dali-dynamic-mode

name	dali-dynamic-mode
description	Use when writing DALI data loading or preprocessing code with `nvidia.dali.experimental.dynamic` (ndd), or when converting DALI pipeline-mode code to dynamic mode, or when the user asks about DALI dynamic mode, imperative DALI, or ndd. Use this skill any time someone mentions 'ndd', 'dynamic mode', or wants to load/augment data with DALI outside of a pipeline definition.

DALI Dynamic Mode

Dynamic mode is DALI's imperative Python API. Call DALI operators as regular Python functions with standard control flow -- no pipeline graph, no pipe.build(), no pipe.run().

import nvidia.dali.experimental.dynamic as ndd

Core Data Types

Tensor -- single sample

t = ndd.tensor(data)           # copy
t = ndd.as_tensor(data)        # wrap, no copy if possible
t.cpu()                        # move to CPU
t.gpu()                        # move to GPU
t.torch(copy=False)            # conversion to PyTorch tensor with no copy (default)
t[1:3]                         # slicing supported
np.asarray(t)                  # NumPy via __array__ (CPU only)

Supports __dlpack__, __cuda_array_interface__, __array__, arithmetic operators.

Batch -- collection of samples (variable shapes OK)

b = ndd.batch([arr1, arr2])    # copy
b = ndd.as_batch(data)         # wrap, no copy if possible

Batch has no __getitem__ -- batch[i] raises TypeError because indexing is ambiguous (sample selection vs. per-sample slicing). Use the explicit APIs instead:

Intent	Method	Returns
Get sample i	batch.select(i)	Tensor
Get subset of samples	batch.select(slice_or_list)	Batch
Slice within each sample	batch.slice[...]	Batch (same batch_size)

.select() picks which samples. .slice indexes inside each sample.

xy = ndd.random.uniform(batch_size=16, range=[0, 1], shape=2)
crop_x = xy.slice[0]       # Batch of 16 scalars, first element from each sample
crop_y = xy.slice[1]       # Batch of 16 scalars, second element from each sample
sample_0 = xy.select(0)    # Tensor, the entire first sample [x, y]

PyTorch conversion:

• batch.torch() -- works for uniform shapes; raises for ragged batches
• batch.torch(pad=True) -- zero-pads ragged batches to max shape (use for variable-length audio, detection boxes, etc.)
• batch.torch(copy=None) is the default (avoids copy if possible)
• Batch has no __dlpack__ -- use ndd.as_tensor(batch) first for DLPack consumers. ndd.as_tensor supports pad as well.
• Tensor.torch(copy=False) is default (no copy)

Iteration: for sample in batch: yields Tensors.

Readers

Readers are stateful objects -- create once, reuse across epochs. This matters because readers track internal state like shuffle order and shard position.

reader = ndd.readers.File(file_root=image_dir, random_shuffle=True)

for epoch in range(num_epochs):
    for jpegs, labels in reader.next_epoch(batch_size=64):
        # jpegs, labels are Batch objects
        ...

Key points:

• Reader outputs (jpegs, labels, etc.) are CPU tensors/batches. Labels typically stay on CPU until you convert them for your framework (e.g. labels.torch().to(device)).
• Reader classes are PascalCase: ndd.readers.File(...), ndd.readers.COCO(...), ndd.readers.TFRecord(...)
• batch_size goes to next_epoch(), not to the reader constructor
• next_epoch(batch_size=N) yields tuples of Batch; next_epoch() without batch_size yields tuples of Tensor
• The iterator from next_epoch() must be fully consumed before calling next_epoch() again
• Once a reader is used with a given batch_size, it cannot be changed. Similarly, a reader used in batch mode cannot switch to sample mode or vice versa.

Sharded reading for distributed training:

reader = ndd.readers.File(
    file_root=image_dir,
    shard_id=rank, num_shards=world_size,
    stick_to_shard=True,
    pad_last_batch=True,
)

Device Handling

• Device is inferred from inputs -- GPU if any input is on GPU
• For hybrid decode: use device="gpu" (NOT "mixed"). The "mixed" keyword is a pipeline-mode concept for implicit CPU-to-GPU transfer; in dynamic mode, passing device="gpu" triggers the same hardware-accelerated decode path.
• Don't call .cpu() before passing to a GPU model -- .torch() gives you a GPU tensor directly. .cpu() is only needed for consumers requiring host memory (numpy, __array__).
• CUDA stream sync between DALI and PyTorch is automatic via DLPack -- no manual stream management needed.

