Add an optional_imports module.

This is a stand-alone module to do the following:

* Don’t import optional things until we need them
* Keep all the optional imports centralized

Author: theengineear

plotly/optional_imports.py

@@ -0,0 +1,25 @@
+"""
+Stand-alone module to provide information about whether optional deps exist.
+
+"""
+from __future__ import absolute_import
+
+from importlib import import_module
+
+_not_importable = set()
+
+
+def get_module(name):
+    """
+    Return module or None. Absolute import is required.
+
+    :param (str) name: Dot-separated module path. E.g., 'scipy.stats'.
+    :raise: (ImportError) Only when exc_msg is defined.
+    :return: (module|None) If import succeeds, the module will be returned.
+
+    """
+    if name not in _not_importable:
+        try:
+            return import_module(name)
+        except ImportError:
+            _not_importable.add(name)

plotly/tests/test_core/test_optional_imports/__init__.py

@@ -0,0 +1,24 @@
+from __future__ import absolute_import
+
+from unittest import TestCase
+
+from plotly.optional_imports import get_module
+
+
+class OptionalImportsTest(TestCase):
+
+    def test_get_module_exists(self):
+        import math
+        module = get_module('math')
+        self.assertIsNotNone(module)
+        self.assertEqual(math, module)
+
+    def test_get_module_exists_submodule(self):
+        import requests.sessions
+        module = get_module('requests.sessions')
+        self.assertIsNotNone(module)
+        self.assertEqual(requests.sessions, module)
+
+    def test_get_module_does_not_exist(self):
+        module = get_module('hoopla')
+        self.assertIsNone(module)