From 23fe5549122edae2101f41c14cedc432f1563cf7 Mon Sep 17 00:00:00 2001
From: Colin Carroll <colcarroll@gmail.com>
Date: Sun, 20 May 2018 16:21:04 -0400
Subject: [PATCH] Add optional dependency class

---
 .pylintrc                    |  3 ++-
 arviz/compat/__init__.py     |  4 ++++
 arviz/compat/optional_dep.py | 32 ++++++++++++++++++++++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 arviz/compat/__init__.py
 create mode 100644 arviz/compat/optional_dep.py

diff --git a/.pylintrc b/.pylintrc
index bfcaede8af..5c7899fbbe 100644
--- a/.pylintrc
+++ b/.pylintrc
@@ -61,7 +61,8 @@ disable=missing-docstring,
         too-many-locals,
         too-many-branches,
         too-many-statements,
-        no-self-use
+        no-self-use,
+        too-few-public-methods
 
 # Enable the message, report, category or checker with the given id(s). You can
 # either give multiple identifier separated by comma (,) or put this option
diff --git a/arviz/compat/__init__.py b/arviz/compat/__init__.py
new file mode 100644
index 0000000000..061b0d2511
--- /dev/null
+++ b/arviz/compat/__init__.py
@@ -0,0 +1,4 @@
+#pylint: disable=invalid-name
+from .optional_dep import OptionalDep
+pymc3 = OptionalDep('pymc3')
+altair = OptionalDep('altair')
diff --git a/arviz/compat/optional_dep.py b/arviz/compat/optional_dep.py
new file mode 100644
index 0000000000..1524a3576c
--- /dev/null
+++ b/arviz/compat/optional_dep.py
@@ -0,0 +1,32 @@
+import importlib
+import sys
+
+
+class OptionalDep(object):
+    """Wrapper for optional library dependencies.
+
+    Note that since only __getattr__ is implemented, if this object implements
+    methods, those will be used *before* the true library is called.
+
+    For example
+
+    class PyMC3(OptionalDep):
+        def trace_to_dataframe(*args, **kwargs):
+            ...
+
+    pm = PyMC3()
+    pm.trace_to_dataframe(trace)  # calls the OptionalDep method
+    pm.Normal('x', 0, 1)  # calls pymc3.Normal
+    """
+    def __init__(self, name):
+        self.__name = name
+        self.__module = None
+
+    def __getattr__(self, name):
+        if self.__module is None:
+            try:
+                self.__module = importlib.import_module(self.__name)
+            except ModuleNotFoundError:
+                raise ModuleNotFoundError(
+                    'Failed to import optional module {}'.format(self.__name), sys.exc_info()[0])
+        return getattr(self.__module, name)