Execution Model

Default mode is eager -- async execution in a background thread, returns immediately.

No .evaluate() needed in most cases. Any data consumption (.torch(), __dlpack__, __array__, .shape, property access, iteration) triggers evaluation automatically.

For debugging, switch to synchronous mode so errors surface at the exact call site rather than later in the async queue:

with ndd.EvalMode.sync_full:
    images = ndd.decoders.image(jpegs, device="gpu")
    images = ndd.resize(images, size=[224, 224])
    # Any error surfaces here, at the exact op that failed

Modes (increasing synchronicity): deferred < eager < sync_cpu < sync_full

Use EvalMode.sync_full for debugging instead of scattering .evaluate() calls -- it's cleaner and catches all issues at once. sync_cpu is often sufficient and lighter than sync_full.

Thread Configuration

ndd.set_num_threads(4)  # Call once at startup, only if necessary to override the defaults

Controls DALI's internal worker threads for CPU operators. Defaults to CPU affinity count or DALI_NUM_THREADS env var. Unrelated to Python-level threading.

RNG

Two approaches (use one, not both):

# Approach 1: set the thread-local default seed (simple, good enough for most cases)
ndd.random.set_seed(42)
angles = ndd.random.uniform(batch_size=64, range=(-30, 30))

# Approach 2: explicit RNG object (finer control, pass rng= to each op)
rng = ndd.random.RNG(seed=42)
values = ndd.random.uniform(batch_size=64, range=[0, 1], shape=2, rng=rng)

When rng= is passed to a random op, the explicit RNG overrides the default seed. Thread-local: each thread has independent random state.

Random ops need an explicit batch_size when working with batches -- there is no pipeline-level batch size to inherit.

Example: Image Classification Pipeline

import nvidia.dali.experimental.dynamic as ndd

ndd.set_num_threads(4)
reader = ndd.readers.File(file_root="/data/imagenet/train", random_shuffle=True)

for epoch in range(num_epochs):
    for jpegs, labels in reader.next_epoch(batch_size=64):
        images = ndd.decoders.image(jpegs, device="gpu")
        images = ndd.resize(images, size=[224, 224])
        images = ndd.crop_mirror_normalize(
            images,
            mean=[0.485 * 255, 0.456 * 255, 0.406 * 255],
            std=[0.229 * 255, 0.224 * 255, 0.225 * 255],
        )
        train_step(images.torch(), labels.torch())

Common Mistakes

Wrong	Right	Why
device="mixed"	device="gpu"	"mixed" is pipeline mode only
batch[i]	batch.select(i)	Batch has no __getitem__
batch.select(0) for per-sample slicing	batch.slice[0]	.select() picks samples; .slice slices within each sample
.evaluate() after every op	Let consumption trigger eval	.torch(), .shape, etc. trigger it automatically
.cpu() before GPU model	.torch() directly	Avoids wasteful D2H + H2D round-trip
Recreate reader each epoch	reader.next_epoch()	Readers are stateful -- create once, reuse
ndd.readers.file(...)	ndd.readers.File(...)	Reader classes are PascalCase
break from next_epoch() loop	Exhaust iterator or create new reader	Iterator must be fully consumed before next next_epoch()
No batch_size to random ops	ndd.random.uniform(batch_size=N, ...)	No pipeline-level batch size to inherit

Pipeline Mode Migration

Pipeline Mode	Dynamic Mode
@pipeline_def / pipe.build() / pipe.run()	Direct function calls in a loop
fn.readers.file(...)	ndd.readers.File(...) (PascalCase, stateful)
fn.decoders.image(jpegs, device="mixed")	ndd.decoders.image(jpegs, device="gpu")
fn.op_name(...)	ndd.op_name(...)
Pipeline-level batch_size=64	reader.next_epoch(batch_size=64) + random ops batch_size=64
Pipeline-level seed=42	ndd.random.set_seed(42) or ndd.random.RNG(seed=42)
Pipeline-level num_threads=4	ndd.set_num_threads(4) at startup
output.at(i)	batch.select(i)
output.as_cpu()	batch.cpu()
pipe.run() returns tuple of TensorList	reader.next_epoch(batch_size=N) yields tuples of Batch