Python函数针对非空列表返回None

Question

I have written a recursive function to make queries to Racktables database and follow the connections between objects and put them in a list.

All the results get appended and extended to a list that I give as argument. The function works up to the return statement, but the list gives None after being returned

I have added some print statements in the function to debug, and the list contains the data I need up to that point:

def plink(Objid, PortId, mylist):

    if len(mylist) < 2:                         #If this is the first run put he initial data in the list
        mylist.append(Objid)
        mylist.append(PortId)

    res = rt.GetLink(PortId)                    #check if port is connected 

    if res == (False, False):
        print 'exiting because not connected'   #debug
        return mylist                           #If not connected return list

    nextObj = rt.GetPortObjid(res[1])
    mylist.extend(res)
    mylist.append(nextObj)

    ispatch = rt.CheckObjType(nextObj, 50080)
    if ispatch[0][0] == 0:                      #If connected to a non-patch-unit, add data to list and exit
        print "exiting because next object {0} is not a patchunit, mylist is {1}".format(nextObj, mylist)  #debug
        return mylist

    patchPorts = rt.GetAllPorts(nextObj)

    if len(patchPorts) != 2:                    #check if the patchunit has the right number of ports
        mylist.append("Error, patch-unit must have exactly two ports")
        return mylist

    if patchPorts[0][2] == res[1]:              #check which port is unseen and call the function again
        print mylist
        plink(nextObj, patchPorts[1][2], mylist)
    else:
        print mylist
        plink(nextObj, patchPorts[0][2], mylist)

results = ['Initial data']
allconn = plink(159, 947, results)
print "The full connection is {0}".format(allconn)

(I have skipped the DB constructs here)

Running this code gives:

['Initial data', 159, 947, 'C150303-056', 4882, 1591L]
['Initial data', 159, 947, 'C150303-056', 4882, 1591L, 'C140917-056', 4689, 727L]
exiting because next object 1114 is not a patchunit, mylist is ['Initial data', 159, 947, 'C150303-056', 4882, 1591L, 'C140917-056', 4689, 727L, 'C140908-001', 3842, 1114L]
The full connection is None

The debug prints show the list being populated exactly as I it is supposed to, but then I get None when printing outside the function, also after assigning to variable beforehand.

I am writing this to run it on a CentOS 6 server with python 2.6. I could run it in virtualenv, as a last resort, but I would avoid it if possible

Answer 1

if patchPorts[0][2] == res[1]:              #check which port is unseen and call the function again
    print mylist
    plink(nextObj, patchPorts[1][2], mylist)
else:
    print mylist
    plink(nextObj, patchPorts[0][2], mylist)

Calling a function recursively does not automatically make the inner call pass the return value up to the outer call. You still need to explicitly return .

if patchPorts[0][2] == res[1]:              #check which port is unseen and call the function again
    print mylist
    return plink(nextObj, patchPorts[1][2], mylist)
else:
    print mylist
    return plink(nextObj, patchPorts[0][2], mylist)