Your PyYAML class had a few problems:
yaml_tag
is case sensitive, so !Env
and !ENV
are different tags.
- So, as per the documentation,
yaml.YAMLObject
uses meta-classes to define itself, and has default to_yaml
and from_yaml
functions for those cases. By default, however, those functions require that your argument to your custom tag (in this case !ENV
) be a mapping. So, to work with the default functions, your defaults.yaml
file must look like this (just for example) instead:
example: !ENV {env_var: "PWD", test: "test"}
Your code will then work unchanged, in my case print(settings)
now results in {'example': /home/Fred}
But you're using load
instead of safe_load
-- in their answer below, Anthon pointed out that this is dangerous because the parsed YAML can overwrite/read data anywhere on the disk.
You can still easily use your YAML file format, example: !ENV foo
—you just have to define an appropriate to_yaml
and from_yaml
in class EnvTag
, ones that can parse and emit scalar variables like the string "foo".
So:
import os
import yaml
class EnvTag(yaml.YAMLObject):
yaml_tag = u'!ENV'
def __init__(self, env_var):
self.env_var = env_var
def __repr__(self):
v = os.environ.get(self.env_var) or ''
return 'EnvTag({}, contains={})'.format(self.env_var, v)
@classmethod
def from_yaml(cls, loader, node):
return EnvTag(node.value)
@classmethod
def to_yaml(cls, dumper, data):
return dumper.represent_scalar(cls.yaml_tag, data.env_var)
# Required for safe_load
yaml.SafeLoader.add_constructor('!ENV', EnvTag.from_yaml)
# Required for safe_dump
yaml.SafeDumper.add_multi_representer(EnvTag, EnvTag.to_yaml)
settings_file = open('defaults.yaml', 'r')
settings = yaml.safe_load(settings_file)
print(settings)
s = yaml.safe_dump(settings)
print(s)
When this program is run, it outputs:
{'example': EnvTag(foo, contains=)}
{example: !ENV 'foo'}
This code has the benefit of (1) using the original pyyaml, so nothing extra to install and (2) adding a representer. :)
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