Make PrettyTable optional

PrettyTable as a depend can have a tendency to conflict with other
things that want to use it, which is a shame for a general purpose
library like futurist. Make it optional to reduce the transitive
dependency burden for consumers.

Change-Id: Ie976f7f4e544f273fccf0cf579b82b659cbd48d1

futurist/periodics.py

@@ -25,7 +25,10 @@ import random
 import threading
 
 from concurrent import futures
-import prettytable
+try:
+    import prettytable
+except ImportError:
+    prettytable = None
 import six
 
 import futurist
@@ -748,10 +751,17 @@ class PeriodicWorker(object):
         cols = list(_DEFAULT_COLS)
         for c in ['Runs in', 'Active', 'Periodicity']:
             cols.remove(c)
-        self._log.debug("Stopped running %s callbacks:\n%s",
-                        len(self._works), self.pformat(columns=cols))
+        self._log.debug(
+            "Stopped running %s callbacks:\n%s",
+            len(self._works),
+            self.pformat(columns=cols) if prettytable
+            else "statistics not available, PrettyTable missing"
+        )
 
     def pformat(self, columns=_DEFAULT_COLS):
+        if prettytable is None:
+            raise ImportError(
+                "PrettyTable is required to use the pformat method")
         # Convert to a list to ensure we maintain the same order when used
         # further in this function (since order will matter)...
         if not isinstance(columns, (list, tuple)):

releasenotes/notes/prettytable-optional-2c9198a9250427b6.yaml

@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    The ``PrettyTable`` dependency has been made optional, as it
+    is only used in the `Periodics.pformat` method. If the use
+    of that method is important, users should depend on
+    ``PrettyTable`` directly.

requirements.txt

@@ -4,4 +4,3 @@
 
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 six>=1.10.0 # MIT
-PrettyTable<0.8,>=0.7.1 # BSD

test-requirements.txt

@@ -14,3 +14,4 @@ oslotest>=3.2.0 # Apache-2.0
 stestr>=2.0.0 # Apache-2.0
 testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=2.2.0 # MIT
+PrettyTable<0.8,>=0.7.1 # BSD