45.8. Explicit Subtransactions
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## 45.8. Explicit Subtransactions

Recovering from errors caused by database access as described in Section 45.7.2 can lead to an undesirable situation where some operations succeed before one of them fails, and after recovering from that error the data is left in an inconsistent state. PL/Python offers a solution to this problem in the form of explicit subtransactions.

### 45.8.1. Subtransaction Context Managers

Consider a function that implements a transfer between two accounts:

    CREATE FUNCTION transfer_funds() RETURNS void AS $$
    try:
        plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
        plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
    except plpy.SPIError, e:
        result = "error transferring funds: %s" % e.args
    else:
        result = "funds transferred correctly"
    plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
    plpy.execute(plan, [result])
    $$ LANGUAGE plpythonu;

If the second `UPDATE` statement results in an exception being raised, this function will report the error, but the result of the first `UPDATE` will nevertheless be committed. In other words, the funds will be withdrawn from Joe's account, but will not be transferred to Mary's account.

To avoid such issues, you can wrap your `plpy.execute` calls in an explicit subtransaction. The `plpy` module provides a helper object to manage explicit subtransactions that gets created with the `plpy.subtransaction()` function. Objects created by this function implement the context manager interface. Using explicit subtransactions we can rewrite our function as:

    CREATE FUNCTION transfer_funds2() RETURNS void AS $$
    try:
        with plpy.subtransaction():
            plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
            plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
    except plpy.SPIError, e:
        result = "error transferring funds: %s" % e.args
    else:
        result = "funds transferred correctly"
    plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
    plpy.execute(plan, [result])
    $$ LANGUAGE plpythonu;

Note that the use of `try`/`except` is still required. Otherwise the exception would propagate to the top of the Python stack and would cause the whole function to abort with a PostgreSQL error, so that the `operations` table would not have any row inserted into it. The subtransaction context manager does not trap errors, it only assures that all database operations executed inside its scope will be atomically committed or rolled back. A rollback of the subtransaction block occurs on any kind of exception exit, not only ones caused by errors originating from database access. A regular Python exception raised inside an explicit subtransaction block would also cause the subtransaction to be rolled back.

### 45.8.2. Older Python Versions

Context managers syntax using the `with` keyword is available by default in Python 2.6. If using PL/Python with an older Python version, it is still possible to use explicit subtransactions, although not as transparently. You can call the subtransaction manager's `__enter__` and `__exit__` functions using the `enter` and `exit` convenience aliases. The example function that transfers funds could be written as:

    CREATE FUNCTION transfer_funds_old() RETURNS void AS $$
    try:
        subxact = plpy.subtransaction()
        subxact.enter()
        try:
            plpy.execute("UPDATE accounts SET balance = balance - 100 WHERE account_name = 'joe'")
            plpy.execute("UPDATE accounts SET balance = balance + 100 WHERE account_name = 'mary'")
        except:
            import sys
            subxact.exit(*sys.exc_info())
            raise
        else:
            subxact.exit(None, None, None)
    except plpy.SPIError, e:
        result = "error transferring funds: %s" % e.args
    else:
        result = "funds transferred correctly"

    plan = plpy.prepare("INSERT INTO operations (result) VALUES ($1)", ["text"])
    plpy.execute(plan, [result])
    $$ LANGUAGE plpythonu;

### Note

Although context managers were implemented in Python 2.5, to use the `with` syntax in that version you need to use a future statement. Because of implementation details, however, you cannot use future statements in PL/Python functions.

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