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I'd like to access the SQLite trace callback from Python to actually see the same queries that SQLite actually gets to see.
The C API of SQLite supports this via the sqlite3_trace function. I added support to this to the sqlite3 module plus unit tests testing that it is called, can be removed again and that unicode arguments arrive correctly at the callback.
Please consider applying the patch. I am not sure if this should go to the pysqlite project on Google Code directly, I am just submitting it here because I worked in the python source tree as I am more used to its build system. I am sorry, Gerhard, if this is the wrong way.

Here is the same change for Python 2.7. I don't expect this to get merged on the Python 2 branch, put perhaps it is useful to somebody.

Thanks for the patch.
A couple of comments:
- this is a new feature, so can only go in in 3.x: no need to post a 2.7 patch (unless this helps Gerhard for his standalone project)
- you need to document the new API in Doc/library/sqlite3.rst
About the patch: looks mostly good!
+ self.assertTrue([x for x in traced_statements if x.find("create table foo") != -1])
This looks a bit complicated, why not something like
`any("create table foo" in x for x in traced_statements)`?
(`y in x` is simper and more readable than `x.find(y) != -1`)
+ sqlite3_trace(self->db, _trace_callback, trace_callback);
+ if (PyDict_SetItem(self->function_pinboard, trace_callback, Py_None) == -1)
+ return NULL;
Shouldn't sqlite3_trace() be called only after PyDict_SetItem() succeeds?

> A couple of comments:
> - this is a new feature, so can only go in in 3.x: no need to post a 2.7 patch (unless this helps Gerhard for his standalone project)
The motivation for the 2.7er patch is mostly that we are still using Python 2.x at work and I made this patch for 2.7 first. Actually I will need to backport it to 2.6 but I guess there are no differences. And maybe I should have submitted this against pysqlite directly which is (AFAICT) also still targetting 2.x.
> - you need to document the new API in Doc/library/sqlite3.rst
Sure. I was just filing the report to have the code on file and this was only fallout from another patch. I will create an updated patch including documentation about the feature.
> About the patch: looks mostly good!
Thanks.
> + self.assertTrue([x for x in traced_statements if x.find("create table foo") != -1])
>
> This looks a bit complicated, why not something like
> `any("create table foo" in x for x in traced_statements)`?
Fine with me. I did not know that "bar" in "foobarbaz" works (I was expecting it to act as if the right hand string is a list of characters). Obviously I was wrong. Further I thought "any" was new in 2.6 and therefore not suitable for use in pysqlite which was also wrong.
> (`y in x` is simper and more readable than `x.find(y) != -1`)
I agree, I just did not know it works for substrings.
> + sqlite3_trace(self->db, _trace_callback, trace_callback);
> + if (PyDict_SetItem(self->function_pinboard, trace_callback, Py_None) == -1)
> + return NULL;
>
> Shouldn't sqlite3_trace() be called only after PyDict_SetItem() succeeds?
Good catch. If SetItem fails, trace_callback may become invalid and the _trace_callback will crash. I did not think about that as I only derived that function from similar code in the module. I will have to fix this as well.

> - you need to document the new API in Doc/library/sqlite3.rst
Included in the updated patch.
> + self.assertTrue([x for x in traced_statements if x.find("create table foo") != -1])
>
> This looks a bit complicated, why not something like
> `any("create table foo" in x for x in traced_statements)`?
Fixed.
> + sqlite3_trace(self->db, _trace_callback, trace_callback);
> + if (PyDict_SetItem(self->function_pinboard, trace_callback, Py_None) == -1)
> + return NULL;
>
> Shouldn't sqlite3_trace() be called only after PyDict_SetItem() succeeds?
Fixed as well. I just reversed the calls. What I dislike about this function pinboard approach is that every function registered as a callback stays pinned to the SQLite connection for the lifetime of the latter. But that belongs into another patch, I guess.

The patch looks good to me, thank you!
Gerhard, would you like to tackle this? Otherwise I'll commit in a couple of days.

> The patch looks good to me, thank you!
> Gerhard, would you like to tackle this? Otherwise I'll commit in a couple of days.
What I am still wondering about is if it would make sense to use the text factory here. It might make sense to pass through the bytes directly without translation into unicode (especially for big amounts).
OTOH, this is mostly a debugging aid and nothing that would be enabled in production. And premature optimization is the root of all evil...

> OTOH, this is mostly a debugging aid and nothing that would be enabled
> in production. And premature optimization is the root of all evil...
Agreed.

New changeset 575ee55081dc by Antoine Pitrou in branch 'default':
Issue #11688: Add sqlite3.Connection.set_trace_callback(). Patch by Torsten Landschoff.
http://hg.python.org/cpython/rev/575ee55081dc 

Patch committed, thank you!

Reopening, a failure appeared on some of the buildbots. I've made the failure message a bit more explicit:
test_sqlite: testing with version '2.6.0', sqlite_version '3.6.12'
[...]
======================================================================
FAIL: CheckUnicodeContent (sqlite3.test.hooks.TraceCallbackTests)
----------------------------------------------------------------------
Traceback (most recent call last):
 File "/Users/buildbot/buildarea/3.x.parc-snowleopard-1/build/Lib/sqlite3/test/hooks.py", line 220, in CheckUnicodeContent
 % (ascii(unicode_value), ', '.join(map(ascii, traced_statements))))
AssertionError: False is not true : Unicode data '\xf6\xe4\xfc\xd6\xc4\xdc\xdf\u20ac' garbled in trace callback: 'create table foo(x)', 'BEGIN ', 'insert into foo(x) values (?)', 'COMMIT'
See e.g. http://www.python.org/dev/buildbot/all/builders/AMD64%20Snow%20Leopard%202%203.x/builds/168/steps/test/logs/stdio 

Ok, it seems that expanding the value of bound parameters in the statement passed to the trace callback is a recent SQLite feature:
http://www.sqlite.org/draft/releaselog/3_6_21.html
 "The SQL output resulting from sqlite3_trace() is now modified to include the values of bound parameters."

New changeset ce37570768f5 by Antoine Pitrou in branch 'default':
Fix TraceCallbackTests to not use bound parameters (followup to issue #11688)
http://hg.python.org/cpython/rev/ce37570768f5 

Should be fixed now.