dynamically pass the columns to select() in python postgresql

Question

Below code works fine

table = Table('user_data',metadata,autoload=True,autoload_with=conn)
stmt = select([table.columns.user_id,table.columns.username])
results = conn.execute(stmt).fetchall()
print(results)

But when i try to pass the columns as list (from postresql db) it fails

cols = get_table_cols() -- it returns a list -> [table.columns.user_id,table.columns.username]
stmt = select(cols )
results = conn.execute(stmt).fetchall()
print(results)

Error:

 sqlalchemy.exc.ArgumentError: Textual column expression 'table.c.username' should be explicitly declared with text('table.c.username'), or use column('table.c.username') for more specificity

Kindly provide inputs on the same

added error details. tried with column () and text() but no luck — Sai
– Sai, Commented Nov 17, 2021 at 13:35

golden_curry2020 · Accepted Answer · 2021-11-17 15:20:51Z

1

I've just recently experienced something similar to this. Keep in mind my engine had the 'Future' attribute set to True (in case this doesn't work for you)

I can't remember why simply unpacking the list like this didn't work (which it might in your case)

stmt = select(*cols)

But this ended up being my solution

cols = get_table_cols()
#assuming cols does return a list
stmt = select(*[c for c in cols])

#alternatively you can get the columns directly from the table object
#stmt = select(*[c for c in table.c])
results = conn.execute(stmt).fetchall()

#session.execute(stmt).all() -> what I did for ORM

print(results)

If you are using declarative classes for ORM, the table object can instead be accessed with

ClassName.__table__

answered Nov 17, 2021 at 15:20

golden_curry2020

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