Jinja2
Jinja2 is a templating language often used in web design, with some similarities to Python. It can be used to make a workflow definition more dynamic.
The Jinja2 Language
In Jinja2 statements are wrapped with {%
characters, i.e:
{% ... %}
Variables are initialised with the set
statement, e.g:
{% set foo = 3 %}
Expressions wrapped with {{
characters will be replaced with
the evaluated expression, e.g:
There are {{ foo }} methods for consolidating the flow.cylc file
Would result in:
There are 3 methods for consolidating the flow.cylc file
Loops are written with for
statements, e.g:
{% for x in range(foo) %}
{{ x }}
{% endfor %}
Would result in:
0
1
2
To enable Jinja2 in the flow.cylc
file, add the following shebang to the
top of the file:
#!Jinja2
For more information see the Jinja2 documentation.
Example
To consolidate the configuration for the get_observations
tasks we could
define a dictionary of station and ID pairs:
{% set stations = {'aldergrove': 3917,
'camborne': 3808,
'heathrow': 3772,
'shetland': 3005} %}
We could then loop over the stations like so:
{% for station in stations %}
{{ station }}
{% endfor %}
After processing, this would result in:
aldergrove
camborne
heathrow
shetland
We could also loop over both the stations and corresponding IDs like so:
{% for station, id in stations.items() %}
{{ station }} - {{ id }}
{% endfor %}
This would result in:
aldergrove - 3917
camborne - 3808
heathrow - 3772
shetland - 3005
Putting this all together, the get_observations
configuration could be
written as follows:
#!Jinja2
{% set stations = {'aldergrove': 3917,
'camborne': 3808,
'heathrow': 3772,
'shetland': 3005} %}
[scheduler]
allow implicit tasks = True
[scheduling]
[[graph]]
T00/PT3H = """
{% for station in stations %}
get_observations_{{station}} => consolidate_observations
{% endfor %}
"""
[runtime]
{% for station, id in stations.items() %}
[[get_observations_{{station}}]]
script = get-observations
[[[environment]]]
SITE_ID = {{ id }}
API_KEY = xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
{% endfor %}
Practical
This practical continues on from the families practical.
Use Jinja2 To Avoid Duplication.
The
API_KEY
environment variable is used by both theget_observations
andget_rainfall
tasks. Rather than writing it out multiple times we will use Jinja2 to centralise this configuration.At the top of the
flow.cylc
file add the Jinja2 shebang line. Then copy the value of theAPI_KEY
environment variable and use it to define anAPI_KEY
Jinja2 variable:#!Jinja2 {% set API_KEY = 'xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx' %}
Next replace the key, where it appears in the workflow, with
{{ API_KEY }}
:[runtime] [[get_observations_heathrow]] script = get-observations [[[environment]]] SITE_ID = 3772 - API_KEY = xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx + API_KEY = {{ API_KEY }} [[get_observations_camborne]] script = get-observations [[[environment]]] SITE_ID = 3808 - API_KEY = xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx + API_KEY = {{ API_KEY }} [[get_observations_shetland]] script = get-observations [[[environment]]] SITE_ID = 3005 - API_KEY = xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx + API_KEY = {{ API_KEY }} [[get_observations_aldergrove]] script = get-observations [[[environment]]] SITE_ID = 3917 - API_KEY = xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx + API_KEY = {{ API_KEY }} [[get_rainfall]] script = get-rainfall [[[environment]]] # The key required to get weather data from the DataPoint service. # To use archived data comment this line out. - API_KEY = xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx + API_KEY = {{ API_KEY }}
Check the result with
cylc config
. The Jinja2 will be processed so you should not see any difference after making these changes.