ENH: raise exception when sqlalchemy is required for database string URI

[ksindi]

• I get a cryptic error AttributeError: 'str' object has no attribute 'cursor' if sqlalchemy is not installed and I pass a database string URI to pandas.read_sql. I think the error should be made more explicit.
• Also I think there is a shadowing issue with_SQLALCHEMY_INSTALLED in _engine_builder.

/opt/conda/lib/python3.4/site-packages/pandas/io/sql.py in read_sql_query(sql, con, index_col, coerce_float, params, parse_dates, chunksize)
    427     return pandas_sql.read_query(
    428         sql, index_col=index_col, params=params, coerce_float=coerce_float,
--> 429         parse_dates=parse_dates, chunksize=chunksize)
    430 
    431 

/opt/conda/lib/python3.4/site-packages/pandas/io/sql.py in read_query(self, sql, index_col, coerce_float, params, parse_dates, chunksize)
   1569 
   1570         args = _convert_params(sql, params)
-> 1571         cursor = self.execute(*args)
   1572         columns = [col_desc[0] for col_desc in cursor.description]
   1573 

/opt/conda/lib/python3.4/site-packages/pandas/io/sql.py in execute(self, *args, **kwargs)
   1532             cur = self.con
   1533         else:
-> 1534             cur = self.con.cursor()
   1535         try:
   1536             if kwargs:

AttributeError: 'str' object has no attribute 'cursor'

[jreback]

this would need some tests

[jorisvandenbossche]

Can you also set this to True when it is imported?

[jorisvandenbossche]

Ah, no, sorry, this is good (the _is_sqlalchemy_connectable does more than just setting the global value to True, so that should still be run)

[jorisvandenbossche]

@ksindi Thanks for the catch!

For adding a test, you can put it in TestSQLiteFallbackApi I think (this class is not skipped if sqlalchemy is not installed), and then let the test itself skip if SQLALCHEMY_INSTALLED is True.
I am only not sure if there is a test build that has no sqlalchemy installed

[ksindi]

@jorisvandenbossche thanks. I'll change to ImportError and run tests.

[jreback]

@jorisvandenbossche status of this?

[ksindi]

@jreback Haven't had a chance to work on it.

[jreback]

pls rebase / update

[jreback]

can you rebase/update

[jreback]

can you rebase / update

[ksindi]

Get same error if mysql driver (e.g. mysqldb) is not installed.

[jorisvandenbossche]

@ksindi Would you have some time to update this?

[ksindi]

@jorisvandenbossche I will get this done this week. Sorry for the delay.

[codecov-io]

Current coverage is 85.29% (diff: 50.00%)

Merging #11920 into master will decrease coverage by <.01%

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[ksindi]

@jorisvandenbossche added test

[ksindi]

@jreback not sure why getting build error. Let me know if there's anything else I need to do on my end.

[jreback]

pls add a whatsnew note.

[jreback]

you have linting errors: https://travis-ci.org/pydata/pandas/jobs/148568926
https://travis-ci.org/pydata/pandas/jobs/148568933 is failing

[ksindi]

What's New

• Raise ImportError when sqlalchemy is not installed and a connection string is used

[ksindi]

@jreback thanks will fix linting error tonight

[ksindi]

@jreback just an fyi I fixed linting errorr

[jreback]

lgtm. @jorisvandenbossche

[jorisvandenbossche]

@ksindi Thanks!