Updated with info about passing callable

README.md

@@ -204,3 +204,54 @@ APIs to make usage of *htlog.cgi* in Perl and Python scripts even easier
         );
 
     }
+
+Note that htlogr also supports passing of callable code instead of `i` or
+`tag`.  Use this if you find yourself constructing them in a non-trivial
+way before every call.
+
+I'll illustrate this with Python API, but of course implementation and use
+is same in both APIs.
+
+Imagine situation when existence of a certain environment variable tells us
+context in which we are running (e.g. a specific test case) and its value
+is the iteration (e.g. Jenkins build number).
+
+This example examines environment for existence of such variable and then
+uses its name as tag and value as i.
+
+The code:
+
+    import htlogr
+    import os
+
+    logger = htlogr('http://192.168.1.1/cgi-bin/htlog.cgi')
+
+    def get_both():
+        for key in ['var1', 'var2']:
+            try:
+                return os.environ[key], key
+            except KeyError:
+                pass
+
+    def get_i():
+        i, tag = get_both()
+        return i
+
+    def get_tag():
+        i, tag = get_both()
+        return tag
+
+    logger.log("hello", tag=get_tag, i=get_i)
+
+Now you can e.g. write a logging wrapper function in a trivial yet flexible way:
+
+    # inside a class:
+
+    def rmsg(self, mesage):
+        self.logger.log(message, i=self.get_i, tag=self.name)
+
+    def rwarn(self, mesage):
+        self.logger.log('warning: ' message, i=self.get_i, tag=self.name)
+
+    def rstats(self, stats):
+        self.logger.data(stats, i=self.get_i, tag=self.name)