Control of whitespace symbols

trim_blocks, lstrip_blocks

Parameter trim_blocks removes the first empty line after block if its value is True (default False).

Effect of using the flag is showed on example templates/env_flags.txt:

router bgp {{ bgp.local_as }}
 {% for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

If cfg_gen.py script starts without trim_blocks, lstrip_blocks:

env = Environment(loader=FileSystemLoader(TEMPLATE_DIR))

The output is:

$ python cfg_gen.py templates/env_flags.txt data_files/router.yml
router bgp 100

 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100

 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

new lines occur because of for block.

{% for ibgp in bgp.ibgp_neighbors %}

By default, the same behavior will be with any other Jinja blocks.

When trim_blocks flag is added:

env = Environment(loader=FileSystemLoader(TEMPLATE_DIR),
                  trim_blocks=True)

The result will be:

$ python cfg_gen.py templates/env_flags.txt data_files/router.yml
router bgp 100
  neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
  neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

Empty lines after block were removed.

In front of neighbor ... remote-as lines two spaces appeared. This is because there is a space in front of for block. Once lstrip_blocks has been disabled, spaces and tabs in front of the block are added to the first line of block.

This does not affect the next lines. Therefore, lines with neighbor ... update-source are displayed with one space.

Parameter lstrip_blocks controls whether spaces and tabs will be removed from the beginning of line to the beginning of block (untill opening curly bracket).

If add lstrip_blocks=True:

env = Environment(loader=FileSystemLoader(TEMPLATE_DIR),
                  trim_blocks=True, lstrip_blocks=True)

The result will be:

$ python cfg_gen.py templates/env_flags.txt data_files/router.yml
router bgp 100
 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

Disabling lstrip_blocks for block

Sometimes you need to disable lstrip_blocks in block.

For example, if lstrip_blocks is set to True in an environment, but must be disabled for the second block in template (templates/flagenv_s2.txt file):

router bgp {{ bgp.local_as }}
 {% for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

router bgp {{ bgp.local_as }}
 {%+ for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

The result will be:

$ python cfg_gen.py templates/env_flags2.txt data_files/router.yml
router bgp 100
 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

router bgp 100
  neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

Plus sign after percent sign disables lstrip_blocks for the block, in this case, only in the beginning.

If done this way (plus is added in the end block expression):

router bgp {{ bgp.local_as }}
 {% for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

router bgp {{ bgp.local_as }}
 {%+ for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {%+ endfor %}

It will be disabled for the end of the block:

$ python cfg_gen.py templates/env_flags2.txt data_files/router.yml
router bgp 100
 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

router bgp 100
  neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
  neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

Removing whitespace from block

Similarly, you can control whitespace removal for a block.

For this example, flags are not set in environment:

env = Environment(loader=FileSystemLoader(TEMPLATE_DIR))

Template templates/env_flags3.txt:

router bgp {{ bgp.local_as }}
 {% for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

router bgp {{ bgp.local_as }}
 {%- for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

Note the minus at the beginning of second block. Minus removes all whitespace characters, in this case, at the beginning of the block.

The result will be:

$ python cfg_gen.py templates/env_flags3.txt data_files/router.yml
router bgp 100

 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100

 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100


router bgp 100
 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100

 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

If you add minus to the end of the block:

router bgp {{ bgp.local_as }}
 {% for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

router bgp {{ bgp.local_as }}
 {%- for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {%- endfor %}

Empty string at the end of the block will be deleted:

$ python cfg_gen.py templates/env_flags3.txt data_files/router.yml
router bgp 100

 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100

 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100


router bgp 100
 neighbor 10.0.0.2 remote-as 100
 neighbor 10.0.0.2 update-source lo100
 neighbor 10.0.0.3 remote-as 100
 neighbor 10.0.0.3 update-source lo100

Try to add minus at the end of expressions describing the block and look at the result:

router bgp {{ bgp.local_as }}
 {% for ibgp in bgp.ibgp_neighbors %}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {% endfor %}

router bgp {{ bgp.local_as }}
 {%- for ibgp in bgp.ibgp_neighbors -%}
 neighbor {{ ibgp }} remote-as {{ bgp.local_as }}
 neighbor {{ ibgp }} update-source {{ bgp.loopback }}
 {%- endfor -%}