.2.0 (unreleased)**
 
+* **DEPRECATION** ``pip install --egg`` have been deprecated and will be
+  removed in the future. This "feature" has a long list of drawbacks where it
+  breaks almost all of pip's other features in subtle and hard to diagnose
+  ways.
+
 * Added Appveyor CI
 
 * Uninstall existing packages when performing an editable installation of

pip/commands/install.py

@@ -179,6 +179,15 @@ class InstallCommand(RequirementCommand):
         cmdoptions.resolve_wheel_no_use_binary(options)
         cmdoptions.check_install_build_global(options)
 
+        if options.as_egg:
+            warnings.warn(
+                "--egg has been deprecated and will be removed in the future. "
+                "This flag is mutually exclusive with large parts of pip, and "
+                "actually using it invalidates pip's ability to manage the "
+                "installation process.",
+                RemovedInPip10Warning,
+            )
+
         if options.allow_external:
             warnings.warn(
                 "--allow-external has been deprecated and will be removed in "

tests/functional/test_install.py

@@ -397,7 +397,7 @@ def test_install_as_egg(script, data):
     Test installing as egg, instead of flat install.
     """
     to_install = data.packages.join("FSPkg")
-    result = script.pip('install', to_install, '--egg', expect_error=False)
+    result = script.pip('install', to_install, '--egg', expect_error=True)
     fspkg_folder = script.site_packages / 'fspkg'
     egg_folder = script.site_packages / 'FSPkg-0.1.dev0-py%s.egg' % pyversion
     assert fspkg_folder not in result.files_created, str(result.stdout)

tests/functional/test_uninstall.py

@@ -341,7 +341,7 @@ def test_uninstall_as_egg(script, data):
     Test uninstall package installed as egg.
     """
     to_install = data.packages.join("FSPkg")
-    result = script.pip('install', to_install, '--egg', expect_error=False)
+    result = script.pip('install', to_install, '--egg', expect_error=True)
     fspkg_folder = script.site_packages / 'fspkg'
     egg_folder = script.site_packages / 'FSPkg-0.1.dev0-py%s.egg' % pyversion
     assert fspkg_folder not in result.files_created, str(result.stdout)