你可以使用Bitbucket的API来根据更改的代码有条件地添加默认审阅者。以下是一个示例解决方案：

import requests

def add_default_reviewer(pr_id, default_reviewer, condition):
    # 获取Pull Request的更改文件列表
    url = f"https://api.bitbucket.org/2.0/repositories//pullrequests/{pr_id}/diffstat"
    response = requests.get(url)
    diffstat = response.json()

    # 根据条件判断是否添加默认审阅者
    if condition:
        # 获取Pull Request的审阅者列表
        url = f"https://api.bitbucket.org/2.0/repositories//pullrequests/{pr_id}/participants"
        response = requests.get(url)
        participants = response.json()

        # 检查默认审阅者是否已经存在于审阅者列表中
        if default_reviewer not in [participant['user']['username'] for participant in participants]:
            # 添加默认审阅者
            url = f"https://api.bitbucket.org/2.0/repositories//pullrequests/{pr_id}/participants"
            data = {
                "user": {
                    "username": default_reviewer
                },
                "role": "REVIEWER"
            }
            response = requests.post(url, json=data)
            if response.status_code == 201:
                print(f"Default reviewer {default_reviewer} added successfully.")
            else:
                print("Failed to add default reviewer.")
        else:
            print(f"Default reviewer {default_reviewer} already exists.")
    else:
        print("Condition not met. Default reviewer not added.")

# 使用示例
pr_id = 12345
default_reviewer = "john.doe"
condition = True

add_default_reviewer(pr_id, default_reviewer, condition)

请确保将替换为你的实际仓库名称。此示例中的add_default_reviewer函数接受三个参数：pr_id为Pull Request的ID，default_reviewer为默认审阅者的用户名，condition为根据更改代码是否满足条件来决定是否添加默认审阅者。你可以根据自己的需求来修改条件判断的逻辑。