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Extensions > API reference > Globals

Globals

Objects, functions and modules registered in the global environment.

Members

all

bool all(elements)

Returns true if all elements evaluate to True or if the collection is empty. Elements are converted to boolean using the bool function.
all(["hello", 3, True]) == True
all([-1, 0, 1]) == False

Parameters

Parameter Description
elements required
A string or a collection of elements.

analysis_test_transition

transition analysis_test_transition(settings)

Creates a configuration transition to be applied on an analysis-test rule's dependencies. This transition may only be applied on attributes of rules with analysis_test = True. Such rules are restricted in capabilities (for example, the size of their dependency tree is limited), so transitions created using this function are limited in potential scope as compared to transitions created using transition.

This function is primarily designed to facilitate the Analysis Test Framework core library. See its documentation (or its implementation) for best practices.

Parameters

Parameter Description
settings required
A dictionary containing information about configuration settings which should be set by this configuration transition. Keys are build setting labels and values are their new post-transition values. All other settings are unchanged. Use this to declare specific configuration settings that an analysis test requires to be set in order to pass.

any

bool any(elements)

Returns true if at least one element evaluates to True. Elements are converted to boolean using the bool function.
any([-1, 0, 1]) == True
any([False, 0, ""]) == False

Parameters

Parameter Description
elements required
A string or a collection of elements.

archive_override

None archive_override(module_name, urls, integrity='', strip_prefix='', patches=[], patch_strip=0)

Specifies that this dependency should come from an archive file (zip, gzip, etc) at a certain location, instead of from a registry. This directive can only be used by the root module; in other words, if a module specifies any overrides, it cannot be used as a dependency by others.

Parameters

Parameter Description
module_name required
The name of the Bazel module dependency to apply this override to.
urls string; or Iterable of strings; required
The URLs of the archive; can be http(s):// or file:// URLs.
integrity default = ''
The expected checksum of the archive file, in Subresource Integrity format.
strip_prefix default = ''
A directory prefix to strip from the extracted files.
patches Iterable of strings; default = []
A list of labels pointing to patch files to apply for this module. The patch files must exist in the source tree of the top level project. They are applied in the list order.
patch_strip default = 0
Same as the --strip argument of Unix patch.

aspect

Aspect aspect(implementation, attr_aspects=[], attrs=None, required_providers=[], required_aspect_providers=[], provides=[], requires=[], fragments=[], host_fragments=[], toolchains=[], incompatible_use_toolchain_transition=False, doc='', *, apply_to_generating_rules=False)

Creates a new aspect. The result of this function must be stored in a global value. Please see the introduction to Aspects for more details.

Parameters

Parameter Description
implementation required
A Starlark function that implements this aspect, with exactly two parameters: Target (the target to which the aspect is applied) and ctx (the rule context which the targetis created from). Attributes of the target are available via the ctx.rule field. This function is evaluated during the analysis phase for each application of an aspect to a target.
attr_aspects sequence of strings; default = []
List of attribute names. The aspect propagates along dependencies specified in the attributes of a target with these names. Common values here include deps and exports. The list can also contain a single string "*" to propagate along all dependencies of a target.
attrs dict; or None; default = None
A dictionary declaring all the attributes of the aspect. It maps from an attribute name to an attribute object, like `attr.label` or `attr.string` (see attr module). Aspect attributes are available to implementation function as fields of ctx parameter.

Implicit attributes starting with _ must have default values, and have type label or label_list.

Explicit attributes must have type string, and must use the values restriction. Explicit attributes restrict the aspect to only be used with rules that have attributes of the same name, type, and valid values according to the restriction.

required_providers default = []
This attribute allows the aspect to limit its propagation to only the targets whose rules advertise its required providers. The value must be a list containing either individual providers or lists of providers but not both. For example, [[FooInfo], [BarInfo], [BazInfo, QuxInfo]] is a valid value while [FooInfo, BarInfo, [BazInfo, QuxInfo]] is not valid.

An unnested list of providers will automatically be converted to a list containing one list of providers. That is, [FooInfo, BarInfo] will automatically be converted to [[FooInfo, BarInfo]].

To make some rule (e.g. some_rule) targets visible to an aspect, some_rule must advertise all providers from at least one of the required providers lists. For example, if the required_providers of an aspect are [[FooInfo], [BarInfo], [BazInfo, QuxInfo]], this aspect can only see some_rule targets if and only if some_rule provides FooInfo *or* BarInfo *or* both BazInfo *and* QuxInfo.

required_aspect_providers default = []
This attribute allows this aspect to inspect other aspects. The value must be a list containing either individual providers or lists of providers but not both. For example, [[FooInfo], [BarInfo], [BazInfo, QuxInfo]] is a valid value while [FooInfo, BarInfo, [BazInfo, QuxInfo]] is not valid.

