python3 import用法_python3 from import?

当我在flask的create_app方法里import同一个模块的两个文件时出现不同的情况。

def create_app(config_name=None):
    ...

    with app.app_context():
        if is_debug_mode() and not is_werkzeug_reloader_process():
            pass
        else:
            from app.sched import etl # 或tasks
            scheduler.start()

        from app.sched import scheduler_config

    init_jobs()

    return app

etl和tasks里面都有：

from app import scheduler

但是，如果是import tasks不会报错，import etl会报如下错误：

importError: cannot import name 'scheduler' from 'app'

那么，etl和tasks到底有啥区别：

task.py模块

from flask import current_app

from app import scheduler
from app.sched.etl import etl_resource_data
from app.sched.scheduler_config import ETL_JOB_PARAMS


@scheduler.task(id="etl_daily_tyedu_st_resource_data", **ETL_JOB_PARAMS,
                args=[{"src": current_app.config.get("SQLALCHEMY_DATAbase_URI"),
                       "dest": current_app.config.get("CH_DATAbase_URI")}])
def etl_resource_data_daily(args):
    etl_resource_data(args['src'], args['dest'])

tasks直接是一个装饰器的task

etl.py模块：代码有点多，省略一下：

import petl as etl

from flask import current_app

from app import scheduler
from app.sched.scheduler_config import ETL_JOB_PARAMS

from sqlalchemy import create_engine, desc
from clickhouse_driver import connect
from sqlalchemy.orm import sessionmaker

from config import EngineConfig
from agents.models import ResourceData


chdb_http = sessionmaker(bind=create_engine(EngineConfig.CH_HTTP_URI))()


def fill_resdata_from_recalldata(datas):
   ...


@scheduler.task(id="etl_daily_tyedu_st_resource_data", **ETL_JOB_PARAMS,
                args=[{"src": current_app.config.get("SQLALCHEMY_DATAbase_URI"),
                       "dest": current_app.config.get("CH_DATAbase_URI")}])
def etl_resource_data_daily(args):
    etl_resource_data(args['src'], args['dest'])

我判断

import tasks的时候，是把@scheduler.task这段代码拿过去了

import etl的时候，

  File "C:Users15871Projectspythonrmsapp__init__.py", line 18, in 
    from app.blueprints.jobs import jobs_bp
  File "C:Users15871Projectspythonrmsappblueprintsjobs.py", line 11, in 
    from app.sched.job_controller import start_job
  File "C:Users15871Projectspythonrmsappschedjob_controller.py", line 11, in 
    from app.sched.etl import etl_recall_data
  File "C:Users15871Projectspythonrmsappschedetl.py", line 13, in 
    from app import scheduler
importError: cannot import name 'scheduler' from 'app'

实际上是在前面，注册blueprint时候就报错。

局部import为什么不报错？因为局部import的是其父级模块（import它的模块）里面import了scheduler对象。

说明局部import代码可以利用父级模块的import对象，因为局部import是父级模块完成了它的import才发生。而全局import是不能这样的，全局import整个import是一块，没有先后。

这是这个问题的原因，理解了吗？

所以，复杂项目的import很复杂，一定要小心。