REGR: ImportError on other dbapi2 connections when sqlalchemy is not installed

[vincent3717]

Pandas version checks

• I have checked that this issue has not already been reported.

• I have confirmed this bug exists on the latest version of pandas.

• I have confirmed this bug exists on the main branch of pandas.

Reproducible Example

import psycopg2 #version 2.9.3
import pandas as pd #version 1.4.0
conn = psycopg2.connect(
        host=XXX,
        dbname=XXX,
        user=XXX,
        password=XXX,
    )
df = pd.read_sql("select 1", conn)

ImportError: Missing optional dependency 'SQLAlchemy'.  Use pip or conda to install SQLAlchemy.

Issue Description

There was no SQLAlchemy dependency in the previous version when using a psycopg2 connection object in read_sql pandas function

Even after installing SQLAlchemy, there is a unexpected warning :

UserWarning: pandas only support SQLAlchemy connectable(engine/connection) ordatabase string URI or sqlite3 DBAPI2 connectionother DBAPI2 objects are not tested, please consider using SQLAlchemy

Expected Behavior

No error neither warning using a psycopg2 connection object in read_sql pandas function

Installed Versions

INSTALLED VERSIONS

commit : bb1f651
python : 3.9.9.final.0
python-bits : 64
OS : Linux
OS-release : 5.13.0-27-generic
Version : #29~20.04.1-Ubuntu SMP Fri Jan 14 00:32:30 UTC 2022
machine : x86_64
processor : x86_64
byteorder : little
LC_ALL : None
LANG : en_US.UTF-8
LOCALE : en_US.UTF-8

pandas : 1.4.0
numpy : 1.22.1
pytz : 2021.3
dateutil : 2.8.2
pip : 20.0.2
setuptools : 44.0.0
Cython : None
pytest : None
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : None
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.9.3
jinja2 : None
IPython : 7.31.1
pandas_datareader: None
bs4 : None
bottleneck : None
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : None
numba : None
numexpr : None
odfpy : None
openpyxl : None
pandas_gbq : None
pyarrow : None
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.7.3
sqlalchemy : 1.4.31
tables : None
tabulate : None
xarray : None
xlrd : None
xlwt : None
zstandard : None

[phofl]

Can you try on master, this was (maybe) partly adressed in #45416

[abowden1989]

I don't think this is addressed by the change in 3418679. When trying to use a different connection object when sqlalchemy is not installed, the code will fail on the line

    sqlalchemy = import_optional_dependency("sqlalchemy")

The change I would propose would be to pass errors="ignore" to the import_optional_dependency call, and then only do the isinstance checks relating to sqlalchemy if the return from import_optional_dependency is not None

This wouldn't address the issue that a warning is shown (which seems reasonable to me), but would mean that sqlalchemy doesn't need to be installed to use other dbapi2 connections (some of which may be compatible with sqlite3)

[jbrockmendel]

pls give the issue an informative title

[sknutsonsf]

I am getting the same warning messages after installing SQLAlchemy

[simonjayhawkins]

Even after installing SQLAlchemy, there is a unexpected warning :

UserWarning: pandas only support SQLAlchemy connectable(engine/connection) ordatabase string URI or sqlite3 DBAPI2 connectionother DBAPI2 objects are not tested, please consider using SQLAlchemy

@fangchenli this is the intended behavior? this issue is closed off by #45679?

[fangchenli]

Even after installing SQLAlchemy, there is a unexpected warning :
UserWarning: pandas only support SQLAlchemy connectable(engine/connection) ordatabase string URI or sqlite3 DBAPI2 connectionother DBAPI2 objects are not tested, please consider using SQLAlchemy

@fangchenli this is the intended behavior? this issue is closed off by #45679?

Yes, this is the intended behavior. See #45416. We don't officially support raw dbapi2 objects other than sqlite3. But if people think we should support it, we can keep this open for discussion.

[simonjayhawkins]

But if people think we should support it, we can keep this open for discussion.

I think maybe could be a separate discussion so closing this one. The ImportError issue is fixed.