An unnested list of providers will automatically be converted to a list containing one list of providers. That is, [FooInfo, BarInfo] will automatically be converted to [[FooInfo, BarInfo]].

To make another aspect (e.g. other_aspect) visible to this aspect, other_aspect must provide all providers from at least one of the lists. In the example of [[FooInfo], [BarInfo], [BazInfo, QuxInfo]], this aspect can only see other_aspect if and only if other_aspect provides FooInfo *or* BarInfo *or* both BazInfo *and* QuxInfo.

provides default = []
A list of providers that the implementation function must return.

It is an error if the implementation function omits any of the types of providers listed here from its return value. However, the implementation function may return additional providers not listed here.

Each element of the list is an *Info object returned by provider(), except that a legacy provider is represented by its string name instead.

requires sequence of Aspects; default = []
List of aspects required to be propagated before this aspect.
fragments sequence of strings; default = []
List of names of configuration fragments that the aspect requires in target configuration.
host_fragments sequence of strings; default = []
List of names of configuration fragments that the aspect requires in host configuration.
toolchains sequence of strings; default = []
If set, the set of toolchains this rule requires. Toolchains will be found by checking the current platform, and provided to the rule implementation via ctx.toolchain.
incompatible_use_toolchain_transition default = False
If set, this aspect will use the toolchain transition for toolchain dependencies. This is ignored if the --incompatible_use_toolchain_transition flag is set.
doc default = ''
A description of the aspect that can be extracted by documentation generating tools.
apply_to_generating_rules default = False
If true, the aspect will, when applied to an output file, instead apply to the output file's generating rule.

For example, suppose an aspect propagates transitively through attribute `deps` and it is applied to target `alpha`. Suppose `alpha` has `deps = [':beta_output']`, where `beta_output` is a declared output of a target `beta`. Suppose `beta` has a target `charlie` as one of its `deps`. If `apply_to_generating_rules=True` for the aspect, then the aspect will propagate through `alpha`, `beta`, and `charlie`. If False, then the aspect will propagate only to `alpha`.

False by default.

bazel_dep

None bazel_dep(name, version, repo_name='', dev_dependency=False)

Declares a direct dependency on another Bazel module.

Parameters

Parameter Description
name required
The name of the module to be added as a direct dependency.
version required
The version of the module to be added as a direct dependency.
repo_name default = ''
The name of the external repo representing this dependency. This is by default the name of the module.
dev_dependency default = False
If true, this dependency will be ignored if the current module is not the root module or `--ignore_dev_dependency` is enabled.

bind

None bind(name, actual=None)

Warning: use of bind() is not recommended. See Consider removing bind for a long discussion if its issues and alternatives.

Gives a target an alias in the //external package.

Parameters

Parameter Description
name required
The label under '//external' to serve as the alias name
actual string; or None; default = None
The real label to be aliased

bool

bool bool(x=False)

Constructor for the bool type. It returns False if the object is None, False, an empty string (""), the number 0, or an empty collection (e.g. (), []). Otherwise, it returns True.

Parameters

Parameter Description
x default = False
The variable to convert.

configuration_field

LateBoundDefault configuration_field(fragment, name)

References a late-bound default value for an attribute of type label. A value is 'late-bound' if it requires the configuration to be built before determining the value. Any attribute using this as a value must be private.

Example usage:

Defining a rule attribute: 

'_foo': attr.label(default=configuration_field(fragment='java', name='toolchain'))

Accessing in rule implementation: 

  def _rule_impl(ctx):
    foo_info = ctx.attr._foo
    ...

Parameters

Parameter Description
fragment required
The name of a configuration fragment which contains the late-bound value.
name required
The name of the value to obtain from the configuration fragment.

depset

depset depset(direct=None, order="default", *, transitive=None)

Creates a depset. The direct parameter is a list of direct elements of the depset, and transitive parameter is a list of depsets whose elements become indirect elements of the created depset. The order in which elements are returned when the depset is converted to a list is specified by the order parameter. See the Depsets overview for more information.

All elements (direct and indirect) of a depset must be of the same type, as obtained by the expression type(x).

Because a hash-based set is used to eliminate duplicates during iteration, all elements of a depset should be hashable. However, this invariant is not currently checked consistently in all constructors. Use the --incompatible_always_check_depset_elements flag to enable consistent checking; this will be the default behavior in future releases; see Issue 10313.

In addition, elements must currently be immutable, though this restriction will be relaxed in future.

The order of the created depset should be compatible with the order of its transitive depsets. "default" order is compatible with any other order, all other orders are only compatible with themselves.

Note on backward/forward compatibility. This function currently accepts a positional items parameter. It is deprecated and will be removed in the future, and after its removal direct will become a sole positional parameter of the depset function. Thus, both of the following calls are equivalent and future-proof:

depset(['a', 'b'], transitive = [...])
depset(direct = ['a', 'b'], transitive = [...])

