Chex
Chex is a library of utilities for helping to write reliable JAX code.
This includes utils to help:
- Instrument your code (e.g. assertions)
- Debug (e.g. transforming
pmapsinvmapswithin a context manager). - Test JAX code across many
variants(e.g. jitted vs non-jitted).
Installation
You can install the latest released version of Chex from PyPI via:
pip install chexor you can install the latest development version from GitHub:
pip install git+https://github.com/deepmind/chex.gitModules Overview
Dataclass (dataclass.py)
Dataclasses are a popular construct introduced by Python 3.7 to allow to easily specify typed data structures with minimal boilerplate code. They are not, however, compatible with JAX and dm-tree out of the box.
In Chex we provide a JAX-friendly dataclass implementation reusing python dataclasses.
Chex implementation of dataclass registers dataclasses as internal PyTree
nodes to ensure
compatibility with JAX data structures.
In addition, we provide a class wrapper that exposes dataclasses as
collections.Mapping descendants which allows to process them
(e.g. (un-)flatten) in dm-tree methods as usual Python dictionaries.
See @mappable_dataclass
docstring for more details.
Example:
@chex.dataclass
class Parameters:
x: chex.ArrayDevice
y: chex.ArrayDevice
parameters = Parameters(
x=jnp.ones((2, 2)),
y=jnp.ones((1, 2)),
)
# Dataclasses can be treated as JAX pytrees
jax.tree_util.tree_map(lambda x: 2.0 * x, parameters)
# and as mappings by dm-tree
tree.flatten(parameters)NOTE: Unlike standard Python 3.7 dataclasses, Chex
dataclasses cannot be constructed using positional arguments. They support
construction arguments provided in the same format as the Python dict
constructor. Dataclasses can be converted to tuples with the from_tuple and
to_tuple methods if necessary.
parameters = Parameters(
jnp.ones((2, 2)),
jnp.ones((1, 2)),
)
# ValueError: Mappable dataclass constructor doesn't support positional args.Assertions (asserts.py)
One limitation of PyType annotations for JAX is that they do not support the
specification of DeviceArray ranks, shapes or dtypes. Chex includes a number
of functions that allow flexible and concise specification of these properties.
E.g. suppose you want to ensure that all tensors t1, t2, t3 have the same
shape, and that tensors t4, t5 have rank 2 and (3 or 4), respectively.
chex.assert_equal_shape([t1, t2, t3])
chex.assert_rank([t4, t5], [2, {3, 4}])More examples:
from chex import assert_shape, assert_rank, ...
assert_shape(x, (2, 3)) # x has shape (2, 3)
assert_shape([x, y], [(), (2,3)]) # x is scalar and y has shape (2, 3)
assert_rank(x, 0) # x is scalar
assert_rank([x, y], [0, 2]) # x is scalar and y is a rank-2 array
assert_rank([x, y], {0, 2}) # x and y are scalar OR rank-2 arrays
assert_type(x, int) # x has type `int` (x can be an array)
assert_type([x, y], [int, float]) # x has type `int` and y has type `float`
assert_equal_shape([x, y, z]) # x, y, and z have equal shapes
assert_trees_all_close(tree_x, tree_y) # values and structure of trees match
assert_tree_all_finite(tree_x) # all tree_x leaves are finite
assert_devices_available(2, 'gpu') # 2 GPUs available
assert_tpu_available() # at least 1 TPU available
assert_numerical_grads(f, (x, y), j) # f^{(j)}(x, y) matches numerical gradsSee asserts.py
documentation to
find all supported assertions.
If you cannot find a specific assertion, please consider making a pull request or openning an issue on the bug tracker.
Optional Arguments
All chex assertions support the following optional kwargs for manipulating the emitted exception messages:
custom_message: A string to include into the emitted exception messages.include_default_message: Whether to include the default Chex message into the emitted exception messages.exception_type: An exception type to use.AssertionErrorby default.
For example, the following code:
dataset = load_dataset()
params = init_params()
for i in range(num_steps):
params = update_params(params, dataset.sample())
chex.assert_tree_all_finite(params,
custom_message=f'Failed at iteration {i}.',
exception_type=ValueError)will raise a ValueError that includes a step number when params get polluted
with NaNs or Nones.
