Fixing issues with install's --target option.

 1. Check for the existence of a directory before copying from temporary directory to final target. If it exists, warn the user.
 2. If the user specifies the --upgrade option and the directory exists, delete it and continue with installation.
 3. Adding tests for above cases.

This resolves pypa#1489, pypa#1710 completely and parts of pypa#1833.

Author: theacodes

pip/commands/install.py

@@ -51,7 +51,10 @@ def __init__(self, *args, **kw):
             dest='target_dir',
             metavar='dir',
             default=None,
-            help='Install packages into <dir>.')
+            help='Install packages into <dir>. '
+                 'By default this will not replace existing files/folders in <dir>.'
+                 'Use --upgrade to replace existing packages in <dir> with new versions.'
+        )
 
         cmd_opts.add_option(
             '-d', '--download', '--download-dir', '--download-directory',
@@ -344,9 +347,19 @@ def run(self, options, args):
                     os.makedirs(options.target_dir)
                 lib_dir = distutils_scheme('', home=temp_target_dir)['purelib']
                 for item in os.listdir(lib_dir):
+                    target_item_dir = os.path.join(options.target_dir, item)
+                    if os.path.exists(target_item_dir):
+                        if not options.upgrade:
+                            logger.warn('Target directory %s already exists. Specify --upgrade to force replacement.' % target_item_dir)
+                            continue
+                        if not os.path.isdir(target_item_dir):
+                            logger.warn('Target directory %s already exists and is not a directory. Please remove in order for replacement.' % target_item_dir)
+                            continue
+                        shutil.rmtree(target_item_dir)
+
                     shutil.move(
                         os.path.join(lib_dir, item),
-                        os.path.join(options.target_dir, item),
+                        target_item_dir
                     )
                 shutil.rmtree(temp_target_dir)
             return requirement_set

tests/functional/test_install.py

@@ -527,6 +527,19 @@ def test_install_package_with_target(script):
         str(result)
     )
 
+    # Test repeated call without --upgrade, no files should have changed
+    result = script.pip('install', '-t', target_dir, "initools==0.1")
+    assert not Path('scratch') / 'target' / 'initools' in result.files_updated
+
+    # Test upgrade call, check that new version is installed
+    result = script.pip('install', '--upgrade', '-t', target_dir, "initools==0.2")
+    assert Path('scratch') / 'target' / 'initools' in result.files_updated, (
+        str(result)
+    )
+    assert Path('scratch') / 'target' / 'INITools-0.2-py%s.egg-info' % pyversion in result.files_created, (
+        str(result)
+    )
+
 
 def test_install_package_with_root(script, data):
     """