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https://github.com/simonw/sqlite-utils/pull/118#issuecomment-655239728 https://api.github.com/repos/simonw/sqlite-utils/issues/118 655239728 MDEyOklzc3VlQ29tbWVudDY1NTIzOTcyOA== 79913 2020-07-08T02:16:42Z 2020-07-08T02:16:42Z CONTRIBUTOR

I fixed my original oops by moving the DELETE FROM $table out of the chunking loop and repushed. I think this change can be considered in isolation from issues around transactions, which I discuss next.

I wanted to make the DELETE + INSERT happen all in the same transaction so it was robust, but that was more complicated than I expected. The transaction handling in the Database/Table classes isn't systematic, and this poses big hurdles to making Table.insert_all (or other operations) consistent and robust in the face of errors.

For example, I wanted to do this (whitespace ignored in diff, so indentation change not highlighted):

diff --git a/sqlite_utils/db.py b/sqlite_utils/db.py
index d6b9ecf..4107ceb 100644
--- a/sqlite_utils/db.py
+++ b/sqlite_utils/db.py
@@ -1028,6 +1028,11 @@ class Table(Queryable):
         batch_size = max(1, min(batch_size, SQLITE_MAX_VARS // num_columns))
         self.last_rowid = None
         self.last_pk = None
+        with self.db.conn:
+            # Explicit BEGIN is necessary because Python's sqlite3 doesn't
+            # issue implicit BEGINs for DDL, only DML.  We mix DDL and DML
+            # below and might execute DDL first, e.g. for table creation.
+            self.db.conn.execute("BEGIN")
             if truncate and self.exists():
                 self.db.conn.execute("DELETE FROM [{}];".format(self.name))
             for chunk in chunks(itertools.chain([first_record], records), batch_size):
@@ -1038,7 +1043,11 @@ class Table(Queryable):
                         # Use the first batch to derive the table names
                         column_types = suggest_column_types(chunk)
                         column_types.update(columns or {})
-                    self.create(
+                        # Not self.create() because that is wrapped in its own
+                        # transaction and Python's sqlite3 doesn't support
+                        # nested transactions.
+                        self.db.create_table(
+                            self.name,
                             column_types,
                             pk,
                             foreign_keys,
@@ -1139,7 +1148,6 @@ class Table(Queryable):
                     flat_values = list(itertools.chain(*values))
                     queries_and_params = [(sql, flat_values)]

-            with self.db.conn:
                 for query, params in queries_and_params:
                     try:
                         result = self.db.conn.execute(query, params)

but that fails in tests because other methods call insert/upsert/insert_all/upsert_all in the middle of their transactions, so the BEGIN statement throws an error (no nested transactions allowed).

Stepping back, it would be nice to make the transaction handling systematic and predictable. One way to do this is to make the sqlite_utils/db.py code generally not begin or commit any transactions, and require the caller to do that instead. This lets the caller mix and match the Python API calls into transactions as appropriate (which is impossible for the API methods themselves to fully determine). Then, make sqlite_utils/cli.py begin and commit a transaction in each @cli.command function, making each command robust and consistent in the face of errors. The big change here, and why I didn't just submit a patch, is that it dramatically changes the Python API to require callers to begin a transaction rather than just immediately calling methods.

There is also the caveat that for each transaction, an explicit BEGIN is also necessary so that DDL as well as DML (as well as SELECTs) are consistent and rolled back on error. There are several bugs.python.org discussions around this particular problem of DDL and some plans to make it better and consistent with DBAPI2, eventually. In the meantime, the sqlite-utils Database class could be a context manager which supports the incantations necessary to do proper transactions. This would still be a Python API change for callers but wouldn't expose them to the weirdness of the sqlite3's default transaction handling.

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