jaxtyping/API.md

Full API

Annotating array types

Each array is denoted by a type dtype[shape], such as f32["batch channels"].

Shape

The shape should be a string of space-separated symbols, such as "a b c d". Each symbol can be either an:

• int: fixed-size axis, e.g. f32["28 28"].
• str: variable-size axis, e.g. f32["channels"].
• A symbolic expression (without spaces!) in terms of other variable-size axes, e.g. def remove_last(x: f32["dim"]) -> f32["dim-1"].

When calling a function, variable-size axes and symbolic axes will be matched up across all arguments and checked for consistency. (See runtime type checking below.)

In addition some modifiers can be applied:

• Prepend * to a dimension to indicate that it can match multiple axes, e.g. f32["*batch c h w"] will match zero or more batch axes.
• Prepend # to a dimension to indicate that it can be that size or equal to one -- i.e. broadcasting is acceptable, e.g. add(x: f32["#foo"], y: f32["#foo"]) -> f32["#foo"].
• Prepend _ to a dimension to disable any runtime checking of that dimension (so that it can be used just as documentation). This can also be used as just _ on its own: e.g. f32["b c _ _"].

The order of these modifiers does not matter.

As a special case:

• ...: anonymous zero or more axes (equivalent to *_) e.g. f32["... c h w"]

Some notes:

• To denote a scalar shape use "", e.g. f32[""].
• To denote an arbitrary shape (and only check dtype) use "...", e.g. f32["..."].
• You cannot have more than one use of multiple-axes, i.e. you can only use ... or *name at most once in each array.
• An example of broadcasting multiple dimensions: add(x: f32["*#foo"], y: f32["*#foo"]) -> f32["*#foo"].
• A symbolic expression cannot be evaluated unless all of the axes sizes it refers to have already been processed. In practice this usually means that they should only be used in annotations for the return type, and only use axes declared in the arguments.

Dtype

The dtype should be any one of (imported from jaxtyping):

• Any dtype at all: Array
  • Boolean: b
  • Any integer, unsigned integer, floating, or complex: n (for number)
    • Any floating or complex: x (for inexact)
      • Any floating point: f
        • Floating point: bf16, f16, f32, f64 (bf16 is bfloat16)
      • Any complex: c
        • Complexes: c64, c128
    • Any integer or unsigned intger: t (for integer)
      • Any unsigned integer: u
        • Unsigned integer: u8, u16, u32, u64
      • Any signed integer: i
        • Signed integer: i8, i16, i32, i64

Unless you really want to force a particular precision, then for most applications you should probably allow any floating-point, any integer, etc. That is, use

from jaxtyping import f
f["some_shape"]

rather than

from jaxtyping import f32
f32["some_shape"]

PyTrees

jaxtyping.PyTree

Each PyTree is denoted by a type PyTree[LeafType], such as PyTree[int] or PyTree[Union[str, f32["b c"]]].

You can leave off the [...], in which case PyTree is simply a suggestively-named alternative to Any. (By definition all types are PyTrees.)

Runtime type checking

Single-argument type checking will work with any runtime type checker out-of-the-box.

To enable multi-argument consistency checks (i.e. that shapes match up between arrays), then you have two options, as discussed below. (And if either are too much magic for you, you can safely use neither and stick to just single-argument type checking.)

Regardless of your choice, this approach synergises beautifully with jax.jit! All shape checks will be performed at trace-time only, and will not impact runtime performance.

jaxtyping.jaxtyped

Decorate a function with this to have shapes checked for consistency across multiple arguments.

Example:

# Import both the annotation and the `jaxtyped` decorator from `jaxtyping`
from jaxtyping import f32, jaxtyped

# Use your favourite typechecker: usually one of the two lines below.
from typeguard import typechecked as typechecker
from beartype import beartype as typechecker

# Write your function. @jaxtyped must be applied above @typechecker!
@jaxtyped
@typechecker
def batch_outer_product(x: f32["b c1"], y: f32["b c2"]) -> f32["b c1 c2"]:
    return x[:, :, None] * y[:, None, :]

Note that @jaxtyped is applied above the type checker.

jaxtyping.jaxtyped for advanced users

Put precisely, all isinstance shape checks are scoped to the thread-local dynamic context of a jaxtyped call. A new dynamic context will allow different dimensions sizes to be bound to the same name. After this new dynamic context is finished then the old one is returned to.

For example, this means you could leave off the @jaxtyped decorator to enforce that this function use the same axes sizes as the function it was called from.

Likewise, this means you can use isinstance checks inside a function body and have them contribute to the same collection of consistency checks performed by a typechecker against its arguments. (Or even forgo a typechecker altogether, and just do your own manual isinstance checks.)

Only isinstance checks that pass will contribute to the store of axis name-size pairs; those that fail will not. As such it is safe to write e.g. assert not isinstance(x, f32["foo"]).

jaxtyping.install_import_hook

It can be a lot of effort to add @jaxtyped decorators all over your codebase. (Not to mention that double-decorators everywhere are a bit ugly.) The easier option is usually to use the import import hook.

Example:

from jaxtyping import install_import_hook
# Plus either one of the following:
install_import_hook("foo", ("typeguard", "typechecked"))  # decorate @jaxtyped and @typeguard.typechecked
install_import_hook("foo", ("beartype", "beartype"))  # decorate @jaxtyped and @beartype.beartype
install_import_hook("foo", None)  # decorate only @jaxtyped (if you have manually applied typechecking decorators)

Any module imported afterwards, whose name begins with the specified string, will automatically have both @jaxtyped and the specified typechecker applied to all of their functions. (E.g. in the above example foo, foo.bar, foo.bar.qux would all be hook'd).

The import hook may be uninstalled after you've imported all the modules you're interested in:

hook = install_import_hook(...)
...  # perform imports
hook.uninstall()

The import hook can be applied to multiple packages via

install_import_hook(["foo", "bar.baz"], ...)

Example: writing an end-user script

### entry_point.py
from jaxtyping import install_import_hook
install_import_hook("do_stuff", ("typeguard", "typechecked"))
import do_stuff

### do_stuff.py
from jaxtyping import f32

def g(x: f32["..."]):
    ...

Example: writing a library

### __init__.py
from jaxtyping import install_import_hook
hook = install_import_hook("my_library_name", ("beartype", "beartype"))
from .subpackage import foo  # full name is my_library_name.subpackage so will be hook'd
from .another_subpackage import bar  # full name is my_library_name.another_subpackage so will be hook'd.
hook.uninstall()
del hook, install_import_hook, jaxtyping  # keep interface tidy

pytest hook

The import hook can be installed at test-time only, as a pytest hook. The syntax is

pytest --jaxtyping-packages=foo,bar.baz,beartype.beartype

which will apply the import hook to all modules whose names start with either foo or bar.baz. The typechecker used in this example is beartype.beartype.

Abstract base classes

jaxtyping.AbstractDtype

The base class of all dtypes. This can be used to create your own custom collection of dtypes (analogous to n, x etc.) For example:

class u8_or_u16(AbstractDtype):
    dtypes = ["uint8", "uint16"]

u8_or_u16["shape"]

which is functionally equivalent to

Union[u8["shape"], u16["shape"]]

jaxtyping.AbstractArray

The base class of all shape-and-dtype-specified arrays, e.g. it's a base class for f32["foo"].