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66 lines
2.5 KiB
Plaintext
66 lines
2.5 KiB
Plaintext
psycopg asynchronous API
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************************
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Program code can initiate an asynchronous query by passing an 'async=1' flag
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to the .execute() method. A very simple example, from the connection to the
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query:
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conn = psycopg.connect(database='test')
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curs = conn.cursor()
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curs.execute("SEECT * from test WHERE fielda > %s", (1971,), async=1)
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From then on any query on other cursors derived from the same connection is
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doomed to fail (and raise an exception) until the original cursor (the one
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executing the query) complete the asynchronous operation. This can happen in
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a number of different ways:
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1) one of the .fetchXXX() methods is called, effectively blocking untill
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data has been sent from the backend to the client, terminating the
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query.
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2) .cancel() is called. This method tries to abort the current query and
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will block until the query is aborted or fully executed. The return
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value is True if the query was successfully aborted or False if it
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was executed. Query result are discarded in both cases.
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3) .execute() is called again on the same cursor (.execute() on a
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different cursor will simply raise an exception.) This waits for the
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complete execution of the current query, discard any data and execute
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the new one.
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Note that calling .execute() two times in a row will not abort the former
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query and will temporarily go to synchronous mode until the first of the two
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queries is executed.
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Cursors now have some extra methods that make them usefull during
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asynchronous queries:
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.fileno()
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Returns the file descriptor associated with the current connection and
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make possible to use a cursor in a context where a file object would be
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expected (like in a select() call.)
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.isbusy()
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Returns True if the backend is still processing the query or false if
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data is ready to be fetched (by one of the .fetchXXX() methods.)
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A code snippet that shows how to use the cursor object in a select() call:
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import psycopg
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import select
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conn = psycopg.connect(database='test')
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curs = conn.cursor()
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curs.execute("SEECT * from test WHERE fielda > %s", (1971,), async=1)
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# wait for input with a maximum timeout of 5 seconds
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query_ended = False
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while not query_ended:
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rread, rwrite, rspec = select([cursor, another_file], [], [], 5)
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if not cursor.isbusy():
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query_ended = True
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# manage input from other sources like other_file, etc.
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print "Query Results:"
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for row in cursor:
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print row
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