Recursive function to read yaml files looking for a key which includes other yaml files

I am attempting to write a function that reads a yaml file looking for a certain key. if the key does not exist, the function should check for an '_include' key and then check that yaml file for the same key until all options are exhausted or it finds the key.

import yaml
import os.path

def load_yaml_data(yaml_path):
    """Read a YAML file and return the data"""
    try:
        with open(yaml_path, 'r', encoding="utf-8") as yaml_file:
            yaml_data = yaml.safe_load(yaml_file)
            return yaml_data
    except FileNotFoundError:
        return None
    except yaml.YAMLError as exc:
        return None

def find_key_in_yaml_recursive(yaml_path, key):
    print(f"opening {yaml_path}")
    yaml_data = load_yaml_data(yaml_path)
    # print(f"Yaml data : {yaml_data}")
    if yaml_data:
        if key in yaml_data:
            print(f"{key} found in {yaml_path}")
            return yaml_path
        elif '_include' in yaml_data:
            for include in yaml_data['_include']:
                include_path = os.path.expandvars(include)
                if not os.path.isabs(include_path):
                    include_path = os.path.abspath('/'.join([os.path.dirname(yaml_path), include_path]))
                return find_key_in_yaml_recursive(include_path, key)

if __name__ == "__main__":
    print(find_key_in_yaml_recursive("./test.yaml", "foo"))

Dir structure:

.
|    test.yaml
└─── test
│   │    test.yaml
│   └─── deeper
│       │   test.yaml
│       │   notexist.yaml

./test.yaml

---
_include:
  - "test/test.yaml"
not_foo:
  - "some data"

./test/test.yaml

---
_include:
  - ./deeper/notexist.yaml
  - ./deeper/test.yaml
foop:
  - "somedata"

./test/deeper/notexist.yaml

---
nothinguseful:
  - data

./test/deeper/test.yaml

---
foo:
 - some data
somethingelse:
  - data

'foo' exists in ./test/deeper/test.yaml. In most circumstances, the function will find it and return the full ./test/deeper/test.yaml path. However, when ./test/test.yaml has an '_include' array that lists deeper/notexist.yaml before deeper/test.yaml the function returns None.
I have not done much recursive programming but i know you need a base case but i am unsure how to implement a base case in this scenario.

I think this should work:

import yaml
import os.path

def load_yaml_data(yaml_path):
    """Read a YAML file and return the data"""
    try:
        with open(yaml_path, 'r', encoding="utf-8") as yaml_file:
            yaml_data = yaml.safe_load(yaml_file)
            return yaml_data
    except FileNotFoundError:
        return None
    except yaml.YAMLError as exc:
        return None

def find_key_in_yaml_recursive(yaml_path, key):
    print(f"opening {yaml_path}")
    yaml_data = load_yaml_data(yaml_path)
    #print(f"Yaml data : {yaml_data}")
    if yaml_data:
        if key in yaml_data:
            print(f"{key} found in {yaml_path}")
            return yaml_path
        elif '_include' in yaml_data:
            for include in yaml_data['_include']:
                include_path = os.path.expandvars(include)
                if not os.path.isabs(include_path):
                    include_path = os.path.abspath('/'.join([os.path.dirname(yaml_path), include_path]))
                var = find_key_in_yaml_recursive(include_path, key)
                if var:
                    return var  
    return None

if __name__ == "__main__":
    print(find_key_in_yaml_recursive("./test.yaml", "foo"))