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# These are hidden from the docs, but these are necessary for doctest # since the inspect module doesn't play nicely with the execution # environment for doctest import torch

original_script = torch.jit.script def script_wrapper(obj, args,kwargs): obj.__module__ = 'FakeMod' return original_script(obj,args, **kwargs)

torch.jit.script = script_wrapper

original_trace = torch.jit.trace def trace_wrapper(obj, args,kwargs): obj.__module__ = 'FakeMod' return original_trace(obj,args, **kwargs)

torch.jit.trace = trace_wrapper

TorchScript Language Reference

TorchScript is a statically typed subset of Python that can either be written directly (using the @torch.jit.script <torch.jit.script> decorator) or generated automatically from Python code via tracing. When using tracing, code is automatically converted into this subset of Python by recording only the actual operators on tensors and simply executing and discarding the other surrounding Python code.

When writing TorchScript directly using @torch.jit.script decorator, the programmer must only use the subset of Python supported in TorchScript. This section documents what is supported in TorchScript as if it were a language reference for a stand alone language. Any features of Python not mentioned in this reference are not part of TorchScript. See Builtin Functions for a complete reference of available Pytorch tensor methods, modules, and functions.

As a subset of Python, any valid TorchScript function is also a valid Python function. This makes it possible to disable TorchScript and debug the function using standard Python tools like pdb. The reverse is not true: there are many valid Python programs that are not valid TorchScript programs. Instead, TorchScript focuses specifically on the features of Python that are needed to represent neural network models in PyTorch.


The largest difference between TorchScript and the full Python language is that TorchScript only supports a small set of types that are needed to express neural net models. In particular, TorchScript supports:

Type Description
Tensor A PyTorch tensor of any dtype, dimension, or backend
Tuple[T0, T1, ..., TN] A tuple containing subtypes T0, T1, etc. (e.g. Tuple[Tensor, Tensor])
bool A boolean value
int A scalar integer
float A scalar floating point number
str A string
List[T] A list of which all members are type T
Optional[T] A value which is either None or type T
Dict[K, V] A dict with key type K and value type V. Only str, int, and float are allowed as key types.
T A TorchScript Class
E A TorchScript Enum
NamedTuple[T0, T1, ...] A collections.namedtuple <collections.namedtuple> tuple type

Unlike Python, each variable in TorchScript function must have a single static type. This makes it easier to optimize TorchScript functions.

Example (a type mismatch)

import torch

@torch.jit.script def an_error(x): if x: r = torch.rand(1) else: r = 4 return r

Traceback (most recent call last):


RuntimeError: ...

Type mismatch: r is set to type Tensor in the true branch and type int in the false branch: @torch.jit.script def an_error(x): if x: ~~~~~ r = torch.rand(1) ~~~~~~~~~~~~~~~~~ else: ~~~~~ r = 4 ~~~~~ <--- HERE return r and was used here: else: r = 4 return r ~ <--- HERE...

Unsupported Typing Constructs

TorchScript does not support all features and types of the typing module. Some of these are more fundamental things that are unlikely to be added in the future while others may be added if there is enough user demand to make it a priority.

These types and features from the typing module are unavailble in TorchScript.

Item Description
typing.Any typing.Any is currently in development but not yet released
typing.NoReturn Not implemented
typing.Union Unlikely to be implemented (however typing.Optional is supported)
typing.Sequence Not implemented
typing.Callable Not implemented
typing.Literal Not implemented
typing.ClassVar Not implemented
typing.Final This is supported for module attributes <Module Attributes> class attribute annotations but not for functions
typing.AnyStr TorchScript does not support bytes so this type is not used
typing.overload typing.overload is currently in development but not yet released
Type aliases Not implemented
Nominal vs structural subtyping Nominal typing is in development, but structural typing is not
NewType Unlikely to be implemented
Generics Unlikely to be implemented

Any other functionality from the typing module not explitily listed in this documentation is unsupported.

