Loops and Conditions for Repeated Configuration

Replacing manual indexing with a loop

Lesson 3 left off with an awkward pattern: device.vlans[0], device.vlans[1], device.vlans[2], hand-written for exactly the number of VLANs one device happened to have. Jinja2's {% for %} tag is the same idea as Python's for loop — "do this once per item" — applied inside a template:

vlan_loop.py

from jinja2 import Template

vlans = [
  {"id": 10, "name": "USERS"},
  {"id": 20, "name": "VOICE"},
]

template = Template("""{% for vlan in vlans %}
vlan {{ vlan.id }}
name {{ vlan.name }}
{% endfor %}""")

print(template.render(vlans=vlans))

{% for vlan in vlans %} ... {% endfor %} runs the block between the tags once per item in vlans, with vlan bound to the current item each time — exactly like Python's for vlan in vlans:.

Quick checkConcept check

Inside {% for vlan in vlans %} ... {% endfor %}, what does vlan refer to on each pass?

The entire vlans list, every timeThe current item from the vlans list for that pass through the loop — the same role a Python for loop's variable playsAlways the first item in the listThe index number of the current position

A real gotcha: stray blank lines

Running the example above produces this — worth looking at closely:

output (naive)


vlan 10
name USERS

vlan 20
name VOICE

Notice the blank line before every 'vlan' block. The {% for %} and {% endfor %} tags each sit on their own line in the template — and by default, Jinja2 keeps the newline after each tag as part of the output.

That's not a mistake in the VLAN data — it's what Jinja2 does by default with block tags on their own line. For CLI configuration text, stray blank lines are usually harmless to a device, but they make diffs, validation, and reviewed output noisier than they need to be.

Verified, not assumed

This is real, reproducible Jinja2 behavior — worth confirming for yourself with the exact code above rather than taking it on faith.

The fix is two Environment options:

vlan_loop_fixed.py

from jinja2 import Environment

env = Environment(trim_blocks=True, lstrip_blocks=True)

template = env.from_string("""{% for vlan in vlans %}
vlan {{ vlan.id }}
name {{ vlan.name }}
{% endfor %}""")

print(template.render(vlans=vlans))

trim_blocks removes the newline immediately after a block tag; lstrip_blocks removes leading whitespace before a block tag on its own line. Together they produce clean output with no template text changes.

output (fixed)

vlan 10
name USERS
vlan 20
name VOICE

Same template text, same data — only the Environment options changed. No blank lines.

Quick checkPredict output

With trim_blocks=True and lstrip_blocks=True, does the exact same template text from the naive example above still produce blank lines between VLAN blocks?

Yes — those options don't affect for loopsNo — both options together remove the stray newlines left by the {% for %} / {% endfor %} tags being on their own line, producing clean outputYes, but only for the last item in the listNo, but only if the VLAN list has exactly two items

Conditional configuration

{% if %} works the same way {% for %} does — the same concept as Python's if, applied inside a template:

conditional_interfaces.py

interfaces = [
  {"name": "GigabitEthernet1/0/1", "description": "Uplink", "enabled": True},
  {"name": "GigabitEthernet1/0/2", "description": "Unused port", "enabled": False},
]

template = env.from_string("""{% for iface in interfaces %}
interface {{ iface.name }}
description {{ iface.description }}
{% if not iface.enabled %}
shutdown
{% endif %}
{% endfor %}""")

print(template.render(interfaces=interfaces))

{% if not iface.enabled %} ... {% endif %} only includes the 'shutdown' line for interfaces where enabled is False — the same Environment(trim_blocks=True, lstrip_blocks=True) keeps this clean too.

This renders:

output

interface GigabitEthernet1/0/1
description Uplink
interface GigabitEthernet1/0/2
description Unused port
shutdown

The first interface (enabled: True) has no shutdown line. The second (enabled: False) does — the same template produced structurally different output per item, driven entirely by data.

Quick checkIdentify the correct code

Which condition correctly adds a 'shutdown' line only for interfaces where enabled is False?

{% if iface.enabled %} shutdown {% endif %}{% if not iface.enabled %} shutdown {% endif %}{% for iface.enabled == False %} shutdown {% endfor %}{{ if not iface.enabled }} shutdown {{ endif }}

One template, many devices

Loops and conditionals together are what make a single template file usable across an entire fleet — the same template renders correctly-different output for every device, because every device supplies its own data:

One template filewritten once — vlan_config.j2
Device 1's datarenders Device 1's VLANs
Device 2's datasame template, renders Device 2's VLANs
Device 3's datasame template, renders Device 3's VLANs

Quick checkScenario

Three devices each have a different number of VLANs (2, 4, and 1). How many different templates are needed to render all three correctly?

Three — one per deviceOne — the {% for %} loop adapts to however many items are actually in each device's own VLAN listFour — one per VLAN count that appearsZero — templates can't handle a varying number of items

Key takeaways

{% for item in list %} ... {% endfor %} renders its contents once per item — the same concept as Python's for loop, inside a template.
{% if condition %} ... {% endif %} conditionally includes template content — the same concept as Python's if, inside a template.
Jinja2's default behavior leaves stray blank lines around block tags on their own line — Environment(trim_blocks=True, lstrip_blocks=True) fixes this, verified directly rather than assumed.
A loop is what actually lets one template serve devices with a different number of VLANs each — no per-device template editing required.
Loops and conditionals together are the mechanism behind 'one template, many devices' — data drives every difference in the output.

What you'll learn

Replacing manual indexing with a loop

A real gotcha: stray blank lines

Conditional configuration

One template, many devices