Add an optional_imports module.

This is a stand-alone module to the following:

* Don’t import things until we need them
* Keep all the information centralized
* No possibility of circular imports by using this module

Author: theengineear

plotly/optional_imports.py

@@ -0,0 +1,41 @@
+"""
+Stand-alone module to provide information about whether optional deps exist.
+
+"""
+from __future__ import absolute_import
+
+import importlib
+
+__all__ = ['get_module']
+
+import_cache = {}
+
+
+def get_module(name, raise_exc=False, msg=None):
+    """
+    Return the module if it's available. Raise if desired.
+
+    A wrapper around importlib.import_module that only allows absolute import.
+
+    :param (str) name: Dot-separated module path. E.g., 'scipy.stats'.
+    :param (bool) raise_exc: Raise ImportError if not found?
+    :param (str|None) msg: Optional message to raise with ImportError.
+    :raises: (ImportError) If `raise_exception` is True, this can happen.
+    :return: (module|None) If import succeeds, the module will be returned.
+
+    """
+    if name in import_cache:
+        return import_cache[name]
+
+    try:
+        module = importlib.import_module(name)  # only allow absolute imports
+    except ImportError:
+        if raise_exc:
+            if msg is None:
+                raise  # just raise the current import error
+            else:
+                raise ImportError(msg)
+    else:
+        import_cache[name] = module
+
+    return import_cache.get(name, None)  # Don't store the None!