Load YAML nested with Jinja2 in Python

Question

I have a YAML file (all.yaml) that looks like:

...
var1: val1
var2: val2
var3: {{var1}}-{{var2}}.txt
...

If I load it in Python like this:

import yaml

f = open('all.yaml')
dataMap = yaml.safe_load(f)
f.close()
print(dataMap["var3"])

the output is {{var1}}-{{var2}}.txt and not val1-val2.txt.

Is it possible to replace the nested vars with the value?

I tried to load it with:

import jinja2
templateLoader = jinja2.FileSystemLoader( searchpath="/path/to/dir" )
templateEnv = jinja2.Environment( loader=templateLoader )
TEMPLATE_FILE = "all.yaml"
template = templateEnv.get_template( TEMPLATE_FILE )

The exception is no longer thrown, now I am stuck and have to research how to proceed.

Answer 1

First define an Undefined class and load yaml to get known values. Then load it again and render with known values.

#!/usr/bin/env python

import yaml
from jinja2 import Template, Undefined

str1 = '''var1: val1
var2: val2
var3: {{var1}}-{{var2}}.txt
'''

class NullUndefined(Undefined):
  def __getattr__(self, key):
    return ''

t = Template(str1, undefined=NullUndefined)
c = yaml.safe_load(t.render())

print t.render(c)

Run it:

$ ./test.py
var1: val1
var2: val2
var3: val1-val2.txt