Parameters

Parameter Description
direct sequence; or None; default = None
A list of direct elements of a depset.
order default = "default"
The traversal strategy for the new depset. See here for the possible values.
transitive sequence of depsets; or None; default = None
A list of depsets whose elements will become indirect elements of the depset.

dict

dict dict(pairs=[], **kwargs)

Creates a dictionary from an optional positional argument and an optional set of keyword arguments. In the case where the same key is given multiple times, the last value will be used. Entries supplied via keyword arguments are considered to come after entries supplied via the positional argument.

Parameters

Parameter Description
pairs default = []
A dict, or an iterable whose elements are each of length 2 (key, value).
kwargs required
Dictionary of additional entries.

dir

list dir(x)

Returns a list of strings: the names of the attributes and methods of the parameter object.

Parameters

Parameter Description
x required
The object to check.

enumerate

list enumerate(list, start=0)

Returns a list of pairs (two-element tuples), with the index (int) and the item from the input sequence. 
enumerate([24, 21, 84]) == [(0, 24), (1, 21), (2, 84)]

Parameters

Parameter Description
list required
input sequence.
start default = 0
start index.

exec_group

exec_group exec_group(toolchains=[], exec_compatible_with=[], copy_from_rule=False)

Creates an execution group which can be used to create actions for a specific execution platform during rule implementation.

Parameters

Parameter Description
toolchains sequence of strings; default = []
The set of toolchains this execution group requires.
exec_compatible_with sequence of strings; default = []
A list of constraints on the execution platform.
copy_from_rule default = False
If set to true, this exec group inherits the toolchains and constraints of the rule to which this group is attached. If set to any other string this will throw an error.

fail

None fail(msg=None, attr=None, *args)

Causes execution to fail with an error.

Parameters

Parameter Description
msg default = None
Deprecated: use positional arguments instead. This argument acts like an implicit leading positional argument.
attr string; or None; default = None
Deprecated. Causes an optional prefix containing this string to be added to the error message.
args required
A list of values, formatted with str and joined with spaces, that appear in the error message.

float

float float(x=unbound)

Returns x as a float value.
  • If x is already a float, float returns it unchanged.
  • If x is a bool, float returns 1.0 for True and 0.0 for False.
  • If x is an int, float returns the nearest finite floating-point value to x, or an error if the magnitude is too large.
  • If x is a string, it must be a valid floating-point literal, or be equal (ignoring case) to NaN, Inf, or Infinity, optionally preceded by a + or - sign.
Any other value causes an error. With no argument, float() returns 0.0.

Parameters

Parameter Description
x default = unbound
The value to convert.

getattr

unknown getattr(x, name, default=unbound)

Returns the struct's field of the given name if it exists. If not, it either returns default (if specified) or raises an error. getattr(x, "foobar") is equivalent to x.foobar.
getattr(ctx.attr, "myattr")
getattr(ctx.attr, "myattr", "mydefault")

Parameters

Parameter Description
x required
The struct whose attribute is accessed.
name required
The name of the struct attribute.
default default = unbound
The default value to return in case the struct doesn't have an attribute of the given name.

git_override

None git_override(module_name, remote, commit='', patches=[], patch_strip=0)

Specifies that a dependency should come from a certain commit of a Git repository. This directive can only be used by the root module; in other words, if a module specifies any overrides, it cannot be used as a dependency by others.

Parameters

Parameter Description
module_name required
The name of the Bazel module dependency to apply this override to.
remote required
The URL of the remote Git repository.
commit default = ''
The commit that should be checked out.
patches Iterable of strings; default = []
A list of labels pointing to patch files to apply for this module. The patch files must exist in the source tree of the top level project. They are applied in the list order.
patch_strip default = 0
Same as the --strip argument of Unix patch.

hasattr

bool hasattr(x, name)

Returns True if the object x has an attribute or method of the given name, otherwise False. Example:
hasattr(ctx.attr, "myattr")

Parameters

Parameter Description
x required
The object to check.
name required
The name of the attribute.

hash

int hash(value)

Return a hash value for a string. This is computed deterministically using the same algorithm as Java's String.hashCode(), namely: 
s[0] * (31^(n-1)) + s[1] * (31^(n-2)) + ... + s[n-1]
Hashing of values besides strings is not currently supported.

