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Showing content from https://cloud.google.com/python/docs/reference/datastore/1.12.0/transactions below:

Python client library | Google Cloud

Create / interact with Google Cloud Datastore transactions.

Bases: google.cloud.datastore.batch.Batch

An abstraction representing datastore Transactions.

Transactions can be used to build up a bulk mutation and ensure all or none succeed (transactionally).

For example, the following snippet of code will put the two save operations (either insert or upsert) into the same mutation, and execute those within a transaction:

>>> with client.transaction():
...     client.put_multi([entity1, entity2])

Because it derives from Batch, Transaction also provides put() and delete() methods:

>>> with client.transaction() as xact:
...     xact.put(entity1)
...     xact.delete(entity2.key)

By default, the transaction is rolled back if the transaction block exits with an error:

>>> with client.transaction():
...     do_some_work()
...     raise SomeException  # rolls back
Traceback (most recent call last):
  ...
SomeException

If the transaction block exits without an exception, it will commit by default.

WARNING: > Inside a transaction, automatically assigned IDs for

entities will not be available at save time! That means, if you try:

>>> with client.transaction():
...     entity = Entity(key=client.key('Thing'))
...     client.put(entity)

entity won’t have a complete key until the transaction is committed.

Once you exit the transaction (or call commit()), the automatically generated ID will be assigned to the entity:

>>> with client.transaction():
...     entity = Entity(key=client.key('Thing'))
...     client.put(entity)
...     print(entity.key.is_partial)  # There is no ID on this key.
...
True
>>> print(entity.key.is_partial)  # There *is* an ID.
False

If you don’t want to use the context manager you can initialize a transaction manually:

>>> transaction = client.transaction()
>>> transaction.begin()
>>>
>>> entity = Entity(key=client.key('Thing'))
>>> transaction.put(entity)
>>>
>>> transaction.commit()

• Parameters

  • client (google.cloud.datastore.client.Client) – the client used to connect to datastore.

  • read_only (bool) – indicates the transaction is read only.

Begins a transaction.

This method is called automatically when entering a with statement, however it can be called explicitly if you don’t want to use a context manager.

• Raises

  ValueError if the transaction has already begun.

Commits the transaction.

This is called automatically upon exiting a with statement, however it can be called explicitly if you don’t want to use a context manager.

This method has necessary side-effects:

• Sets the current transaction’s ID to None.

Return the topmost transaction.

NOTE: If the topmost element on the stack is not a transaction, returns None.

• Return type

  google.cloud.datastore.transaction.Transaction or None

• Returns

  The current transaction (if any are active).

Remember a key to be deleted during commit().

• Parameters

  key (google.cloud.datastore.key.Key) – the key to be deleted.

• Raises

  ValueError if the batch is not in progress, if key is not complete, or if the key’s project does not match ours.

Getter for the transaction ID.

• Return type

  str

• Returns

  The ID of the current transaction.

Getter for the changes accumulated by this batch.

Every batch is committed with a single commit request containing all the work to be done as mutations. Inside a batch, calling put() with an entity, or delete() with a key, builds up the request by adding a new mutation. This getter returns the protobuf that has been built-up so far.

• Return type

  iterable

• Returns

  The list of datastore_pb2.Mutation protobufs to be sent in the commit request.

Getter for namespace in which the batch will run.

• Return type

  str

• Returns

  The namespace in which the batch will run.

Getter for project in which the batch will run.

• Return type

  str

• Returns

  The project in which the batch will run.

Adds an entity to be committed.

Ensures the transaction is not marked readonly. Please see documentation at put()

• Parameters

  entity (Entity) – the entity to be saved.

• Raises

  RuntimeError if the transaction is marked ReadOnly

Rolls back the current transaction.

This method has necessary side-effects:

• Sets the current transaction’s ID to None.

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