CODEWITHSUNDEEP
python
misteryes
misteryes

Reputation: 2287

Python: only 1 loop excuted in a nested for loop

I have a python script, with a nested for loop

    HostList = file('trylist.txt')
    DestHostList = file('trylist2.txt')
    for DestHost in DestHostList:
            DestHost = DestHost.strip()
            for HostName in HostList:
                    HostName = HostName.strip()
                    try:
                            if DestHost!=HostName:
                                    print HostName,DestHost

                            print "one loop finishes"
                    except Exception, e:
                            ExceptionHost.write(HostName+' -  '+DestHost+':    '+str(e)+'\n')       
                            print "exception"
                            #traceback.print_exc(file=sys.stdout)

The outer for loop appears to be only executed once. What could be the potential problem?

Upvotes: 2

Views: 4745

Answers (2)

shx2
shx2

Reputation: 64368

The outer loop executes more than once, but only in the first iteration it has things to do. In the rest, the inner loop does not execute, leaving you with the impression it's only running once.

The problem is that you open the first file, trylist.txt, and read it entirely in the first iteration of the outer loop. On the second iteration, the file object (which is iterator-like) has already been "consumed".

Try:

HostList = file('trylist.txt').readlines()  # now it's a list of lines, not an iterator
DestHostList = file('trylist2.txt')
for DestHost in DestHostList:
    DestHost = DestHost.strip()
    for HostName in HostList:

If the file is huge and you want to avoid storing it in memory, you can reopen it every time:

DestHostList = file('trylist2.txt')
for DestHost in DestHostList:
    DestHost = DestHost.strip()
    HostList = file('trylist.txt')
    for HostName in HostList:

Also, it is good practice to open files in python using a with statement.

Upvotes: 4

nad2000
nad2000

Reputation: 4955

The problem is that you have exhausted the second file in the inner loop after the first pass.

As I can see you were trying to troubleshoot the issue with try ... except .... But since the file read operation is outside the block, you missed it.

Here is a solution with reading in advance all file content. I hope it will help:

host_list = file("trylist.txt").readlines()
dest_host_list = file("trylist2.txt").readlines()

for h in host_list:
    for d in dest_host_list:
        h,d = h.strip(),d.strip()
        if h != d:
            print h,d

Upvotes: 0

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