output: Allow the LoggingFile to use different prefixes per logger

We are about to need to allow different log prefixes per logger in our
LoggingFile helper class. Let's allow it by using list of prefixes,
instead of a single prefix.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/core/output.py

@@ -607,22 +607,23 @@ class LoggingFile(object):
     File-like object that will receive messages pass them to logging.
     """
 
-    def __init__(self, prefix='', level=logging.DEBUG,
+    def __init__(self, prefixes=None, level=logging.DEBUG,
                  loggers=None):
         """
         Constructor. Sets prefixes and which loggers are going to be used.
 
-        :param prefix - The prefix for each line logged by this object.
+        :param prefixes: Prefix per logger to be prefixed to each line.
         :param level: Log level to be used when writing messages.
         :param loggers: Loggers into which write should be issued. (list)
         """
-
-        self._prefix = prefix
         if not loggers:
             loggers = [logging.getLogger()]
         self._level = level
         self._buffer = []
         self._loggers = loggers
+        if prefixes is None:
+            prefixes = [""] * len(loggers)
+        self._prefixes = prefixes
 
     def write(self, data):
         """"
@@ -653,8 +654,8 @@ class LoggingFile(object):
         """
         Passes lines of output to the logging module.
         """
-        for lg in self._loggers:
-            lg.log(self._level, self._prefix + line)
+        for logger, prefix in zip(self._loggers, self._prefixes):
+            logger.log(self._level, prefix + line)
 
     def _flush_buffer(self):
         if self._buffer:
@@ -667,11 +668,14 @@ class LoggingFile(object):
     def isatty(self):
         return False
 
-    def add_logger(self, logger):
+    def add_logger(self, logger, prefix=""):
         self._loggers.append(logger)
+        self._prefixes.append(prefix)
 
     def rm_logger(self, logger):
+        idx = self._loggers.index(logger)
         self._loggers.remove(logger)
+        self._prefixes = self._prefixes[:idx] + self._prefixes[idx+1:]
 
 
 class Throbber(object):