Parameters

Parameter Description
value required
String value to hash.

int

int int(x, base=unbound)

Returns x as an int value.
  • If x is already an int, int returns it unchanged.
  • If x is a bool, int returns 1 for True and 0 for False.
  • If x is a string, it must have the format <sign><prefix><digits>. <sign> is either "+", "-", or empty (interpreted as positive). <digits> are a sequence of digits from 0 up to base - 1, where the letters a-z (or equivalently, A-Z) are used as digits for 10-35. In the case where base is 2/8/16, <prefix> is optional and may be 0b/0o/0x (or equivalently, 0B/0O/0X) respectively; if the base is any other value besides these bases or the special value 0, the prefix must be empty. In the case where base is 0, the string is interpreted as an integer literal, in the sense that one of the bases 2/8/10/16 is chosen depending on which prefix if any is used. If base is 0, no prefix is used, and there is more than one digit, the leading digit cannot be 0; this is to avoid confusion between octal and decimal. The magnitude of the number represented by the string must be within the allowed range for the int type.
  • If x is a float, int returns the integer value of the float, rounding towards zero. It is an error if x is non-finite (NaN or infinity).
This function fails if x is any other type, or if the value is a string not satisfying the above format. Unlike Python's int function, this function does not allow zero arguments, and does not allow extraneous whitespace for string arguments.

Examples:

int("123") == 123
int("-123") == -123
int("+123") == 123
int("FF", 16) == 255
int("0xFF", 16) == 255
int("10", 0) == 10
int("-0x10", 0) == -16
int("-0x10", 0) == -16
int("123.456") == 123

Parameters

Parameter Description
x required
The string to convert.
base default = unbound
The base used to interpret a string value; defaults to 10. Must be between 2 and 36 (inclusive), or 0 to detect the base as if x were an integer literal. This parameter must not be supplied if the value is not a string.

len

int len(x)

Returns the length of a string, sequence (such as a list or tuple), dict, or other iterable.

Parameters

Parameter Description
x required
The value whose length to report.

list

list list(x=[])

Returns a new list with the same elements as the given iterable value.
list([1, 2]) == [1, 2]
list((2, 3, 2)) == [2, 3, 2]
list({5: "a", 2: "b", 4: "c"}) == [5, 2, 4]

Parameters

Parameter Description
x default = []
The object to convert.

local_path_override

None local_path_override(module_name, path)

Specifies that a dependency should come from a certain directory on local disk. This directive can only be used by the root module; in other words, if a module specifies any overrides, it cannot be used as a dependency by others.

Parameters

Parameter Description
module_name required
The name of the Bazel module dependency to apply this override to.
path required
The path to the directory where this module is.

max

unknown max(*args)

Returns the largest one of all given arguments. If only one argument is provided, it must be a non-empty iterable.It is an error if elements are not comparable (for example int with string), or if no arguments are given. 
max(2, 5, 4) == 5
max([5, 6, 3]) == 6

Parameters

Parameter Description
args required
The elements to be checked.

min

unknown min(*args)

Returns the smallest one of all given arguments. If only one argument is provided, it must be a non-empty iterable. It is an error if elements are not comparable (for example int with string), or if no arguments are given. 
min(2, 5, 4) == 2
min([5, 6, 3]) == 3

Parameters

Parameter Description
args required
The elements to be checked.

module

None module(name='', version='', compatibility_level=0, execution_platforms_to_register=[], toolchains_to_register=[])

Declares certain properties of the Bazel module represented by the current Bazel repo. These properties are either essential metadata of the module (such as the name and version), or affect behavior of the current module and its dependents.

It should be called at most once. It can be omitted only if this module is the root module (as in, if it's not going to be depended on by another module).

Parameters

Parameter Description
name default = ''
The name of the module. Can be omitted only if this module is the root module (as in, if it's not going to be depended on by another module).
version default = ''
The version of the module. Can be omitted only if this module is the root module (as in, if it's not going to be depended on by another module).
compatibility_level default = 0
The compatibility level of the module; this should be changed every time a major incompatible change is introduced. This is essentially the "major version" of the module in terms of SemVer, except that it's not embedded in the version string itself, but exists as a separate field. Modules with different compatibility levels participate in version resolution as if they're modules with different names, but the final dependency graph cannot contain multiple modules with the same name but different compatibility levels (unless multiple_version_override is in effect; see there for more details).
execution_platforms_to_register Iterable of strings; default = []
A list of already-defined execution platforms to be registered when this module is selected. Should be a list of absolute target patterns (ie. beginning with either @ or //). See toolchain resolution for more information.
toolchains_to_register Iterable of strings; default = []
A list of already-defined toolchains to be registered when this module is selected. Should be a list of absolute target patterns (ie. beginning with either @ or //). See toolchain resolution for more information.

module_extension

unknown module_extension(implementation, *, tag_classes={}, doc='')

Creates a new module extension. Store it in a global value, so that it can be exported and used in a MODULE.bazel file.

Parameters

Parameter Description
implementation required
The function that implements this module extension. Must take a single parameter, module_ctx. The function is called once at the beginning of a build to determine the set of available repos.
tag_classes default = {}
A dictionary to declare all the tag classes used by the extension. It maps from the name of the tag class to a tag_class object.
doc default = ''
A description of the module extension that can be extracted by documentation generating tools.

multiple_version_override

None multiple_version_override(module_name, versions, registry='')

Specifies that a dependency should still come from a registry, but multiple versions of it should be allowed to coexist. This directive can only be used by the root module; in other words, if a module specifies any overrides, it cannot be used as a dependency by others.

