simpletrace: made Analyzer into context-manager

Instead of explicitly calling `begin` and `end`, we can change the class
to use the context-manager paradigm. This is mostly a styling choice,
used in modern Python code. But it also allows for more advanced analyzers
to handle exceptions gracefully in the `__exit__` method (not
demonstrated here).

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Mads Ynddal <m.ynddal@samsung.com>
Message-id: 20230926103436.25700-9-mads@ynddal.dk
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

scripts/simpletrace.py

@@ -122,12 +122,13 @@ def read_trace_records(event_mapping, event_id_to_name, fobj):
 
             yield rec
 
-class Analyzer(object):
+class Analyzer:
     """A trace file analyzer which processes trace records.
 
     An analyzer can be passed to run() or process().  The begin() method is
     invoked, then each trace record is processed, and finally the end() method
-    is invoked.
+    is invoked. When Analyzer is used as a context-manager (using the `with`
+    statement), begin() and end() are called automatically.
 
     If a method matching a trace event name exists, it is invoked to process
     that trace record.  Otherwise the catchall() method is invoked.
@@ -165,6 +166,15 @@ class Analyzer(object):
         """Called at the end of the trace."""
         pass
 
+    def __enter__(self):
+        self.begin()
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if exc_type is None:
+            self.end()
+        return False
+
 def process(events, log, analyzer, read_header=True):
     """Invoke an analyzer on each event in a log.
     Args:
@@ -226,15 +236,14 @@ def process(events, log, analyzer, read_header=True):
             # Just arguments, no timestamp or pid
             return lambda _, rec: fn(*rec[3:3 + event_argcount])
 
-    analyzer.begin()
-    fn_cache = {}
-    for rec in read_trace_records(event_mapping, event_id_to_name, log):
-        event_num = rec[0]
-        event = event_mapping[event_num]
-        if event_num not in fn_cache:
-            fn_cache[event_num] = build_fn(analyzer, event)
-        fn_cache[event_num](event, rec)
-    analyzer.end()
+    with analyzer:
+        fn_cache = {}
+        for rec in read_trace_records(event_mapping, event_id_to_name, log):
+            event_num = rec[0]
+            event = event_mapping[event_num]
+            if event_num not in fn_cache:
+                fn_cache[event_num] = build_fn(analyzer, event)
+            fn_cache[event_num](event, rec)
 
     if close_log:
         log.close()