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From: Karsten Hilbert <Karsten.Hilbert@gmx.net>
Newsgroups: comp.lang.python
Subject: psycopg2: proper positioning of .commit() within try: except: blocks
Date: Sat, 7 Sep 2024 17:48:01 +0200
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Dear all,
unto now I had been thinking this is a wise idiom (in code
that needs not care whether it fails to do what it tries to
do^1):
conn =3D psycopg2.connection(...)
curs =3D conn.cursor()
try:
curs.execute(SOME_SQL)
except PSYCOPG2-Exception:
some logging being done, and, yes, I
can safely inhibit propagation^1
finally:
conn.commit() # will rollback, if SOME_SQL failed
conn.close()
So today I head to learn that conn.commit() may very well
raise a DB related exception, too:
psycopg2.errors.SerializationFailure: could not serialize access due to r=
ead/write dependencies among transactions
DETAIL: Reason code: Canceled on identification as a pivot, during commi=
t attempt.
TIP: The transaction might succeed if retried.
Now, what is the proper placement of the .commit() ?
(doing "with ... as conn:" does not free me of committing appropriately)
Should I
try:
curs.execute(SOME_SQL)
conn.commit()
except PSYCOPG2-Exception:
some logging being done, and, yes, I
can safely inhibit propagation^1
finally:
conn.close() # which should .rollback() automagically in case we had n=
ot reached to .commit()
?
Thanks for insights,
Karsten
#-------------------------------
^1:
This particular code is writing configuration defaults
supplied in-code when no value is yet to be found in the
database. If it fails, no worries, the supplied default
is used by follow-on code and storing it is re-tried next
time around.
#-------------------------------
Exception details:
Traceback (most recent call last):
File "/usr/share/gnumed/Gnumed/wxpython/gmGuiMain.py", line 3472, in On=
Init
frame =3D gmTopLevelFrame(None, id =3D -1, title =3D _('GNUmed client=
'), size =3D (640, 440))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^=
^^^^^^^^^^^^^^^^
File "/usr/share/gnumed/Gnumed/wxpython/gmGuiMain.py", line 191, in __i=
nit__
self.LayoutMgr =3D gmHorstSpace.cHorstSpaceLayoutMgr(self, -1)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/share/gnumed/Gnumed/wxpython/gmHorstSpace.py", line 215, in =
__init__
self.top_panel =3D gmTopPanel.cTopPnl(self, -1)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/share/gnumed/Gnumed/wxpython/gmTopPanel.py", line 52, in __i=
nit__
wxgTopPnl.wxgTopPnl.__init__(self, *args, **kwargs)
File "/usr/share/gnumed/Gnumed/wxGladeWidgets/wxgTopPnl.py", line 33, i=
n __init__
self._TCTRL_patient_selector =3D cActivePatientSelector(self, wx.ID_A=
NY, "")
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^=
^^^^^
File "/usr/share/gnumed/Gnumed/wxpython/gmPatSearchWidgets.py", line 12=
95, in __init__
cfg.get2 (
File "/usr/share/gnumed/Gnumed/pycommon/gmCfg.py", line 248, in get2
self.set (
File "/usr/share/gnumed/Gnumed/pycommon/gmCfg.py", line 367, in set
rw_conn.commit() # will rollback if transaction failed
^^^^^^^^^^^^^^^^
psycopg2.errors.SerializationFailure: could not serialize access due to r=
ead/write dependencies among transactions
DETAIL: Reason code: Canceled on identification as a pivot, during commi=
t attempt.
TIP: The transaction might succeed if retried.
2024-08-20 22:17:04 INFO gm.cfg [140274204403392 UpdChkThrea=
d-148728] (/usr/share/gnumed/Gnumed/pycommon/gmCfg.py::get2() #148): crea=
ting option [horstspace.update.consider_latest_branch] with default [True]
2024-08-20 22:17:04 DEBUG gm.db_pool [140274459512896 MainThread]=
========== REMAINDER OF ARTICLE TRUNCATED ==========