Parameters

Parameter Description
module_name required
The name of the Bazel module dependency to apply this override to.
versions Iterable of strings; required
Explicitly specifies the versions allowed to coexist. These versions must already be present in the dependency graph pre-selection. Dependencies on this module will be "upgraded" to the nearest higher allowed version at the same compatibility level, whereas dependencies that have a higher version than any allowed versions at the same compatibility level will cause an error.
registry default = ''
Overrides the registry for this module; instead of finding this module from the default list of registries, the given registry should be used.

print

None print(sep=" ", *args)

Prints args as debug output. It will be prefixed with the string "DEBUG" and the location (file and line number) of this call. The exact way in which the arguments are converted to strings is unspecified and may change at any time. In particular, it may be different from (and more detailed than) the formatting done by str() and repr().

Using print in production code is discouraged due to the spam it creates for users. For deprecations, prefer a hard error using fail() whenever possible.

Parameters

Parameter Description
sep default = " "
The separator string between the objects, default is space (" ").
args required
The objects to print.

provider

unknown provider(doc='', *, fields=None, init=None)

Defines a provider symbol. The provider may be instantiated by calling it, or used directly as a key for retrieving an instance of that provider from a target. Example:
MyInfo = provider()
...
def _my_library_impl(ctx):
    ...
    my_info = MyInfo(x = 2, y = 3)
    # my_info.x == 2
    # my_info.y == 3
    ...

See Rules (Providers) for a comprehensive guide on how to use providers.

Returns a Provider callable value if init is not specified.

If init is specified, returns a tuple of 2 elements: a Provider callable value and a raw constructor callable value. See Rules (Custom initialization of custom providers) and the discussion of the init parameter below for details.

Parameters

Parameter Description
doc default = ''
A description of the provider that can be extracted by documentation generating tools.
fields sequence of strings; or dict; or None; default = None
If specified, restricts the set of allowed fields.
Possible values are:
  • list of fields:
    provider(fields = ['a', 'b'])

  • dictionary field name -> documentation:
    provider(
       fields = { 'a' : 'Documentation for a', 'b' : 'Documentation for b' })
All fields are optional.
init callable; or None; default = None
An optional callback for preprocessing and validating the provider's field values during instantiation. If init is specified, provider() returns a tuple of 2 elements: the normal provider symbol and a raw constructor.

A precise description follows; see Rules (Custom initialization of providers) for an intuitive discussion and use cases.

Let P be the provider symbol created by calling provider(). Conceptually, an instance of P is generated by calling a default constructor function c(*args, **kwargs), which does the following:

  • If args is non-empty, an error occurs.
  • If the fields parameter was specified when provider() was called, and if kwargs contains any key that was not listed in fields, an error occurs.
  • Otherwise, c returns a new instance that has, for each k: v entry in kwargs, a field named k with value v.
In the case where an init callback is not given, a call to the symbol P itself acts as a call to the default constructor function c; in other words, P(*args, **kwargs) returns c(*args, **kwargs). For example,
MyInfo = provider()
m = MyInfo(foo = 1)
will straightforwardly make it so that m is a MyInfo instance with m.foo == 1.

But in the case where init is specified, the call P(*args, **kwargs) will perform the following steps instead:

  1. The callback is invoked as init(*args, **kwargs), that is, with the exact same positional and keyword arguments as were passed to P.
  2. The return value of init is expected to be a dictionary, d, whose keys are field name strings. If it is not, an error occurs.
  3. A new instance of P is generated as if by calling the default constructor with d's entries as keyword arguments, as in c(**d).

NB: the above steps imply that an error occurs if *args or **kwargs does not match init's signature, or the evaluation of init's body fails (perhaps intentionally via a call to fail()), or if the return value of init is not a dictionary with the expected schema.

In this way, the init callback generalizes normal provider construction by allowing positional arguments and arbitrary logic for preprocessing and validation. It does not enable circumventing the list of allowed fields.

When init is specified, the return value of provider() becomes a tuple (P, r), where r is the raw constructor. In fact, the behavior of r is exactly that of the default constructor function c discussed above. Typically, r is bound to a variable whose name is prefixed with an underscore, so that only the current .bzl file has direct access to it:

MyInfo, _new_myinfo = provider(init = ...)

range

sequence range(start_or_stop, stop_or_none=None, step=1)

Creates a list where items go from start to stop, using a step increment. If a single argument is provided, items will range from 0 to that element.
range(4) == [0, 1, 2, 3]
range(3, 9, 2) == [3, 5, 7]
range(3, 0, -1) == [3, 2, 1]

Parameters

Parameter Description
start_or_stop required
Value of the start element if stop is provided, otherwise value of stop and the actual start is 0
stop_or_none int; or None; default = None
optional index of the first item not to be included in the resulting list; generation of the list stops before stop is reached.
step default = 1
The increment (default is 1). It may be negative.

register_execution_platforms

None register_execution_platforms(*platform_labels)

Register an already-defined platform so that Bazel can use it as an execution platform during toolchain resolution.

