Parsing one field of PCAP file into a column in Python

Question

I am currently work on a PCAP file and want to export of the one particular field(['TCP'].srcport) into a separate variable. The ['TCP'].srcport is not present for every row. it is present for the first 3 rows and not present for the remaining 3 row. The following is the code which I am using,

a = []
l = 0
for i in pcap_file:
    try :
        print l
        print(i['TCP'].srcport)
        a[l] = i['TCP'].srcport
        l = l+1
    except:
         print l
         print 'None'
         a[l] = 0
         l = l+1
         continue

The problem here is the ['TCP'].srcport is not present in all the rows of the PCAP file. I want to parse the value wherever it is available and when it is not available, I want to give a value as "None". I am able to print something as follows,

64
64
64
None
None
None

But when I assign it to a variable, I am getting the following error,

--> 13         a[l] = "
     14         l = l+1
     15         continue

IndexError: list assignment index out of range

I would ideally want something like in a variable called l,

l

64 64 64 None None None

Can anybody help me in handling the exceptions and placing all the values into a variable?

Answer 1

Use a list comprehension with a conditional expression:

a = [i['TCP'] if 'TCP' in i else None for i in pcap_file]

Or use a .get() call; it allows you to specify a default value to return, but the default for that is None so there's no need to specify it.

a = [i.get('TCP') for i in pcap_file]