Python with sqlalchemy db retrival

Question

I am quite new to python and to sqlalchemy.I have written the following netowrk program.

class SourcetoPort(Base):
    """"""
    __tablename__ = 'source_to_port'
    id = Column(Integer, primary_key=True)
    port_no        = Column(Integer)
    src_address    = Column(String)

    #----------------------------------------------------------------------
    def __init__(self, src_address,port_no):
        """"""
        self.src_address = src_address
        self.port_no     = port_no

#

if session.query(SourcetoPort).filter_by(src_address='packet.dst').all():

In the above code the line

 if session.query(SourcetoPort).filter_by(src_address='packet.dst').all(): 

does not work as I expect it.What I expect is that it should retrive the entry from sqlalchemy database and if it succeeds (as the output is not NONE) it should execute the following code.

When I try to print that line using print

session query  []

The output that I get is

 session query [] 

Am I doing something wrong.It would be great if someone could point this out.

If I change the above line based on the suggestion as

creating a new db entry
RECIEVED FROM PORT  2 SOURCE  96:74:ba:a9:92:b9
creating a new db entry

ERROR:core:Exception while handling Connection!PacketIn...
Traceback (most recent call last):
  File "ws_thesis/pox/pox/lib/revent/revent.py", line 234, in raiseEventNoErrors
    return self.raiseEvent(event, *args, **kw)
  File "ws_thesis/pox/pox/lib/revent/revent.py", line 281, in raiseEvent
    rv = event._invoke(handler, *args, **kw)
  File "ws_thesis/pox/pox/lib/revent/revent.py", line 159, in _invoke
    return handler(self, *args, **kw)
  File "ws_thesis/pox/tutorial.py", line 137, in _handle_PacketIn
    self.act_like_switch(packet, packet_in)
  File "ws_thesis/pox/tutorial.py", line 101, in act_like_switch
    print session.query(SourcetoPort).filter_by(src_address=packet.dst).count()
  File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2400, in count
    return self.from_self(col).scalar()
  File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2045, in scalar
    ret = self.one()
  File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2014, in one
    ret = list(self)
  File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2057, in __iter__
    return self._execute_and_instances(context)
  File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2072, in _execute_and_instances
    result = conn.execute(querycontext.statement, self._params)
  File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1405, in execute

I get the following error:

 creating a new db entry RECIEVED FROM PORT 2 SOURCE 96:74:ba:a9:92:b9 creating a new db entry ERROR:core:Exception while handling Connection!PacketIn... Traceback (most recent call last): File "ws_thesis/pox/pox/lib/revent/revent.py", line 234, in raiseEventNoErrors return self.raiseEvent(event, *args, **kw) File "ws_thesis/pox/pox/lib/revent/revent.py", line 281, in raiseEvent rv = event._invoke(handler, *args, **kw) File "ws_thesis/pox/pox/lib/revent/revent.py", line 159, in _invoke return handler(self, *args, **kw) File "ws_thesis/pox/tutorial.py", line 137, in _handle_PacketIn self.act_like_switch(packet, packet_in) File "ws_thesis/pox/tutorial.py", line 101, in act_like_switch print session.query(SourcetoPort).filter_by(src_address=packet.dst).count() File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2400, in count return self.from_self(col).scalar() File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2045, in scalar ret = self.one() File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2014, in one ret = list(self) File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2057, in __iter__ return self._execute_and_instances(context) File "/usr/lib/python2.7/dist-packages/sqlalchemy/orm/query.py", line 2072, in _execute_and_instances result = conn.execute(querycontext.statement, self._params) File "/usr/lib/python2.7/dist-packages/sqlalchemy/engine/base.py", line 1405, in execute 

Answer 1

If you want to filter items by the contents of a variable , you need to use that variable directly:

session.query(SourcetoPort).filter_by(src_address=packet.dst)

You were instead trying to filter by the string 'packet.dst' .

You made the same mistake when creating the SourcetoPort entry; you probably wanted to store result of the packet.src expression, not the string 'packet.src' :

SourcetoPort(src_address=packet.src, port_no=packet_in.in_port)

Note that calling .all() retrieves all matching entries from the database, but you are only using it in a if test. It would be more efficient (potentially much more so) to use .count() instead of .all() to let the database tell us how many items match:

if session.query(SourcetoPort).filter_by(src_address=packet.dst).count():