Default Types

By default, all parameters to a TorchScript function are assumed to be Tensor. To specify that an argument to a TorchScript function is another type, it is possible to use MyPy-style type annotations using the types listed above.

import torch

@torch.jit.script def foo(x, tup): # type: (int, Tuple[Tensor, Tensor]) -> Tensor t0, t1 = tup return t0 + t1 + x

print(foo(3, (torch.rand(3), torch.rand(3))))



It is also possible to annotate types with Python 3 type hints from the typing module.

import torch from typing import Tuple

@torch.jit.script def foo(x: int, tup: Tuple[torch.Tensor, torch.Tensor]) -> torch.Tensor: t0, t1 = tup return t0 + t1 + x

print(foo(3, (torch.rand(3), torch.rand(3))))


An empty list is assumed to be List[Tensor] and empty dicts Dict[str, Tensor]. To instantiate an empty list or dict of other types, use Python 3 type hints.

Example (type annotations for Python 3):

import torch import torch.nn as nn from typing import Dict, List, Tuple

class EmptyDataStructures(torch.nn.Module):
def __init__(self):

super(EmptyDataStructures, self).__init__()

def forward(self, x: torch.Tensor) -> Tuple[List[Tuple[int, float]], Dict[str, int]]:

# This annotates the list to be a List[Tuple[int, float]] my_list: List[Tuple[int, float]] = [] for i in range(10): my_list.append((i, x.item()))

my_dict: Dict[str, int] = {} return my_list, my_dict

x = torch.jit.script(EmptyDataStructures())

Optional Type Refinement

TorchScript will refine the type of a variable of type Optional[T] when a comparison to None is made inside the conditional of an if-statement or checked in an assert. The compiler can reason about multiple None checks that are combined with and, or, and not. Refinement will also occur for else blocks of if-statements that are not explicitly written.

The None check must be within the if-statement's condition; assigning a None check to a variable and using it in the if-statement's condition will not refine the types of variables in the check. Only local variables will be refined, an attribute like self.x will not and must assigned to a local variable to be refined.

Example (refining types on parameters and locals):

import torch import torch.nn as nn from typing import Optional

class M(nn.Module):

z: Optional[int]

def __init__(self, z):

super(M, self).__init__() # If z is None, its type cannot be inferred, so it must # be specified (above) self.z = z

def forward(self, x, y, z):

# type: (Optional[int], Optional[int], Optional[int]) -> int if x is None: x = 1 x = x + 1

# Refinement for an attribute by assigning it to a local z = self.z if y is not None and z is not None: x = y + z

# Refinement via an assert assert z is not None x += z return x

module = torch.jit.script(M(2)) module = torch.jit.script(M(None))

TorchScript Classes


TorchScript class support is experimental. Currently it is best suited for simple record-like types (think a NamedTuple with methods attached).

Python classes can be used in TorchScript if they are annotated with @torch.jit.script <torch.jit.script>, similar to how you would declare a TorchScript function:

@torch.jit.script class Foo: def __init__(self, x, y): self.x = x

def aug_add_x(self, inc):

self.x += inc

This subset is restricted:

After a class is defined, it can be used in both TorchScript and Python interchangeably like any other TorchScript type:

# Declare a TorchScript class
class Pair:
  def __init__(self, first, second):
    self.first = first
    self.second = second

def sum_pair(p):
  # type: (Pair) -> Tensor
  return p.first + p.second

p = Pair(torch.rand(2, 3), torch.rand(2, 3))

TorchScript Enums

Python enums can be used in TorchScript without any extra annotation or code:

from enum import Enum

class Color(Enum):
    RED = 1
    GREEN = 2

def enum_fn(x: Color, y: Color) -> bool:
    if x == Color.RED:
        return True

    return x == y

After an enum is defined, it can be used in both TorchScript and Python interchangeably like any other TorchScript type. The type of the values of an enum must be int, float, or str. All values must be of the same type; heterogenous types for enum values are not supported.

Named Tuples

Types produced by collections.namedtuple <collections.namedtuple> can be used in TorchScript.

import torch import collections

Point = collections.namedtuple('Point', ['x', 'y'])

@torch.jit.script def total(point): # type: (Point) -> Tensor return point.x + point.y

p = Point(x=torch.rand(3), y=torch.rand(3)) print(total(p))



Some functions (for example, zip and enumerate) can only operate on iterable types. Iterable types in TorchScript include Tensors, lists, tuples, dictionaries, strings, torch.nn.ModuleList and torch.nn.ModuleDict.


