bpo-34011: Update code copying DLLs and init.tcl into venvs. (GH-8253)

Lib/venv/__init__.py

@@ -9,6 +9,7 @@ import os
 import shutil
 import subprocess
 import sys
+import sysconfig
 import types
 
 logger = logging.getLogger(__name__)
@@ -207,7 +208,10 @@ class EnvBuilder:
                     copier(context.env_exe, path, relative_symlinks_ok=True)
                     if not os.path.islink(path):
                         os.chmod(path, 0o755)
-        else:
+        elif sysconfig.is_python_build(True):
+            # See bpo-34011. This copying code should only be needed when a
+            # venv is created from a source Python build (i.e. not an installed
+            # Python)
             subdir = 'DLLs'
             include = self.include_binary
             files = [f for f in os.listdir(dirname) if include(f)]

Misc/NEWS.d/next/Windows/2018-07-11-15-58-06.bpo-34011.Ho_d5T.rst

+A suite of code has been changed which copied across DLLs and init.tcl from
+the running Python location into a venv being created. These copies are needed
+only when running from a Python source build, and the copying code is now only
+run when that is the case, rather than whenever a venv is created.