Fix for #770: pip install -U shouldn't look at pypi if not needed

If an exact version is specified for install, and that version is
already installed, then there is no point going to pypi as no install
is needed.

Adds a test called
`test_upgrade_no_look_at_pypi_if_exact_version_installed`.

This is a rework of PR #771, because that PR is old and has merge
conflicts that were easier to fix by applying the changes manually.

pip/req/req_set.py

@@ -225,7 +225,12 @@ class RequirementSet(object):
             if not self.ignore_installed and not req_to_install.editable:
                 req_to_install.check_if_exists()
                 if req_to_install.satisfied_by:
-                    if self.upgrade:
+                    # check that we don't already have an exact version match
+                    # i.e. with at least one strict req operator
+                    strict_req = set(('==', '===')) & set(
+                        op for op, _ in req_to_install.req.specs)
+                    if self.upgrade and (not strict_req or
+                                         self.force_reinstall):
                         if not (self.force_reinstall or req_to_install.link):
                             try:
                                 link = finder.find_requirement(

tests/unit/test_req.py

@@ -8,6 +8,7 @@ import pytest
 from mock import Mock, patch, mock_open
 from pip.exceptions import (
     PreviousBuildDirError, InvalidWheelFilename, UnsupportedWheel,
+    BestVersionAlreadyInstalled,
 )
 from pip.download import PipSession
 from pip.index import PackageFinder
@@ -54,6 +55,60 @@ class TestRequirementSet(object):
             finder,
         )
 
+    @patch(
+        'pip.req.req_install.pkg_resources.get_distribution',
+        lambda x: pkg_resources.Distribution(
+            project_name='Pygments',
+            version='2.0.2',
+            location='/python',
+        )
+    )
+    def test_upgrade_no_look_at_pypi_if_exact_version_installed(
+            self, data):
+        """
+        If an exact version is specified for install, and that version is
+        already installed, then there is no point going to pypi as no install
+        is needed.
+        """
+        reqset = self.basic_reqset()
+        reqset.upgrade = True
+        req = InstallRequirement.from_line('pygments==2.0.2')
+        req.url = None
+        reqset.add_requirement(req)
+        finder = PackageFinder([data.find_links], [], session=PipSession())
+        with patch.object(finder, 'find_requirement') as find_requirement:
+            find_requirement.side_effect = AssertionError(
+                'find_requirement should NOT be called')
+            reqset.prepare_files(finder)
+
+    @patch(
+        'pip.req.req_install.pkg_resources.get_distribution',
+        lambda x: pkg_resources.Distribution(
+            project_name='Pygments',
+            version='2.0.2',
+            location='/python',
+        )
+    )
+    def test_upgrade_look_at_pypi_if_exact_version_installed_and_force(
+            self, data):
+        """
+        If an exact version is specified for install, and that version is
+        already installed, but --force-reinstall was provided, we should hit
+        PyPI.
+        """
+        reqset = self.basic_reqset()
+        reqset.upgrade = True
+        reqset.force_reinstall = True
+        req = InstallRequirement.from_line('pygments==2.0.2')
+        req.url = None
+        reqset.add_requirement(req)
+        finder = PackageFinder([data.find_links], [], session=PipSession())
+        with patch.object(finder, 'find_requirement') as find_requirement:
+            find_requirement.side_effect = BestVersionAlreadyInstalled
+            with pytest.raises(BestVersionAlreadyInstalled):
+                reqset.prepare_files(finder)
+                find_requirement.assert_called_once()
+
     def test_environment_marker_extras(self, data):
         """
         Test that the environment marker extras are used with