The following Python Expressions are supported.


'string literals'
"string literals"
3  # interpreted as int
3.4  # interpreted as a float

List Construction

An empty list is assumed have type List[Tensor]. The types of other list literals are derived from the type of the members. See Default Types for more details.

[3, 4]
[torch.rand(3), torch.rand(4)]

Tuple Construction

(3, 4)

Dict Construction

An empty dict is assumed have type Dict[str, Tensor]. The types of other dict literals are derived from the type of the members. See Default Types for more details.

{'hello': 3}
{'a': torch.rand(3), 'b': torch.rand(4)}


See Variable Resolution for how variables are resolved.


Arithmetic Operators

a + b
a - b
a * b
a / b
a ^ b
a @ b

Comparison Operators

a == b
a != b
a < b
a > b
a <= b
a >= b

Logical Operators

a and b
a or b
not b

Subscripts and Slicing

t[0, 1]
t[0, 1:2]
t[0, :1]
t[-1, 1:, 0]
t[1:, -1, 0]
t[i:j, i]

Function Calls

Calls to builtin functions

torch.rand(3, dtype=torch.int)

Calls to other script functions:

import torch

@torch.jit.script def foo(x): return x + 1

@torch.jit.script def bar(x): return foo(x)

Method Calls

Calls to methods of builtin types like tensor: x.mm(y)

On modules, methods must be compiled before they can be called. The TorchScript compiler recursively compiles methods it sees when compiling other methods. By default, compilation starts on the forward method. Any methods called by forward will be compiled, and any methods called by those methods, and so on. To start compilation at a method other than forward, use the @torch.jit.export <torch.jit.export> decorator (forward implicitly is marked @torch.jit.export).

Calling a submodule directly (e.g. self.resnet(input)) is equivalent to calling its forward method (e.g. self.resnet.forward(input)).

import torch import torch.nn as nn import torchvision

class MyModule(nn.Module):
def __init__(self):

super(MyModule, self).__init__() means = torch.tensor([103.939, 116.779, 123.68]) self.means = torch.nn.Parameter(means.resize(1, 3, 1, 1)) resnet = torchvision.models.resnet18() self.resnet = torch.jit.trace(resnet, torch.rand(1, 3, 224, 224))

def helper(self, input):

return self.resnet(input - self.means)

def forward(self, input):

return self.helper(input)

# Since nothing in the model calls top_level_method, the compiler # must be explicitly told to compile this method @torch.jit.export def top_level_method(self, input): return self.other_helper(input)

def other_helper(self, input):

return input + 10

# my_script_module will have the compiled methods forward, helper, # top_level_method, and other_helper my_script_module = torch.jit.script(MyModule())

Ternary Expressions

x if x > y else y



Accessing Module Parameters



TorchScript supports the following types of statements:

Simple Assignments

a = b
a += b # short-hand for a = a + b, does not operate in-place on a
a -= b

Pattern Matching Assignments

a, b = tuple_or_list
a, b, *c = a_tuple

Multiple Assignments :

a = b, c = tup
print("the result of an add:", a + b)

If Statements

if a < 4:
    r = -a
elif a < 3:
    r = a + a
    r = 3 * a

In addition to bools, floats, ints, and Tensors can be used in a conditional and will be implicitly casted to a boolean.