Static and Value (aka Runtime) Assertions
Chex divides all assertions into 2 classes: static and value assertions.
-
static assertions use anything except concrete values of tensors. Examples:
assert_shape,assert_trees_all_equal_dtypes,assert_max_traces. -
value assertions require access to tensor values, which are not available during JAX tracing (see HowJAX primitives work), thus such assertion need special treatment in a jitted code.
To enable value assertions in a jitted function, it can be decorated with
chex.chexify() wrapper. Example:
@chex.chexify
@jax.jit
def logp1_abs_safe(x: chex.Array) -> chex.Array:
chex.assert_tree_all_finite(x)
return jnp.log(jnp.abs(x) + 1)
logp1_abs_safe(jnp.ones(2)) # OK
logp1_abs_safe(jnp.array([jnp.nan, 3])) # FAILS (in async mode)
# The error will be raised either at the next line OR at the next
# `logp1_abs_safe` call. See the docs for more detain on async mode.
logp1_abs_safe.wait_checks() # Wait for the (async) computation to complete.See
this docstring
for more detail on chex.chexify().
JAX Tracing Assertions
JAX re-traces JIT'ted function every time the structure of passed arguments
changes. Often this behavior is inadvertent and leads to a significant
performance drop which is hard to debug. @chex.assert_max_traces
decorator asserts that the function is not re-traced more than n times during
program execution.
Global trace counter can be cleared by calling
chex.clear_trace_counter(). This function be used to isolate unittests relying
on @chex.assert_max_traces.
Examples:
@jax.jit
@chex.assert_max_traces(n=1)
def fn_sum_jitted(x, y):
return x + y
fn_sum_jitted(jnp.zeros(3), jnp.zeros(3)) # tracing for the 1st time - OK
fn_sum_jitted(jnp.zeros([6, 7]), jnp.zeros([6, 7])) # AssertionError!Can be used with jax.pmap() as well:
def fn_sub(x, y):
return x - y
fn_sub_pmapped = jax.pmap(chex.assert_max_traces(fn_sub, n=10))See HowJAX primitives work section for more information about tracing.
Test variants (variants.py)
JAX relies extensively on code transformation and compilation, meaning that it can be hard to ensure that code is properly tested. For instance, just testing a python function using JAX code will not cover the actual code path that is executed when jitted, and that path will also differ whether the code is jitted for CPU, GPU, or TPU. This has been a source of obscure and hard to catch bugs where XLA changes would lead to undesirable behaviours that however only manifest in one specific code transformation.
Variants make it easy to ensure that unit tests cover different ‘variations’ of a function, by providing a simple decorator that can be used to repeat any test under all (or a subset) of the relevant code transformations.
E.g. suppose you want to test the output of a function fn with or without jit.
You can use chex.variants to run the test with both the jitted and non-jitted
version of the function by simply decorating a test method with
@chex.variants, and then using self.variant(fn) in place of fn in the body
of the test.
def fn(x, y):
return x + y
...
class ExampleTest(chex.TestCase):
@chex.variants(with_jit=True, without_jit=True)
def test(self):
var_fn = self.variant(fn)
self.assertEqual(fn(1, 2), 3)
self.assertEqual(var_fn(1, 2), fn(1, 2))If you define the function in the test method, you may also use self.variant
as a decorator in the function definition. For example:
class ExampleTest(chex.TestCase):
@chex.variants(with_jit=True, without_jit=True)
def test(self):
@self.variant
def var_fn(x, y):
return x + y
self.assertEqual(var_fn(1, 2), 3)Example of parameterized test:
from absl.testing import parameterized
# Could also be:
# `class ExampleParameterizedTest(chex.TestCase, parameterized.TestCase):`
# `class ExampleParameterizedTest(chex.TestCase):`
class ExampleParameterizedTest(parameterized.TestCase):
@chex.variants(with_jit=True, without_jit=True)
@parameterized.named_parameters(
('case_positive', 1, 2, 3),
('case_negative', -1, -2, -3),
)
def test(self, arg_1, arg_2, expected):
@self.variant
def var_fn(x, y):
return x + y
self.assertEqual(var_fn(arg_1, arg_2), expected)Chex currently supports the following variants:
with_jit-- appliesjax.jit()transformation to the function.without_jit-- uses the function as is, i.e. identity transformation.with_device-- places all arguments (except specified inignore_argnumsargument) into device memory before applying the function.without_device-- places all arguments in RAM before applying the function.with_pmap-- appliesjax.pmap()transformation to the function (see notes below).
