Example python plugin (#825)

* Add example python plugin
* Fix log incorrectly detecting as progress level

pkg/plugin/examples/python/log.py

@@ -0,0 +1,44 @@
+import sys
+
+# Log messages sent from a plugin instance are transmitted via stderr and are
+# encoded with a prefix consisting of special character SOH, then the log
+# level (one of t, d, i, w, e, or p - corresponding to trace, debug, info,
+# warning, error and progress levels respectively), then special character
+# STX.
+#
+# The LogTrace, LogDebug, LogInfo, LogWarning, and LogError methods, and their equivalent
+# formatted methods are intended for use by plugin instances to transmit log
+# messages. The LogProgress method is also intended for sending progress data.
+#
+
+def __prefix(levelChar):
+    startLevelChar = b'\x01'
+    endLevelChar = b'\x02'
+
+    ret = startLevelChar + levelChar + endLevelChar
+    return ret.decode()
+
+def __log(levelChar, s):
+    if levelChar == "":
+        return
+
+    print(__prefix(levelChar) + s + "\n", file=sys.stderr, flush=True)
+
+def LogTrace(s):
+    __log(b't', s)
+
+def LogDebug(s):
+    __log(b'd', s)
+
+def LogInfo(s):
+    __log(b'i', s)
+
+def LogWarning(s):
+    __log(b'w', s)
+
+def LogError(s):
+    __log(b'e', s)
+
+def LogProgress(p):
+    progress = min(max(0, p), 1)
+    __log(b'p', str(progress))