Move optional dependencies to extras

README.rst

@@ -110,7 +110,10 @@ Installation
 ------------
 ::
 
-    pip install smart_open
+    pip install smart_open  // Install with no cloud dependencies
+    pip install smart_open[aws] // Install AWS deps
+    pip install smart_open[gcp] // Install GCP deps
+    pip install smart_open[all] // Installs all cloud dependencies
 
 Or, if you prefer to install from the `source tar.gz <http://pypi.python.org/pypi/smart_open>`_::
 
@@ -122,6 +125,11 @@ The tests are also run automatically with `Travis CI <https://travis-ci.org/RaRe
 
 If you're upgrading from ``smart_open`` versions 1.8.0 and below, please check out the `Migration Guide <MIGRATING_FROM_OLDER_VERSIONS.rst>`_.
 
+Version ``2.0`` will introduce a backwards incompatible installation method with regards to the cloud dependencies. A migration path to minimize breaking
+was introduced in version ``x.x.x``. If you want to maintain backwards compatibility (installing all dependencies) install this package via ``smart_open[all]`` now
+and once the change is made you should not have any issues. If all you care about is AWS dependencies for example you can install via ``smart_open[aws]`` and
+once the dependency change is made you will simply drop the unwanted dependencies. You can read more about the motivations `here <https://github.com/RaRe-Technologies/smart_open/issues/443>`_
+
 
 Built-in help
 -------------

setup.py

@@ -53,10 +53,13 @@ def read(fname):
 
 install_requires = [
     'requests',
-    'boto3',
-    'google-cloud-storage',
 ]
 
+aws_deps = ['boto3']
+gcp_deps = ['google-cloud-storage']
+
+all_deps = install_requires + aws_deps + gcp_deps
+
 setup(
     name='smart_open',
     version=__version__,
@@ -81,10 +84,16 @@ def read(fname):
     license='MIT',
     platforms='any',
 
-    install_requires=install_requires,
+    # Concatenating the lists together is temporary and will
+    # eventually simply be install_requires dropping the cloud
+    # dependencies from being installed without explicitly being declared.
+    install_requires=install_requires + aws_deps + gcp_deps,
     tests_require=tests_require,
     extras_require={
         'test': tests_require,
+        'aws': aws_deps,
+        'gcp': gcp_deps,
+        'all': all_deps,
     },
 
     test_suite="smart_open.tests",