don't create _ListenerCollection objects if not needed?

[Mike Bayer]

This would most ideally apply across the board. Here's a patch that does it in an awkward way just for schema events:

diff -r e15fc03a16a322298e27c416dce3f3fe869bdf1a lib/sqlalchemy/event.py
--- a/lib/sqlalchemy/event.py   Mon Jun 18 10:06:49 2012 -0400
+++ b/lib/sqlalchemy/event.py   Wed Jun 20 16:03:04 2012 -0400
@@ -115,6 +115,14 @@
     def __reduce__(self):
         return _UnpickleDispatch(), (self._parent_cls, )

+    def _has(self, name, cls):
+        cls_dispatch = getattr(self.__class__, name)
+        if cls in cls_dispatch._clslevel and \
+            cls_dispatch._clslevel[cls](cls):
+            return True
+        else:
+            return name in self.__dict__
+
     def _update(self, other, only_propagate=True):
         """Populate from the listeners in another :class:`_Dispatch`
             object."""
diff -r e15fc03a16a322298e27c416dce3f3fe869bdf1a lib/sqlalchemy/events.py
--- a/lib/sqlalchemy/events.py  Mon Jun 18 10:06:49 2012 -0400
+++ b/lib/sqlalchemy/events.py  Wed Jun 20 16:03:04 2012 -0400
@@ -230,9 +230,11 @@
         raise NotImplementedError()

     def _set_parent_with_dispatch(self, parent):
-        self.dispatch.before_parent_attach(self, parent) 
+        if self.dispatch._has('before_parent_attach', self.__class__):
+            self.dispatch.before_parent_attach(self, parent) 
         self._set_parent(parent) 
-        self.dispatch.after_parent_attach(self, parent) 
+        if self.dispatch._has('after_parent_attach', self.__class__):
+            self.dispatch.after_parent_attach(self, parent)

 class PoolEvents(event.Events):
     """Available events for :class:`.Pool`.

but perhaps we can alter _DispatchDescriptor.get to not actually generate a _ListnerCollection. _ListenerCollection can be created only when listen() is called and we can issue a separate codepath for that.

[Mike Bayer]

the attached patch will do it. here's a proof of concept:

from sqlalchemy.event import _ListenerCollection as old_collection
from sqlalchemy import event
from collections import defaultdict

from sqlalchemy.event import _EmptyListener as old_empty

class _ListenerCollection(old_collection):
    canary = defaultdict(int)
    def __init__(self, parent, target_cls):
        _ListenerCollection.canary[parent.__name__)]((target_cls,) += 1
        old_collection.__init__(self, parent, target_cls)

event._ListenerCollection = _ListenerCollection

class _EmptyListener(old_empty):
    canary = defaultdict(int)
    def __init__(self, parent, target_cls):
        _EmptyListener.canary[parent.__name__)]((target_cls,) += 1
        old_empty.__init__(self, parent, target_cls)
event._EmptyListener = _EmptyListener

from sqlalchemy import *

metadata = MetaData()

for i in xrange(60):
    Table('t%d' % i, metadata,
        Column('id', Integer, primary_key=True),
        *[           Column("c%d" % j, Integer)
            for j in xrange(10)
        ](
)
    )

print "-----------"
for (cls, name), count in _ListenerCollection.canary.items():
    print "Class: %r Event name: %s  Count: %s" % (cls.__name__, name, count)
print "Total: %d" % sum(_ListenerCollection.canary.values())

print "-----------"
for (cls, name), count in _EmptyListener.canary.items():
    print "Class: %r Event name: %s  Count: %s" % (cls.__name__, name, count)
print "Total: %d" % sum(_EmptyListener.canary.values())

[Mike Bayer]

this came out great, some modifications vs. the patch here. d25be81ba57766e9948eba408b1115a1570a7737 40098941007ff3aa1593e834915c4042c1668dc2

[Mike Bayer]

major regression, not found by any existing tests....good thing I caught this

    def test_bool_clslevel(self):
        def listen_one(x, y):
            pass
        event.listen(self.Target, "event_one", listen_one)
        t = self.Target()
        assert t.dispatch.event_one

[Mike Bayer]

e5accce97bae1b35eeb1eb26f8281a98c58d2cef

088cf5e622e8e60136394b4759d31dd774c22cc7

[Mike Bayer]

Removing milestone: 0.7.9 (automated comment)