Nested For Loop Issue in Python

Question

I am a newbie in python area ; i was trying to write a simple program to run multiple commands on multiple devices , Input from 2 different text files but the output is a bit wierd and i am not sure what is the issue sample code as below:

commandsfile = input ("Please Enter CommandsFile path as c:/example/ \n :")
hostsfile = input ("Please Enter Hosts path as c:/example/ \n :")
commands1 = open( (commandsfile), "r")
hosts = open((hostsfile) , "r")
for host in hosts:
 print ("1") 
 for cmd in commands1:
 print ("2 ")

i have 2 devices saved in hosts.txt "aa" "bb" and 2 commands saved in commands.txt "11" "22"

the output for above code is 1 2 2 1

how ever i was expecting 1 2 2 1 2 2

any advice how to fix :(

• hobenkr: Do you intend for print("2 ") to be within the second for loop? It would also help to provide verbatim copies of your two files since they seem to be short. The question as you asked it is a little confusing.
• Aquiles: The problem right there is that you are reading a file so on the second loop the file pointer does not come back to the start when you do the for again
• Saadi381: @aquiles ;is there anyway to fix this ??
• Neo: Yes, using seek(). See my answer

Answer 1

I think the problem is that when you iterate the actual file it moves the cursor or something, so when you iterate it again you're at the end of file.
However, using .readlines() after every open() solves it (maybe because you don't iterate over the file itself but over a list created from it).

commandsfile = input ("Please Enter CommandsFile path as c:/example/ \n :")
hostsfile = input ("Please Enter Hosts path as c:/example/ \n :")
commands1 = open( (commandsfile), "r").readlines()
hosts = open((hostsfile) , "r").readlines()
for host in hosts:
 print ("1") 
 for cmd in commands1:
 print ("2 ")

If you want to iterate over the actual file, you can use seek() to change cursor position

commandsfile = input ("Please Enter CommandsFile path as c:/example/ \n :")
hostsfile = input ("Please Enter Hosts path as c:/example/ \n :")
commands1 = open( (commandsfile), "r")
hosts = open((hostsfile) , "r")
for host in hosts:
 print ("1") 
 for cmd in commands1:
 print ("2 ")
 commands1.seek(0, 0)

• Neo: Happy to help... Why not approved?

Answer 2

An easy and quick fix would be to move the commands line into the first loop:

hosts = open((hostsfile) , "r")
for host in hosts:
 print("1") 
 commands1 = open((commandsfile), "r")
 for cmd in commands1:
 print("2")

Answer 3

Your commands1 is exhausted in the first iteration of the for loop. When you do commands1 = open( (commandsfile), "r") you get an iterator in commands1 and will be exhausted in the for loop. You can convert the commamds1 iterator to a list by doing commamds1 = list(commamds1)

commandsfile = input ("Please Enter CommandsFile path as c:/example/ \n :")
hostsfile = input ("Please Enter Hosts path as c:/example/ \n :")
commands1 = open( (commandsfile), "r")
commands1 = list(commands1)
hosts = open((hostsfile) , "r")
for host in hosts:
 print ("1") 
 for cmd in commands1:
 print ("2 ")

• Saadi381: i havent tried this but i believe this will also fix the issue...thanks alot