ftplugin: Only load the python plugin once

This makes opening wiki pages significantly faster and also prevents
clearing the entire cache every time.

ftplugin/vimwiki/taskwiki.vim

@@ -21,12 +21,18 @@ if exists("g:taskwiki_disable")
   finish
 endif
 
+if !exists("g:did_python_taskwiki")
   " Determine the plugin path
   let s:plugin_path = escape(expand('<sfile>:p:h:h:h'), '\')
 
   " Execute the main body of taskwiki source
   execute g:taskwiki_pyfile . s:plugin_path . '/taskwiki/main.py'
 
+  let g:did_python_taskwiki = 1
+endif
+
+execute g:taskwiki_py . "WholeBuffer.update_from_tw()"
+
 augroup taskwiki
     autocmd! * <buffer>
     " Update to TW upon saving

taskwiki/main.py

@@ -32,7 +32,7 @@ class WholeBuffer(object):
         Updates all the incomplete tasks in the vimwiki file if the info from TW is different.
         """
 
-        c = cache()
+        c = cache.load_current()
         c.reset()
         c.load_tasks()
         c.load_presets()
@@ -51,7 +51,7 @@ class WholeBuffer(object):
         Updates all tasks that differ from their TaskWarrior representation.
         """
 
-        c = cache()
+        c = cache.load_current()
         c.reset()
         c.load_tasks()
         c.load_presets()
@@ -637,6 +637,5 @@ class ChooseSplitTags(CallbackSplitMixin, SplitTags):
 
 
 if __name__ == '__main__':
-    WholeBuffer.update_from_tw()
     Meta().integrate_tagbar()
     Meta().set_proper_colors()

tests/test_ftplugin.py

@@ -0,0 +1,18 @@
+import os
+from tests.base import IntegrationTest
+
+
+class TestCachePreserved(IntegrationTest):
+
+    def execute(self):
+        testfile2 = os.path.join(self.dir, "testwiki2.txt")
+
+        self.command("w", regex="written$", lines=1)
+        self.command("w {}".format(testfile2), regex="written$", lines=1)
+
+        self.client.edit(testfile2)
+
+        # check that all buffers have a cache entry
+        cache_keys = self.client.eval('py3eval("sorted(cache.caches.keys())")')
+        buffers = self.client.eval('py3eval("sorted(b.number for b in vim.buffers)")')
+        assert cache_keys == buffers