More intelligently insert the pip path into sys.path

news/8214.bugfix

@@ -0,0 +1 @@
+Fix installing packages which have a pyproject.toml when enum34 is installed.

src/pip/__main__.py

@@ -2,6 +2,7 @@
 
 import os
 import sys
+import sysconfig
 
 # Remove '' and current working directory from the first entry
 # of sys.path, if present to avoid using current directory
@@ -13,12 +14,27 @@
 # If we are running from a wheel, add the wheel to sys.path
 # This allows the usage python pip-*.whl/pip install pip-*.whl
 if __package__ == '':
+    # Retrieve the index in sys.path where all stdlib paths reside
+    # (DESTSHARED is where some extension modules like math live).
+    stdlib_path_indexes = []
+    stdlib_paths = (sysconfig.get_path('stdlib'),
+                    sysconfig.get_path('platstdlib'),
+                    sysconfig.get_config_var('DESTSHARED'))
+    for path in (p for p in stdlib_paths if p is not None):
+        try:
+            stdlib_path_indexes.append(sys.path.index(path))
+        except ValueError:
+            continue
+
     # __file__ is pip-*.whl/pip/__main__.py
     # first dirname call strips of '/__main__.py', second strips off '/pip'
     # Resulting path is the name of the wheel itself
     # Add that to sys.path so we can import pip
-    path = os.path.dirname(os.path.dirname(__file__))
-    sys.path.insert(0, path)
+    pip_installed_path = os.path.dirname(os.path.dirname(__file__))
+
+    # Insert this pip's library path directly after the stdlib so that
+    # we import this pip's library even if another pip is installed.
+    sys.path.insert(max(stdlib_path_indexes) + 1, pip_installed_path)
 
 from pip._internal.cli.main import main as _main  # isort:skip # noqa