在Python中加载与Jinja2嵌套的YAML

问题描述：

我有一个YAML文件（ all.yaml ），看起来像：

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

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

如果我像这样在Python中加载它：

If I load it in Python like this:

import yaml

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

输出为 {{var1}}-{{var2}}。txt 而不是 val1-val2.txt 。

是否可以用值替换嵌套的变量？

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

我尝试加载：

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

不再抛出异常，现在我被卡住了，必须研究

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

答

首先定义一个 Undefined 类并加载yaml获得已知值。然后再次加载它并渲染为已知值。

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)

运行它：

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

在Python中加载与Jinja2嵌套的YAML

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