Leverage Python f-strings with Yaml files?

Question

If I have a yaml file containing a string with the bracket notation {} used in partnership with python f-strings, how might a leverage the f-string interpolation here? Take for example this simple yaml file:

# tmp.yaml
k1: val1
k2: val2 as well as {x}

If x = 'val3', I would like the value of the k2 to reflect val2 as well as val3

# app.py
x = 'val3'
with open('tmp.yaml', 'rt') as f:
    conf = yaml.safe_load(f)

print(conf)
{'k1': 'val1', 'k2': 'val2 as well as {x}'}

This could be accomplished pretty easily with format strings...

print(conf['k2'].format(x=x))
val2 as well as val3

But how to do the same with f-strings?

Probably dupe: [Is there YAML syntax for sharing part of a list or map?](https://stackoverflow.com/q/9254178/674039) — wim, Oct 29 '18 at 01:58
Not seeing how this is a duplicate. Can you elaborate, please? — user9074332, Oct 29 '18 at 02:02
@user9074332 The closest feature that YAML has is using anchors and references (described in linked question). — wim, Oct 29 '18 at 04:23

score 3 · Answer 1 · answered Oct 29 '18 at 10:05

3

You can define a custom constructor:

import yaml

values = { 'x': 'val3' }

def format_constructor(loader, node):
  return loader.construct_scalar(node).format(**values)

yaml.SafeLoader.add_constructor(u'!format', format_constructor)

conf = yaml.safe_load("""
k1: val1
k2: !format val2 as well as {x}
""")

print(conf)

If you don't want to use the tag !format, you can also use add_constructor with u'tag:yaml.org,2002:str' as tag. This will override the default string constructor with yours.

answered Oct 29 '18 at 10:05

flyx

35,506
7
89
126

Could you provide some background on this please? – user9074332 Nov 01 '18 at 01:51
1

@user9074332 What happens is that you put the local tag `!format` on the YAML scalar `val2 as well as {x}` and then tell PyYAML *„when you encounter a node tagged with `!format`, use this function to construct the target value from it“*. In the function, you first construct a string using PyYAML's usual constructor, and then call `format` on it, giving the `value` dict that contains the replacements you want to do. You could also just `...format(x='val3')` but the method I show can be generalized to larger YAML files with multiple replacement templates. – flyx Nov 02 '18 at 10:11

user9074332 · Accepted Answer · 2019-08-20T02:51:48.880

I found that jinja2 provides the easiest solution to this problem.

Source yaml file:

# tmp.yaml
k1: val1
k2: val2 as well as {{ x }}

Read the file and render using jinja templating:

with open('tmp.yaml', 'rt') as f:
    conf = f.read().rstrip()

print(conf)
# 'k1: val1\nk2: val2 as well as {{ x }}'

import jinja2
template = Template(conf)
conf = template.render(x='val3')
config = yaml.safe_load(conf)

print(config)
# {'k1': 'val1', 'k2': 'val2 as well as val3'}

Leverage Python f-strings with Yaml files?

2 Answers2

Source yaml file:

Read the file and render using jinja templating:

Linked