See the guide: Training > Distributed execution
Structure to create or gather pieces commonly needed to train a model.
When you build a model for training you usually need ops to initialize
Saver to checkpoint them, an op to collect summaries for
the visualizer, and so on.
Various libraries built on top of the core TensorFlow library take care of
creating some or all of these pieces and storing them in well known
collections in the graph. The
Scaffold class helps pick these pieces from
the graph collections, creating and adding them to the collections if needed.
If you call the scaffold constructor without any arguments, it will pick
pieces from the collections, creating default ones if needed when
scaffold.finalize() is called. You can pass arguments to the constructor to
provide your own pieces. Pieces that you pass to the constructor are not
added to the graph collections.
The following pieces are directly accessible as attributes of the
tf.train.Saverobject taking care of saving the variables. Picked from and stored into the
SAVERScollection in the graph by default.
init_op: An op to run to initialize the variables. Picked from and stored into the
INIT_OPcollection in the graph by default.
ready_op: An op to verify that the variables are initialized. Picked from and stored into the
READY_OPcollection in the graph by default.
ready_for_local_init_op: An op to verify that global state has been initialized and it is alright to run
local_init_op. Picked from and stored into the
READY_FOR_LOCAL_INIT_OPcollection in the graph by default. This is needed when the initialization of local variables depends on the values of global variables.
local_init_op: An op to initialize the local variables. Picked from and stored into the
LOCAL_INIT_OPcollection in the graph by default.
summary_op: An op to run and merge the summaries in the graph. Picked from and stored into the
SUMMARY_OPcollection in the graph by default.
global_step: A tensor containing the global step counter. Picked from and stored into the
GLOBAL_STEPcollection in the graph by default.
You can also pass the following additional pieces to the constructor:
init_feed_dict: A session feed dictionary that should be used when running the init op.
init_fn: A callable to run after the init op to perform additional initializations. The callable will be called as
__init__( init_op=None, init_feed_dict=None, init_fn=None, ready_op=None, ready_for_local_init_op=None, local_init_op=None, summary_op=None, saver=None, copy_from_scaffold=None )
Create a scaffold.
init_op: Optional op for initializing variables.
init_feed_dict: Optional session feed dictionary to use when running the init_op.
init_fn: Optional function to use to initialize the model after running the init_op. Will be called as
ready_op: Optional op to verify that the variables are initialized. Must return an empty 1D string tensor when the variables are initialized, or a non-empty 1D string tensor listing the names of the non-initialized variables.
ready_for_local_init_op: Optional op to verify that the global variables are initialized and
local_init_opcan be run. Must return an empty 1D string tensor when the global variables are initialized, or a non-empty 1D string tensor listing the names of the non-initialized global variables.
local_init_op: Optional op to initialize local variables.
summary_op: Optional op to gather all summaries. Must return a scalar string tensor containing a serialized
tf.train.Saverobject to use to save and restore variables.
copy_from_scaffold: Optional scaffold object to copy fields from. Its fields will be overwritten by the provided fields in this function.
Returns an op that groups the default local init ops.
This op is used during session initialization when a Scaffold is
initialized without specifying the local_init_op arg. It includes
tf.tables_initializer, and also
initializes local session resources.
The default Scaffold local init op.
Creates operations if needed and finalizes the graph.
@staticmethod get_or_default( arg_name, collection_key, default_constructor )
Get from cache or create a default operation.