diff tests/test-ctxmanager.py @ 27703:4e27c0a70574

util: introduce ctxmanager, to avoid nested try/finally blocks

This is similar in spirit to contextlib.nested in Python <= 2.6,
but uses an extra level of indirection to avoid its inability to
clean up if an __enter__ method raises an exception.

Why add this mechanism?  It greatly simplifies scoped resource
management, and lets us eliminate several hundred lines of try/finally
blocks.  In many of these cases the "finally" is separated from the
"try" by hundreds of lines of code, which makes the connection
between resource acquisition and disposal difficult to follow.

(The preferred mechanism would be the "multi-with" syntax of 2.7+,
but Mercurial can't move to 2.7 for a while.)

Intended use:

>>> with ctxmanager(lambda: file('foo'), lambda: file('bar')) as c:
>>>    f1, f2 = c()

This will open both foo and bar when c() is invoked, and will close
both upon exit from the block.  If the attempt to open bar raises
an exception, the block will not be entered - but foo will still
be closed.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-ctxmanager.py	Mon Jan 11 15:25:43 2016 -0800
@@ -0,0 +1,77 @@
+from __future__ import absolute_import
+
+import silenttestrunner
+import unittest
+
+from mercurial.util import ctxmanager
+
+class contextmanager(object):
+    def __init__(self, name, trace):
+        self.name = name
+        self.entered = False
+        self.exited = False
+        self.trace = trace
+
+    def __enter__(self):
+        self.entered = True
+        self.trace(('enter', self.name))
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.exited = exc_type, exc_val, exc_tb
+        self.trace(('exit', self.name))
+
+    def __repr__(self):
+        return '<ctx %r>' % self.name
+
+class ctxerror(Exception):
+    pass
+
+class raise_on_enter(contextmanager):
+    def __enter__(self):
+        self.trace(('raise', self.name))
+        raise ctxerror(self.name)
+
+class raise_on_exit(contextmanager):
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        self.trace(('raise', self.name))
+        raise ctxerror(self.name)
+
+def ctxmgr(name, trace):
+    return lambda: contextmanager(name, trace)
+
+class test_ctxmanager(unittest.TestCase):
+    def test_basics(self):
+        trace = []
+        addtrace = trace.append
+        with ctxmanager(ctxmgr('a', addtrace), ctxmgr('b', addtrace)) as c:
+            a, b = c()
+            c.atexit(addtrace, ('atexit', 'x'))
+            c.atexit(addtrace, ('atexit', 'y'))
+        self.assertEqual(trace, [('enter', 'a'), ('enter', 'b'),
+                                 ('atexit', 'y'), ('atexit', 'x'),
+                                 ('exit', 'b'), ('exit', 'a')])
+
+    def test_raise_on_enter(self):
+        trace = []
+        addtrace = trace.append
+        with self.assertRaises(ctxerror):
+            with ctxmanager(ctxmgr('a', addtrace),
+                           lambda: raise_on_enter('b', addtrace)) as c:
+                c()
+                addtrace('unreachable')
+        self.assertEqual(trace, [('enter', 'a'), ('raise', 'b'), ('exit', 'a')])
+
+    def test_raise_on_exit(self):
+        trace = []
+        addtrace = trace.append
+        with self.assertRaises(ctxerror):
+            with ctxmanager(ctxmgr('a', addtrace),
+                           lambda: raise_on_exit('b', addtrace)) as c:
+                c()
+                addtrace('running')
+        self.assertEqual(trace, [('enter', 'a'), ('enter', 'b'), 'running',
+                                 ('raise', 'b'), ('exit', 'a')])
+
+if __name__ == '__main__':
+    silenttestrunner.main(__name__)