检查是否<sqlalchemy.engine.result.ResultProxy>对象为空

Question

I'm downloading some data from a SQL Server database through a library that leverages pymssql in the back-end. The result of a curson.execute("""<QUERY BODY>""") is a sqlalchemy.engine.result.ResultProxy object. How can I check if the result of the query was empty, so there are no rows?

cur = ff.sql.create_engine(server=dw.address, db=dw.BI_DW,
                               login=":".join([os.environ["SQL_USER"],
                                               os.environ["SQL_PASSWD"]]))
for n in range(100):
    result = cur.execute("""QUERY BODY;""")
    if result:
        break

Unfortunately, result will never be None even when no rows were returned by the SQL query.

What's the best way to check for that?

Answer 1

The ResultProxy object does not contain any rows yet. Therefore it has no information about the total amount of them, or even whether there are any. ResultProxy is just a "pointer" to the database. You get your rows only when you explicitly fetch them via ResultProxy . You can do that via iteration over this object, or via .first() method, or via .fetchall() method.

Bottom line : you cannot know the amount of fethced rows until you actually fetch all of them and the ResultProxy object is exhausted.

Approach #1

You can fetch all the rows at once and count them and then do whatever you need with them:

rows = result.fetchall()
if len(rows):
    # do something with rows

The downside of this method is that we load all rows into memory at once ( rows is a Python list containing all the fetched rows). This may not be desirable if the amount of fetched rows is very large and/or if you only need to iterate over the rows one-by-one independently (usually that's the case).

Approach #2

If loading all fetched rows into memory at once is not acceptable, then we can do this:

rows_amount = 0
for row in result:
    rows_amount += 1
    # do something with row
if not rows_amount:
    print('There were zero rows')
else:
    print('{} rows were fetched and processed'.format(rows_amount))

Answer 2

SQLAlchemy < 1.2: You can always turn the ResultProxy into an iterator:

res = engine.execute(...)
rp_iter = iter(res)
row_count = 0
try:
    row = next(rp_iter)
    row_count += 1
except StopIteration:
    # end of data
    if not row_count:
      # no rows returned, StopIteration was raised on first attempt

In SQLAlchemy >= 1.2, the ResultProxy implements both .next() and .__next__() , so you do not need to create the iterator:

res = engine.execute()
row_count = 0
try:
    row = next(res)
    row_count += 1
except StopIteration:
    ...