Parameters

Parameter Description
platform_labels sequence of strings; required
The labels of the platforms to register.

register_toolchains

None register_toolchains(*toolchain_labels)

Register an already-defined toolchain so that Bazel can use it during toolchain resolution. See examples of defining and registering toolchains.

Parameters

Parameter Description
toolchain_labels sequence of strings; required
The labels of the toolchains to register.

repository_rule(implementation, attrs, local, environ, configure, remotable, doc)

callable repository_rule(implementation, *, attrs=None, local=False, environ=[], configure=False, remotable=False, doc='')

Creates a new repository rule. Store it in a global value, so that it can be loaded and called from the WORKSPACE file.

Parameters

Parameter Description
implementation required
the function that implements this rule. Must have a single parameter, repository_ctx. The function is called during the loading phase for each instance of the rule.
attrs dict; or None; default = None
dictionary to declare all the attributes of the rule. It maps from an attribute name to an attribute object (see attr module). Attributes starting with _ are private, and can be used to add an implicit dependency on a label to a file (a repository rule cannot depend on a generated artifact). The attribute name is implicitly added and must not be specified.
local default = False
Indicate that this rule fetches everything from the local system and should be reevaluated at every fetch.
environ sequence of strings; default = []
Provides a list of environment variable that this repository rule depends on. If an environment variable in that list change, the repository will be refetched.
configure default = False
Indicate that the repository inspects the system for configuration purpose
remotable default = False
Experimental. This parameter is experimental and may change at any time. Please do not depend on it. It may be enabled on an experimental basis by setting ---experimental_repo_remote_exec
Compatible with remote execution
doc default = ''
A description of the repository rule that can be extracted by documentation generating tools.

repository_rule(implementation, attrs, local, environ, configure, remotable, doc)

callable repository_rule(implementation, *, attrs=None, local=False, environ=[], configure=False, remotable=False, doc='')

Creates a new repository rule. Store it in a global value, so that it can be loaded and called from the WORKSPACE file.

Parameters

Parameter Description
implementation required
the function that implements this rule. Must have a single parameter, repository_ctx. The function is called during the loading phase for each instance of the rule.
attrs dict; or None; default = None
dictionary to declare all the attributes of the rule. It maps from an attribute name to an attribute object (see attr module). Attributes starting with _ are private, and can be used to add an implicit dependency on a label to a file (a repository rule cannot depend on a generated artifact). The attribute name is implicitly added and must not be specified.
local default = False
Indicate that this rule fetches everything from the local system and should be reevaluated at every fetch.
environ sequence of strings; default = []
Provides a list of environment variable that this repository rule depends on. If an environment variable in that list change, the repository will be refetched.
configure default = False
Indicate that the repository inspects the system for configuration purpose
remotable default = False
Experimental. This parameter is experimental and may change at any time. Please do not depend on it. It may be enabled on an experimental basis by setting ---experimental_repo_remote_exec
Compatible with remote execution
doc default = ''
A description of the repository rule that can be extracted by documentation generating tools.

repr

string repr(x)

Converts any object to a string representation. This is useful for debugging.
repr("ab") == '"ab"'

Parameters

Parameter Description
x required
The object to convert.

reversed

list reversed(sequence)

Returns a new, unfrozen list that contains the elements of the original iterable sequence in reversed order.
reversed([3, 5, 4]) == [4, 5, 3]

Parameters

Parameter Description
sequence required
The iterable sequence (e.g. list) to be reversed.

rule

callable rule(implementation, test=False, attrs=None, outputs=None, executable=False, output_to_genfiles=False, fragments=[], host_fragments=[], _skylark_testable=False, toolchains=[], incompatible_use_toolchain_transition=False, doc='', *, provides=[], exec_compatible_with=[], analysis_test=False, build_setting=None, cfg=None, exec_groups=None, compile_one_filetype=None, name=None)

Creates a new rule, which can be called from a BUILD file or a macro to create targets.

Rules must be assigned to global variables in a .bzl file; the name of the global variable is the rule's name.

Test rules are required to have a name ending in _test, while all other rules must not have this suffix. (This restriction applies only to rules, not to their targets.)

