Class variable is NoneType even though attribute was defined

Question

I have a class that looks like this:

class DatabaseTest:

    def __init__(self):

        self.headers = {
            "host": os.environ["HOST"],
            "username": os.environ["USERNAME"],
            "password": os.environ["PASSWORD"]
        }

        self.db = None
        self.cursor = None
        asyncio.create_task(self._connect())

    
    async def _connect(self):
        
        self.db = await aiomysql.connect(self.headers['host'], self.headers['username'], self.headers['password'])
        self.cursor = await self.db.cursor()
        await self.cursor.execute('use bot')


    
    async def _insertData(self, guildid):

        sql = f'insert into data(guild) values ({guildid})'

        await self.cursor.execute(sql)

Whenever I do await DatabaseTest()._insertData(123456789) it gives an error saying that self.cursor is NoneType when I clearly redefined what self.cursor is in _connect. I know that _connect was called because I put a print statement right at the end and it printed. How would I fix this?

The traceback is this (im using it for a discord bot):

Traceback (most recent call last):
  File "C:\Users\Name\PycharmProjects\compscibot\venv\lib\site-packages\discord\ext\commands\core.py", line 85, in wrapped
    ret = await coro(*args, **kwargs)
  File "C:\Users\Name\PycharmProjects\compscibot\cogs\startup\Help.py", line 57, in cmdchannel
    channelid = await DatabaseTest().getData(ctx.guild.id,'cmdchannel')
  File "C:\Users\Name\PycharmProjects\compscibot\cogs\dbtest.py", line 75, in getData
    await self.cursor.execute(sql)
AttributeError: 'NoneType' object has no attribute 'execute'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "C:\Users\Name\PycharmProjects\compscibot\venv\lib\site-packages\discord\ext\commands\bot.py", line 903, in invoke
    await ctx.command.invoke(ctx)
  File "C:\Users\Name\PycharmProjects\compscibot\venv\lib\site-packages\discord\ext\commands\core.py", line 859, in invoke
    await injected(*ctx.args, **ctx.kwargs)
  File "C:\Users\Name\PycharmProjects\compscibot\venv\lib\site-packages\discord\ext\commands\core.py", line 94, in wrapped
    raise CommandInvokeError(exc) from exc
discord.ext.commands.errors.CommandInvokeError: Command raised an exception: AttributeError: 'NoneType' object has no attribute 'execute'
None

Are you calling `_connect()` first and made sure to await it before `_insertData`? Have you checked your `_connect` and verified `db.cursor()` *does* return a cursor? — Gino Mempin, Jan 14 '21 at 00:57
@GinoMempin shouldn't the __init__ call the _connect() method? — Atomization, Jan 14 '21 at 01:17
@zvone The connect method was executed. I put a print statement at the end and it printed — Atomization, Jan 14 '21 at 01:17
I think it's a timing issue? I think you need to wait for the `_connect` task in `__init__` to actually finish before calling other cursor-dependent methods. Maybe cursor is *still* None by the time you called `insertData`. — Gino Mempin, Jan 14 '21 at 10:11
Add a debug log at the start of _insertData. Is it called *after* that debug log you got for _connect? — Gino Mempin, Jan 14 '21 at 10:12

score 2 · Accepted Answer · answered Jan 14 '21 at 17:02

The context is not switched until await.

asyncio.create_task() just schedules an execution.

Using asyncio.sleep():

db = DatabaseTest()
await asyncio.sleep(0)
db._insertData(123456789)

or refactor:

class DatabaseTest:
   def __init__(self):
        self.headers = {
            "host": os.environ["HOST"],
            "username": os.environ["USERNAME"],
            "password": os.environ["PASSWORD"]
        }

    async def __aenter__(self):
        await self._connect()
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        # cleanup
        await self.cursor.close()
        self.db.close()

    ...

async with DatabaseTest() as db:
    await db._insertData(123456789)

The refactoring method works wonders. Thanks a lot kyungmin! — Atomization, Jan 14 '21 at 18:48

Class variable is NoneType even though attribute was defined

1 Answers1