Remove existing package before develop mode install

setuptools has changed its default behaviour whe installing packages
installed in "develop mode" (i.e. pip -e) from being inserted first in
sys.path to being added last (for details, see the bug).

This means that if "pip install -e" a git tree to work on something
but have the original package installed from PyPi (etc.), you will not
actually be importing the development version.  During interactive
development you'll probably notice that your changes aren't applying
and start digging around, but this behaviour change could be very bad
in a CI situation where you *think* you've installed a development
version but really, with setuptools>=25, you're just testing the
release version.

This change removes an existing install before deploying in develop
mode.  This way we only have one copy of the package and avoid
ordering issues.  A new test is added.

Fixes #3874

Author: ianw

pip/req/req_set.py

@@ -718,6 +718,7 @@ def install(self, install_options, global_options=(), *args, **kwargs):
         the packages)
         """
         to_install = self._to_install()
+        editable_reinstall = False
 
         if to_install:
             logger.info(
@@ -734,6 +735,23 @@ def install(self, install_options, global_options=(), *args, **kwargs):
                     )
                     with indent_log():
                         requirement.uninstall(auto_confirm=True)
+
+                if requirement.installed_version and requirement.editable:
+                    # If we have an existing installation but are
+                    # re-installing as editable, make sure we remove
+                    # the existing version.  Note that with
+                    # setuptools<25 this is not strictly required, as
+                    # there setuptools will modify easy-install.pth to
+                    # put the "develop" mode install first in sys.path
+                    # (see SETUPTOOLS_SYS_PATH_TECHNIQUE), effectively
+                    # overwriting the prior installed version.
+                    logger.info(
+                        'Found prior install %s, reinstalling for develop mode'
+                        % requirement.installed_version)
+                    with indent_log():
+                        requirement.uninstall(auto_confirm=True)
+                        editable_reinstall = True
+
                 try:
                     requirement.install(
                         install_options,
@@ -743,12 +761,12 @@ def install(self, install_options, global_options=(), *args, **kwargs):
                     )
                 except:
                     # if install did not succeed, rollback previous uninstall
-                    if (requirement.conflicts_with and not
-                            requirement.install_succeeded):
+                    if ((requirement.conflicts_with or editable_reinstall) and
+                            not requirement.install_succeeded):
                         requirement.rollback_uninstall()
                     raise
                 else:
-                    if (requirement.conflicts_with and
+                    if ((requirement.conflicts_with or editable_reinstall) and
                             requirement.install_succeeded):
                         requirement.commit_uninstall()
                 requirement.remove_temporary_source()

tests/functional/test_install.py

@@ -12,7 +12,7 @@
 from tests.lib import (pyversion, pyversion_tuple,
                        _create_test_package, _create_svn_repo, path_to_url,
                        requirements_file)
-from tests.lib.local_repos import local_checkout
+from tests.lib.local_repos import local_checkout, local_repo
 from tests.lib.path import Path
 
 
@@ -1026,3 +1026,25 @@ def test_double_install_fail(script, data):
     msg = ("Double requirement given: pip==7.1.2 (already in pip==*, "
            "name='pip')")
     assert msg in result.stderr
+
+
+@pytest.mark.network
+def test_update_to_develop(script, tmpdir):
+    """
+    Test updating from install to develop mode removes original package
+    """
+    path = local_repo(
+        'git+http://github.com/pypa/pip-test-package.git',
+        tmpdir.join("cache"),
+    )
+
+    # regular install, then re-install with develop mode
+    result = script.pip(
+        'install', '%s' % path, expect_error=False)
+    result = script.pip(
+        'install', '-e', '%s' % path, expect_error=False)
+
+    # make sure we have overwritten the non-develop install with the
+    # git version.
+    result = script.pip('freeze', expect_stderr=True)
+    assert 'pip-test-package.git' in result.stdout