Regex to extract a list from a yaml string

Question

I have contents of yaml file as a string. I want to look through these and fetch any occurrence of a list named users

yaml_file =  base64.b64decode(encoded_data).decode('utf-8')
users = []
# fetch users from yaml_file

The list can appear anywhere in the files, such as parent element or a child of any degree, hence reading the file as yaml and parsing it won’t be useful since there is no one structure.

...
users:
  - user1
  - user2
  - user3
...

Is there a regex that I can use to fetch only the list of name users from a yaml string?

Answer 1

Parsing structured data with regex is almost always going to result in unreliable behaviors.

You can instead use a function that traverses sub-dicts and sub-lists recursively until a sub-list under a key with a specified name is found:

def find_named_list(data, name):
    if isinstance(data, dict):
        for key, value in data.items():
            if key == name and isinstance(value, list):
                return value
            if (lst := find_named_list(value, name)) is not None:
                return lst
    elif isinstance(data, list):
        for value in data:
            if (lst := find_named_list(value, name)) is not None:
                return lst

so that:

import yaml

data = '''---
foo:
- hello: 1
  world:
    users:
      - user1
      - user2
      - user3
- stack: overflow
bar: ''
'''

print(find_named_list(yaml.safe_load(data), 'users'))

outputs:

['user1', 'user2', 'user3']

Demo: https://ideone.com/Mdjixm

To find all sub-lists under a key with a specified name, create a generator using the yield statement instead:

def find_named_list(data, name):
    if isinstance(data, dict):
        for key, value in data.items():
            if key == name and isinstance(value, list):
                yield value
            yield from find_named_list(value, name)
    elif isinstance(data, list):
        for value in data:
            yield from find_named_list(value, name)

so that:

data = '''---
foo:
- hello: 1
  world:
    users:
    - user1
    - user2
    - user3
- stack: overflow
bar:
  users:
  - user_a
  - user_b
  - user_c
'''

print(list(find_named_list(yaml.safe_load(data), 'users')))

outputs:

[['user1', 'user2', 'user3'], ['user_a', 'user_b', 'user_c']]

Demo: https://ideone.com/eJvw4M