Switch default upgrade strategy to only-if-needed

docs/user_guide.rst

@@ -486,9 +486,9 @@ strategies supported:
   the new parent requirements
 - ``only-if-needed``: upgrades a dependency only if it does not satisfy the new
   parent requirements
- 
-Currently, the default strategy is ``eager``, which was the strategy prior to 
-the ``--upgrade-strategy`` option being added.
+
+The default strategy is ``only-if-needed``. This was changed in pip 10.0 due to
+the breaking nature of ``eager`` when upgrading conflicting dependencies.
 
 As an historic note, an earlier "fix" for getting the ``only-if-needed``
 behaviour was::

news/4500.feature

@@ -0,0 +1 @@
+Switch the default upgrade strategy to be 'only-if-needed'

pip/commands/install.py

@@ -110,7 +110,7 @@ def __init__(self, *args, **kw):
         cmd_opts.add_option(
             '--upgrade-strategy',
             dest='upgrade_strategy',
-            default='eager',
+            default='only-if-needed',
             choices=['only-if-needed', 'eager'],
             help='Determines how dependency upgrading should be handled '
                  '(default: %(default)s). '