deleting (killing) a python object from inside a function

Question

Hi I'm using a bloomberg API (for learning purposes and not too relevant to the question - simply as context).

Part of the process is to create "requests" that are sent as queries to a server that responds.

Each query stays "alive" until the object is killed. I'm using a function to call down the data from each request since this is code I use very frequently and the shape of the request can change considerably with each call.

def calldata(req): # doesnt currently work. Dont know why
    session.sendRequest(req)
    while True:
        ev = session.nextEvent()
        tmp = []
        if ev.eventType() == blpapi.Event.RESPONSE or blpapi.Event.PARTIAL_RESPONSE:
            for msg in ev:
                if msg.hasElement('securityData'):
                    data = msg.getElement('securityData')
                    tmp.append(data)
        break
    del req
    return(tmp)

where the req could be for example

# Data for clean multiple data ::= cleanref
r = svc.createRequest('ReferenceDataRequest')
r.append('securities','MSFT US Equity')
r.append('fields','bid')
cleanref = calldata(r)


# Dirty reference for multiple data - both bad security and bad field 
# ::= errorref
req = svc.createRequest("ReferenceDataRequest")
req.append('securities','Rhubarb Curncy')
req.append('fields','PX_LAST')
req.append('securities','SGD Curncy')
req.append('fields','desc')
errorref= calldata(req) 

# Historical data request ::= histdata
request = svc.createRequest("HistoricalDataRequest")
request.getElement("securities").appendValue("IBM US Equity")
request.getElement("securities").appendValue("MSFT US Equity")
request.getElement("fields").appendValue("PX_LAST")
request.getElement("fields").appendValue("OPEN")
request.set("periodicityAdjustment", "ACTUAL")
request.set("periodicitySelection", "MONTHLY")
request.set("startDate", "20060101")
request.set("endDate", "20061231")
request.set("maxDataPoints", 100)

histdata = calldata(request)

# erroneous historical data request ::= histerr
requ = svc.createRequest("HistoricalDataRequest")
requ.getElement("securities").appendValue("IBM US Equity")
requ.getElement("securities").appendValue("MSFT US Equity")
requ.getElement("fields").appendValue("PX_LAST")
requ.getElement("fields").appendValue("Desc")
requ.set("periodicityAdjustment", "ACTUAL")
requ.set("periodicitySelection", "MONTHLY")
requ.set("startDate", "20060101")
requ.set("endDate", "20061231")
requ.set("maxDataPoints", 100)

histerr = calldata(requ)

So I need to kill each of the requests in the function to be able to reuse it and I cant figure it out. I suspect it may be something to do with locals() but any help would be gratefully recieved.

Answer 1

you have to call del r after each call of calldata(r), like this:

cleanref = calldata(r)
del r

del ref inside your function only delete reference inside your function.

Answer 2

del does not delete objects. It deletes names. So when you call

del req

All you do is remove the name req from the local namespace.

Objects are deleted by the garbage collector. The object that req refers to will not be deleted until all names/variables that refer to the object has been deleted (and even then it may not be deleted, that's an implementation issue). In this case you may be able to do del request after the call to calldata(request). However, there is no guarantee that the request object is deleted even then.

So I suggest you change your code so it doesn't rely on objects being garbage collected at specific times. It is also unclear to me how, why or even if your code actually relies on this, or if you just assume it does.