Parameters

Parameter Description
implementation required
the Starlark function implementing this rule, must have exactly one parameter: ctx. The function is called during the analysis phase for each instance of the rule. It can access the attributes provided by the user. It must create actions to generate all the declared outputs.
test default = False
Whether this rule is a test rule, that is, whether it may be the subject of a bazel test command. All test rules are automatically considered executable; it is unnecessary (and discouraged) to explicitly set executable = True for a test rule. See the Rules page for more information.
attrs dict; or None; default = None
dictionary to declare all the attributes of the rule. It maps from an attribute name to an attribute object (see attr module). Attributes starting with _ are private, and can be used to add an implicit dependency on a label. The attribute name is implicitly added and must not be specified. Attributes visibility, deprecation, tags, testonly, and features are implicitly added and cannot be overridden. Most rules need only a handful of attributes. To limit memory usage, the rule function imposes a cap on the size of attrs.
outputs dict; or None; or function; default = None
Deprecated. This parameter is deprecated and will be removed soon. Please do not depend on it. It is disabled with ---incompatible_no_rule_outputs_param. Use this flag to verify your code is compatible with its imminent removal.
This parameter has been deprecated. Migrate rules to use OutputGroupInfo or attr.output instead.

A schema for defining predeclared outputs. Unlike output and output_list attributes, the user does not specify the labels for these files. See the Rules page for more on predeclared outputs.

The value of this argument is either a dictionary or a callback function that produces a dictionary. The callback works similar to computed dependency attributes: The function's parameter names are matched against the rule's attributes, so for example if you pass outputs = _my_func with the definition def _my_func(srcs, deps): ..., the function has access to the attributes srcs and deps. Whether the dictionary is specified directly or via a function, it is interpreted as follows.

Each entry in the dictionary creates a predeclared output where the key is an identifier and the value is a string template that determines the output's label. In the rule's implementation function, the identifier becomes the field name used to access the output's File in ctx.outputs. The output's label has the same package as the rule, and the part after the package is produced by substituting each placeholder of the form "%{ATTR}" with a string formed from the value of the attribute ATTR:

  • String-typed attributes are substituted verbatim.
  • Label-typed attributes become the part of the label after the package, minus the file extension. For example, the label "//pkg:a/b.c" becomes "a/b".
  • Output-typed attributes become the part of the label after the package, including the file extension (for the above example, "a/b.c").
  • All list-typed attributes (for example, attr.label_list) used in placeholders are required to have exactly one element. Their conversion is the same as their non-list version (attr.label).
  • Other attribute types may not appear in placeholders.
  • The special non-attribute placeholders %{dirname} and %{basename} expand to those parts of the rule's label, excluding its package. For example, in "//pkg:a/b.c", the dirname is a and the basename is b.c.

In practice, the most common substitution placeholder is "%{name}". For example, for a target named "foo", the outputs dict {"bin": "%{name}.exe"} predeclares an output named foo.exe that is accessible in the implementation function as ctx.outputs.bin.

executable default = False
Whether this rule is considered executable, that is, whether it may be the subject of a bazel run command. See the Rules page for more information.
output_to_genfiles default = False
If true, the files will be generated in the genfiles directory instead of the bin directory. Unless you need it for compatibility with existing rules (e.g. when generating header files for C++), do not set this flag.
fragments sequence of strings; default = []
List of names of configuration fragments that the rule requires in target configuration.
host_fragments sequence of strings; default = []
List of names of configuration fragments that the rule requires in host configuration.
_skylark_testable default = False
(Experimental)

If true, this rule will expose its actions for inspection by rules that depend on it via an Actions provider. The provider is also available to the rule itself by calling ctx.created_actions().

This should only be used for testing the analysis-time behavior of Starlark rules. This flag may be removed in the future.
toolchains sequence of strings; default = []
If set, the set of toolchains this rule requires. Toolchains will be found by checking the current platform, and provided to the rule implementation via ctx.toolchain.
incompatible_use_toolchain_transition default = False
If set, this rule will use the toolchain transition for toolchain dependencies. This is ignored if the --incompatible_use_toolchain_transition flag is set.
doc default = ''
A description of the rule that can be extracted by documentation generating tools.
provides default = []
A list of providers that the implementation function must return.

It is an error if the implementation function omits any of the types of providers listed here from its return value. However, the implementation function may return additional providers not listed here.

Each element of the list is an *Info object returned by provider(), except that a legacy provider is represented by its string name instead.

exec_compatible_with sequence of strings; default = []
A list of constraints on the execution platform that apply to all targets of this rule type.
analysis_test default = False
If true, then this rule is treated as an analysis test.

Note: Analysis test rules are primarily defined using infrastructure provided in core Starlark libraries. See Testing for guidance.

