Double For Loop Iteration. Two lists comparison

Question

I'm a beginner.

Could you tell me why this script works even though the i variable is not defined in the ip not in i case?

The script compares a list of ARP table match_data (which I get from a paramiko ssh connection), and my ip list match_ip. I just want to print the IP MAC, and if the IP is not in the ARP list table to print ip + "NO MAC ADDRESS".

for ip in match_ip:
    for i in match_data:
        if ip in i:
            print  re.search((ip+'\s+'),i).group(0),mac.search(i).group(0)
            break
    if ip not in i:
        print ip + '           NO MAC ADDRESS'

Output

C:\Python27\python.exe C:/Python2/Get_mac_from_arp.py
!!!Connecting SSH!!!
10.240.184.103           30e4.db80.b699
10.240.184.104          7c2f.802d.61e5
10.240.184.105          7c2f.8072.40fc
10.240.184.106          e05f.b982.5720
10.240.178.11           NO MAC ADDRESS
10.240.184.177          e05f.b982.5752
10.240.184.178          7c2f.802a.782a
10.240.184.179          30e4.db80.b6ec
10.240.184.180          7c2f.802a.782b

Process finished with exit code 0

What do you mean by "the i variable is not defined?" In Python you don't declare variables — John Coleman
– John Coleman, Commented May 11, 2018 at 11:23
Why do you say that "the "i" variable is not defined in the (ip not in i) case". i still has the value it had at the end of the for loop. — PM 2Ring
– PM 2Ring, Commented May 11, 2018 at 11:24
Because, given that your indentation is correct, the i in if ip not in i: takes the last value of i it had in for i in match_data: — zipa
– zipa, Commented May 11, 2018 at 11:25

PM 2Ring · Accepted Answer · 2018-05-11 11:55:22Z

1

The name i is still valid at that point. It has the last value that it got assigned in the for loop. However, there's no need to actually have that if test following the loop. Python for and while loops take an optional else clause which is entered if the loop terminates normally, i.e. it isn't exited early due to a break statement (or a return statement).

So we can re-write that code as:

for ip in match_ip:
    for i in match_data:
        if ip in i:
            print  re.search((ip+'\s+'),i).group(0),mac.search(i).group(0)
            break
    else:
        # We can only get here if the `break` didn't happen
        print ip + '           NO MAC ADDRESS'

I suppose you expected a for loop to create a new scope, with the loop index and other variables created in the loop only existing in that scope. In some languages (especially those closely related to C) a new block creates a new scope, but that doesn't happen in Python with normal for loops. Python is more conservative with creating scopes, because they aren't as "cheap" to do as they are in C. A function definition creates a new local scope, and so does a class definition. However, a generator expression does run in its own scope, and so does a list comprehension in Python 3, in Python 2 a list comprehension runs in the scope of the surrounding code.

You can read about scopes and namespaces in the official tutorial.

BTW, you should seriously consider migrating to Python 3, Python 2 reaches its official End of Life some time in 2020.

edited May 11, 2018 at 11:55

answered May 11, 2018 at 11:33

PM 2Ring

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2 Comments

Dover Over a year ago

I just merely understand why the "else" case is not expected under the "if ip in i" as an alternative action =) And why is "break" word is necessary here (without break the script doesnt work properly)

PM 2Ring Over a year ago

@Dover The Python interpreter knows that the else belongs to the for and not the preceding if because of the indentation. So you need to be very careful not to mess the indentation up when using else with for or while! The break is necessary because otherwise control will automatically go into the else block when the for terminates. Also break makes the code more efficient because we don't want to keep testing for matches after we've already found a match.

bruno desthuilliers · Accepted Answer · 2018-05-11 11:24:21Z

1

even though the "i" variable is not defined in the (ip not in i) case?

It is actually defined - by the for i in match_data statement.

answered May 11, 2018 at 11:24

bruno desthuilliers

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4 Comments

Joonatan Samuel Over a year ago

This is example why one should name their variables properly instead of using i and j in every for loop.

Dover Over a year ago

Isnt the last "if" case must be under the "for i in match_data"?

Chris_Rands Over a year ago

@Dover Loops do not create a scope in Python

bruno desthuilliers Over a year ago

@Dover: depends on the expected result, but that's another question. Note that if you want it in the inner loop, it would be better written as an else clause.

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