See documentation in variants.py for more details on the supported variants. More examples can be found in variants_test.py.
Variants notes
-
Test classes that use
@chex.variantsmust inherit fromchex.TestCase(or any other base class that unrolls tests generators withinTestCase, e.g.absl.testing.parameterized.TestCase). -
[
jax.vmap] All variants can be applied to a vmapped function; please see an example in variants_test.py (test_vmapped_fn_named_paramsandtest_pmap_vmapped_fn). -
[
@chex.all_variants] You can get all supported variants by using the decorator@chex.all_variants. -
[
with_pmapvariant]jax.pmap(fn)(doc) performs parallel map offnonto multiple devices. Since most tests run in a single-device environment (i.e. having access to a single CPU or GPU), in which casejax.pmapis a functional equivalent tojax.jit,with_pmapvariant is skipped by default (although it works fine with a single device). Below we describe a way to properly testfnif it is supposed to be used in multi-device environments (TPUs or multiple CPUs/GPUs). To disable skippingwith_pmapvariants in case of a single device, add--chex_skip_pmap_variant_if_single_device=falseto your test command.
Fakes (fake.py)
Debugging in JAX is made more difficult by code transformations such as jit
and pmap, which introduce optimizations that make code hard to inspect and
trace. It can also be difficult to disable those transformations during
debugging as they can be called at several places in the underlying
code. Chex provides tools to globally replace jax.jit with a no-op
transformation and jax.pmap with a (non-parallel) jax.vmap, in order to more
easily debug code in a single-device context.
For example, you can use Chex to fake pmap and have it replaced with a vmap.
This can be achieved by wrapping your code with a context manager:
with chex.fake_pmap():
@jax.pmap
def fn(inputs):
...
# Function will be vmapped over inputs
fn(inputs)The same functionality can also be invoked with start and stop:
fake_pmap = chex.fake_pmap()
fake_pmap.start()
... your jax code ...
fake_pmap.stop()In addition, you can fake a real multi-device test environment with a multi-threaded CPU. See section Faking multi-device test environments for more details.
See documentation in fake.py and examples in fake_test.py for more details.
Faking multi-device test environments
In situations where you do not have easy access to multiple devices, you can still test parallel computation using single-device multi-threading.
In particular, one can force XLA to use a single CPU's threads as separate devices, i.e. to fake a real multi-device environment with a multi-threaded one. These two options are theoretically equivalent from XLA perspective because they expose the same interface and use identical abstractions.
Chex has a flag chex_n_cpu_devices that specifies a number of CPU threads to
use as XLA devices.
To set up a multi-threaded XLA environment for absl tests, define
setUpModule function in your test module:
def setUpModule():
chex.set_n_cpu_devices()Now you can launch your test with python test.py --chex_n_cpu_devices=N to run
it in multi-device regime. Note that all tests within a module will have an
access to N devices.
More examples can be found in variants_test.py, fake_test.py and fake_set_n_cpu_devices_test.py.
Using named dimension sizes.
Chex comes with a small utility that allows you to package a collection of dimension sizes into a single object. The basic idea is:
dims = chex.Dimensions(B=batch_size, T=sequence_len, E=embedding_dim)
...
chex.assert_shape(arr, dims['BTE'])String lookups are translated integer tuples. For instance, let's say
batch_size == 3, sequence_len = 5 and embedding_dim = 7, then
dims['BTE'] == (3, 5, 7)
dims['B'] == (3,)
dims['TTBEE'] == (5, 5, 3, 7, 7)
...You can also assign dimension sizes dynamically as follows:
dims['XY'] = some_matrix.shape
dims.Z = 13For more examples, see chex.Dimensions documentation.
Citing Chex
This repository is part of the DeepMind JAX Ecosystem, to cite Chex please use the DeepMind JAX Ecosystem citation.