def get_field(key_value_pair_list, key):
    """
    Given a list of key-value pairs (dicts with keys 'key' and 'value'), 
    find the entry that has the provided `key` and return its value.
    
    If no `key` is found, return None.
    
    It assumes that each `key` only appears one in `key_value_pair_list`,
    so the first appearance is returned.
    """
    entry = list(filter(lambda d: d['key'] == key, key_value_pair_list))
    
    if len(entry) == 0:
        return None
    
    return entry[0]['value']