seems to work?

TODO

@@ -6,6 +6,10 @@ Should probably do before it hits users
 Things that I want to do, but would put me past my deadline:
 
 * coverage: 155, 209, 226, 230-232, 303-304, 384, 388
+* coverage: 155, 208, 225, 229-231, 338, 368, 372
+
+* populate wheels into the cache during build
+    see also: https://github.com/pypa/pip/pull/1572
 
 * On ubuntu stock python2.7 I have to rm -rf $VIRTUAL_ENV/local
     to avoid the AssertionError('unexpected third case')

requirements.txt

@@ -1 +1,2 @@
 -r requirements.d/prod.txt
+pep8==1.0

venv_update.py

@@ -135,6 +135,26 @@ def _get_page(self, link, req):
         PackageFinder._get_page = orig_packagefinder['_get_page']
 
 
+def pip(args):
+    """Run pip, in-process."""
+    import pip as pipmodule
+
+    # pip<1.6 needs its logging config reset on each invocation, or else we get duplicate outputs -.-
+    pipmodule.logger.consumers = []
+
+    from sys import stdout
+    stdout.write(colorize(('pip',) + args))
+    stdout.write('\n')
+    stdout.flush()
+
+    with faster_pip_packagefinder():
+        result = pipmodule.main(list(args))
+
+    if result != 0:
+        # pip exited with failure, then we should too
+        exit(result)
+
+
 def pip_get_installed():
     """Code extracted from the middle of the pip freeze command.
     """
@@ -172,24 +192,78 @@ def pip_parse_requirements(requirement_files):
     return required
 
 
+def is_absolute(requirement):
+    # TODO: unit-test
+    if not requirement:
+        # url-style requirement
+        return False
+
+    for qualifier, dummy_version in requirement.specs:
+        if qualifier == '==':
+            return True
+    return False
+
+
+def exactly_satisfied(pipreq):
+    if not is_absolute(pipreq.req):
+        return False
+
+    return pipreq.check_if_exists()
+
+
+def filter_exactly_satisfied(reqs):
+    result = {}
+    for pkg, req in reqs.items():
+        if exactly_satisfied(req):
+            print('Requirement already up-to-date: %s' % req)
+        else:
+            result[pkg] = req
+    return result
+
+
+def format_req(pipreq):
+    """un-parse a pip requirement back to commandline arguments"""
+    # TODO: unit-test
+    if pipreq.editable:
+        editable = ('-e',)
+    else:
+        editable = ()
+
+    if pipreq.url:
+        spec = (pipreq.url,)
+    else:
+        spec = (str(pipreq.req),)
+
+    return editable + spec
+
+
 def pip_install(args):
     """Run pip install, and return the set of packages installed.
     """
-    from sys import stdout
-    stdout.write(colorize(('pip', 'install') + args))
-    stdout.write('\n')
-    stdout.flush()
-
     from pip.commands.install import InstallCommand
-    install = InstallCommand()
-    options, args = install.parse_args(list(args))
 
-    with faster_pip_packagefinder():
-        successfully_installed = install.run(options, args)
-    if successfully_installed is None:
+    orig_installcommand = vars(InstallCommand).copy()
+
+    class _nonlocal(object):
+        successfully_installed = None
+
+    def install(self, options, args):
+        """capture the list of successfully installed packages as they pass through"""
+        result = orig_installcommand['run'](self, options, args)
+        _nonlocal.successfully_installed = result
+        return result
+
+    # A poor man's dependency injection: monkeypatch :(
+    InstallCommand.run = install
+    try:
+        pip(('install',) + args)
+    finally:
+        InstallCommand.run = orig_installcommand['run']
+
+    if _nonlocal.successfully_installed is None:
         return {}
     else:
-        return dict(successfully_installed.requirements)
+        return dict(_nonlocal.successfully_installed.requirements)
 
 
 def trace_requirements(requirements):
@@ -251,12 +325,17 @@ def venv_uses_system_site_packages():
 
 def do_install(reqs):
     from os import environ
-    from sys import executable as python
 
     previously_installed = pip_get_installed()
     required = pip_parse_requirements(reqs)
-    requirements_as_options = tuple(
-        '--requirement={0}'.format(requirement) for requirement in reqs
+
+    unsatisfied = filter_exactly_satisfied(required)
+    requirements_as_options = sum(
+        (
+            format_req(req)
+            for pkg, req in unsatisfied.items()
+        ),
+        ()
     )
 
     # We put the cache in the directory that pip already uses.
@@ -273,7 +352,7 @@ def do_install(reqs):
         '--find-links=file://' + pip_download_cache,
     )
 
-    # 3) Install: Use our well-populated cache (only) to do the installations.
+    # 3) Install: Use our well-populated cache to do the installations.
     # --use-wheel is somewhat redundant here, but it means we get an error if we have a bad version of pip/setuptools.
     install_opts = ('--upgrade', '--use-wheel',) + cache_opts
     if venv_uses_system_site_packages():
@@ -298,7 +377,7 @@ def do_install(reqs):
 
     # 4) Uninstall any extraneous packages.
     if extraneous:
-        run((python, '-m', 'pip', 'uninstall', '--yes') + tuple(sorted(extraneous)))
+        pip(('uninstall', '--yes') + tuple(sorted(extraneous)))
 
     return 0  # posix:success!