Python For Loop

Python For Loops at a Glance

A for loop takes one item at a time from a sequence and runs its indented body for that item. This repeated processing is called iteration.

1. Python For Loop Visual Summary

Use this infographic to review the loop flow, syntax, control statements, nested loops, and common network automation uses.

2. For Loop

Python supports two common loop types: while and for. A for loop is the natural choice when you want to process each item in a sequence. The sequence can be a string, list, tuple, dictionary, or another iterable.

Start with a list of firewall names:

firewalls = ["FW1", "FW2", "FW3"]

for fw in firewalls:
    print(fw)

The code produces this output:

FW1
FW2
FW3

The loop starts with for, followed by a new variable named fw. On each iteration, fw represents the next item in firewalls. The indented print() call therefore runs three times.

3. Break Statement

The break statement exits the nearest loop before all items have been processed. It is useful when a search has found its target or a condition makes further work unnecessary.

firewalls = ["FW1", "FW2", "FW3"]

for fw in firewalls:
    if fw == "FW3":
        break
    print(fw)

FW1
FW2

The loop prints FW1 and FW2. When fw equals FW3, break exits the loop before the final print() call.

4. Continue Statement

The continue statement stops only the current iteration. Python skips the remaining statements in the loop body and moves to the next item.

firewalls = ["FW1", "FW2", "FW3"]

for fw in firewalls:
    if fw == "FW2":
        continue
    print(fw)

FW1
FW3

The iteration for FW2 reaches continue, so its print() call is skipped. The loop itself continues and processes FW3.

5. Else Statement

A for loop can include an else block. It runs when the sequence is exhausted and the loop finishes normally.

ip_addresses = ["192.168.1.1", "192.168.1.2"]

for ip in ip_addresses:
    print(ip)
else:
    print("Completed the loop.")

192.168.1.1
192.168.1.2
Completed the loop.

The final message appears because the loop processed both addresses without encountering break. If break exits the loop, the else block does not run. This behavior is useful when searching a sequence: the else block can report that no matching item was found.

6. Nested Loop

A nested loop is a loop inside another loop. The inner loop runs once for every iteration of the outer loop. In this example, every show command is paired with every IP address:

show_commands = ["show run", "show ip route"]
ip_addresses = ["192.168.1.1", "192.168.1.2"]

for ip in ip_addresses:
    print("Connecting to IP address %s" % ip)
    for command in show_commands:
        print("Execute command '%s' on device with IP: %s" % (command, ip))

Connecting to IP address 192.168.1.1
Execute command 'show run' on device with IP: 192.168.1.1
Execute command 'show ip route' on device with IP: 192.168.1.1
Connecting to IP address 192.168.1.2
Execute command 'show run' on device with IP: 192.168.1.2
Execute command 'show ip route' on device with IP: 192.168.1.2

The outer loop iterates over ip_addresses, and the inner loop iterates over show_commands. Two devices multiplied by two commands produce four command-and-device combinations.

7. Guided Practice

8. Conclusion

You learned how to use Python for loops:

• A for loop iterates through a sequence and executes code for each item.
• break stops the loop before it processes every item.
• continue skips the current iteration and moves to the next item.
• A loop else block runs after normal completion, but not after break.
• A nested for loop runs one loop inside another.

These patterns let network automation scripts process device inventories, configuration lines, interfaces, commands, and validation results without repeating the same code by hand.