Suggestion: Creation of a dummy placeholder so that mypy doesn't return a "already defined" error when the re-definition is required.

[jcrmatos]

Hello,

I would like to suggest the creation of a dummy placeholder (eg. AlreadyDefined or some symbol like ! or ...) so that mypy doesn't return a "already defined" error when the re-definition is required.

Here are 2 examples:

row = 0  # type: int
# do some processing with the row var
for row, event in enumerate(events_lst):  # type: int, List[str]
    # do some processing with row and event vars

row = 0  # type: int
# do some processing with the row var
row, event = some_func()  # type: int, List[str]

With the examples code above, mypy tells me that
data_processing.py:144: error: Name 'row' already defined
which is correct, but annoying, because to identify the type of event, I must re-identify row.

I know that I could disable the "already defined" errors with a command line flag, but I would like to receive them except when I have to re-identify some vars due to others, like in the examples above.

Thanks,

JM

[ilevkivskyi]

Thanks for reporting!

For your case you can write

row = 0  # type: int
# do some processing with the row var
event = None  # type: List[str]
for row, event in enumerate(events_lst):
    # do some processing with row and event vars

For "throw away variables" there are already issues #465 and #1649

[jcrmatos]

Hello,

Thanks for the solution.

I'll keep the suggestion open.

Best regards,

JM

[jcrmatos]

Hello,

Tried your solution, but mypy returns this error

data_processing.py:154: error: Incompatible types in assignment (expression has
type None, variable has type List[str])

had to change it to
event = [] # type: List[str]

Thanks,

JM

[gvanrossum]

@ilevkivskyi @ddfisher The failure from the last comment (presumably with strict optional) seems to be in violation of what PEP 484 says about this...

[ddfisher]

Yeah, we decided to punt on the implementation of that until mypy had undefined checking. (We discussed this at the time, but that was a while ago now.)

[jcrmatos]

Hello,

Maybe _ could be the placeholder?

row: int = 0
# do some processing with the row var
row, event = some_func()  # type: int, List[str]

would become this

row: int = 0
# do some processing with the row var
row, event = some_func()  # type: _, List[str]

Thanks,

JM

[thatrandomfrenchdude]

Has there been any update on this issue to provide a placeholder value?

Im getting that same error ('"None" has no attribute cursor') when using a placeholder of None in my init() function that gets redefined in the the connect() function.

Not sure how I can resolve this. Thanks in advance for the help!

import sqlite3

class DB:
    def __init__(self, db_path: str) -> None:
        self.db_path = db_path
        self.conn = None
        self.cursor = None

    def connect(self) -> bool:
        try:
            self.conn = sqlite3.connect(self.db_path) # warning here
            self.conn.row_factory = sqlite3.Row # warning here
            self.cursor = self.conn.cursor() # warning here
            print(f"Connected to database at {self.db_path}")
            return True
        except sqlite3.Error as e:
            print(f"Error connecting to database: {e}")
            return False

[hauntsaninja]

Add a type hint in __init__, e.g. self.conn = Optional[<connection type>] = None

Closing the original issue since type comments are discouraged and there's no analogue for the above use case in PEP 526

[thatrandomfrenchdude]

Thanks @hauntsaninja !

[thatrandomfrenchdude]

For anyone coming across this later, this is what I did:

import sqlite3
from typing import Optional


class DB:
    def __init__(self, db_path: str) -> None:
        self.db_path = db_path
        self.conn: Optional[sqlite3.Connection] = None
        self.cursor: Optional[sqlite3.Cursor] = None

    def connect(self) -> bool:
        try:
            self.conn = sqlite3.connect(self.db_path)
            self.conn.row_factory = sqlite3.Row
            self.cursor = self.conn.cursor()
            print(f"Connected to database at {self.db_path}")
            return True
        except sqlite3.Error as e:
            print(f"Error connecting to database: {e}")
            return False