While Loops

a = 0
while a < 4:
    a += 1

For loops with range

x = 0
for i in range(10):
    x *= i

For loops over tuples

These unroll the loop, generating a body for each member of the tuple. The body must type-check correctly for each member.

tup = (3, torch.rand(4))
for x in tup:

For loops over constant nn.ModuleList

To use a nn.ModuleList inside a compiled method, it must be marked constant by adding the name of the attribute to the __constants__ list for the type. For loops over a nn.ModuleList will unroll the body of the loop at compile time, with each member of the constant module list.

class SubModule(torch.nn.Module):
def __init__(self):

super(SubModule, self).__init__() self.weight = nn.Parameter(torch.randn(2))

def forward(self, input):

return self.weight + input

class MyModule(torch.nn.Module):

__constants__ = ['mods']

def __init__(self):

super(MyModule, self).__init__() self.mods = torch.nn.ModuleList([SubModule() for i in range(10)])

def forward(self, v):
for module in self.mods:

v = module(v)

return v

m = torch.jit.script(MyModule())

Break and Continue

for i in range(5):
    if i == 1:
    if i == 3:


return a, b

Variable Resolution

TorchScript supports a subset of Python's variable resolution (i.e. scoping) rules. Local variables behave the same as in Python, except for the restriction that a variable must have the same type along all paths through a function. If a variable has a different type on different branches of an if statement, it is an error to use it after the end of the if statement.

Similarly, a variable is not allowed to be used if it is only defined along some paths through the function.


@torch.jit.script def foo(x): if x < 0: y = 4 print(y)

Traceback (most recent call last):


RuntimeError: ...

y is not defined in the false branch... @torch.jit.script... def foo(x): if x < 0: ~~~~~~~~~ y = 4 ~~~~~ <--- HERE print(y) and was used here: if x < 0: y = 4 print(y) ~ <--- HERE...

Non-local variables are resolved to Python values at compile time when the function is defined. These values are then converted into TorchScript values using the rules described in Use of Python Values.

Use of Python Values

To make writing TorchScript more convenient, we allow script code to refer to Python values in the surrounding scope. For instance, any time there is a reference to torch, the TorchScript compiler is actually resolving it to the torch Python module when the function is declared. These Python values are not a first class part of TorchScript. Instead they are de-sugared at compile-time into the primitive types that TorchScript supports. This depends on the dynamic type of the Python valued referenced when compilation occurs. This section describes the rules that are used when accessing Python values in TorchScript.


TorchScript can call Python functions. This functionality is very useful when incrementally converting a model to TorchScript. The model can be moved function-by-function to TorchScript, leaving calls to Python functions in place. This way you can incrementally check the correctness of the model as you go.


Attribute Lookup On Python Modules

TorchScript can lookup attributes on modules. Builtin functions like torch.add are accessed this way. This allows TorchScript to call functions defined in other modules.

Python-defined Constants

TorchScript also provides a way to use constants that are defined in Python. These can be used to hard-code hyper-parameters into the function, or to define universal constants. There are two ways of specifying that a Python value should be treated as a constant.

  1. Values looked up as attributes of a module are assumed to be constant:

import math import torch

@torch.jit.script def fn(): return math.pi

  1. Attributes of a ScriptModule can be marked constant by annotating them with Final[T]
import torch
import torch.nn as nn

class Foo(nn.Module):
    # `Final` from the `typing_extensions` module can also be used
    a : torch.jit.Final[int]

    def __init__(self):
        super(Foo, self).__init__()
        self.a = 1 + 4

    def forward(self, input):
        return self.a + input

f = torch.jit.script(Foo())

Supported constant Python types are

Module Attributes

The torch.nn.Parameter wrapper and register_buffer can be used to assign tensors to a module. Other values assigned to a module that is compiled will be added to the compiled module if their types can be inferred. All types available in TorchScript can be used as module attributes. Tensor attributes are semantically the same as buffers. The type of empty lists and dictionaries and None values cannot be inferred and must be specified via PEP 526-style class annotations. If a type cannot be inferred and is not explicilty annotated, it will not be added as an attribute to the resulting ScriptModule.


from typing import List, Dict

class Foo(nn.Module):

# words is initialized as an empty list, so its type must be specified words: List[str]

# The type could potentially be inferred if a_dict (below) was not # empty, but this annotation ensures some_dict will be made into the # proper type some_dict: Dict[str, int]

def __init__(self, a_dict):

super(Foo, self).__init__() self.words = [] self.some_dict = a_dict

# `int`s can be inferred self.my_int = 10

def forward(self, input):

# type: (str) -> int self.words.append(input) return self.some_dict[input] + self.my_int

f = torch.jit.script(Foo({'hi': 2}))