If a rule is defined as an analysis test rule, it becomes allowed to use configuration transitions defined using analysis_test_transition on its attributes, but opts into some restrictions:

  • Targets of this rule are limited in the number of transitive dependencies they may have.
  • The rule is considered a test rule (as if test=True were set). This supercedes the value of test
  • The rule implementation function may not register actions. Instead, it must register a pass/fail result via providing AnalysisTestResultInfo.
build_setting BuildSetting; or None; default = None
If set, describes what kind of build setting this rule is. See the config module. If this is set, a mandatory attribute named "build_setting_default" is automatically added to this rule, with a type corresponding to the value passed in here.
cfg default = None
If set, points to the configuration transition the rule will apply to its own configuration before analysis.
exec_groups dict; or None; default = None
Dict of execution group name (string) to exec_groups. If set, allows rules to run actions on multiple execution platforms within a single target. See execution groups documentation for more info.
compile_one_filetype sequence of strings; or None; default = None
Used by --compile_one_dependency: if multiple rules consume the specified file, should we choose this rule over others.
name string; or None; default = None
The name of this rule, as understood by Bazel and reported in contexts such as logging, native.existing_rule(...)[kind], and bazel query. Usually this is the same as the Starlark identifier that gets bound to this rule; for instance a rule called foo_library would typically be declared as foo_library = rule(...) and instantiated in a BUILD file as foo_library(...).

If this parameter is omitted, the rule's name is set to the name of the first Starlark global variable to be bound to this rule within its declaring .bzl module. Thus, foo_library = rule(...) need not specify this parameter if the name is foo_library.

Specifying an explicit name for a rule does not change where you are allowed to instantiate the rule.

select

unknown select(x, no_match_error='')

select() is the helper function that makes a rule attribute configurable. See build encyclopedia for details.

Parameters

Parameter Description
x required
A dict that maps configuration conditions to values. Each key is a Label or a label string that identifies a config_setting or constraint_value instance. See the documentation on macros for when to use a Label instead of a string.
no_match_error default = ''
Optional custom error to report if no condition matches.

single_version_override

None single_version_override(module_name, version='', registry='', patches=[], patch_strip=0)

Specifies that a dependency should still come from a registry, but its version should be pinned, or its registry overridden, or a list of patches applied. This directive can only be used by the root module; in other words, if a module specifies any overrides, it cannot be used as a dependency by others.

Parameters

Parameter Description
module_name required
The name of the Bazel module dependency to apply this override to.
version default = ''
Overrides the declared version of this module in the dependency graph. In other words, this module will be "pinned" to this override version. This attribute can be omitted if all one wants to override is the registry or the patches.
registry default = ''
Overrides the registry for this module; instead of finding this module from the default list of registries, the given registry should be used.
patches Iterable of strings; default = []
A list of labels pointing to patch files to apply for this module. The patch files must exist in the source tree of the top level project. They are applied in the list order.
patch_strip default = 0
Same as the --strip argument of Unix patch.

sorted

list sorted(iterable, *, key=None, reverse=False)

Returns a new sorted list containing all the elements of the supplied iterable sequence. An error may occur if any pair of elements x, y may not be compared using x < y. The elements are sorted into ascending order, unless the reverse argument is True, in which case the order is descending. Sorting is stable: elements that compare equal retain their original relative order. 
sorted([3, 5, 4]) == [3, 4, 5]

Parameters

Parameter Description
iterable required
The iterable sequence to sort.
key default = None
An optional function applied to each element before comparison.
reverse default = False
Return results in descending order.

str

string str(x)

Converts any object to string. This is useful for debugging.
str("ab") == "ab"
str(8) == "8"

Parameters

Parameter Description
x required
The object to convert.

tag_class

tag_class tag_class(attrs={}, *, doc='')

Creates a new tag_class object, which defines an attribute schema for a class of tags, which are data objects usable by a module extension.

Parameters

Parameter Description
attrs default = {}
A dictionary to declare all the attributes of this tag class. It maps from an attribute name to an attribute object (see attr module).
doc default = ''
A description of the tag class that can be extracted by documentation generating tools.

tuple

tuple tuple(x=())

Returns a tuple with the same elements as the given iterable value.
tuple([1, 2]) == (1, 2)
tuple((2, 3, 2)) == (2, 3, 2)
tuple({5: "a", 2: "b", 4: "c"}) == (5, 2, 4)

Parameters

Parameter Description
x default = ()
The object to convert.

type

string type(x)

Returns the type name of its argument. This is useful for debugging and type-checking. Examples:
type(2) == "int"
type([1]) == "list"
type(struct(a = 2)) == "struct"
This function might change in the future. To write Python-compatible code and be future-proof, use it only to compare return values: 
if type(x) == type([]):  # if x is a list

Parameters

Parameter Description
x required
The object to check type of.

use_extension

module_extension_proxy use_extension(extension_bzl_file, extension_name, *, dev_dependency=False)

Returns a proxy object representing a module extension; its methods can be invoked to create module extension tags.

Parameters

Parameter Description
extension_bzl_file required
A label to the Starlark file defining the module extension.
extension_name required
The name of the module extension to use. A symbol with this name must be exported by the Starlark file.
dev_dependency default = False
If true, this usage of the module extension will be ignored if the current module is not the root module or `--ignore_dev_dependency` is enabled.

use_repo

None use_repo(extension_proxy, *args, **kwargs)

Imports one or more repos generated by the given module extension into the scope of the current module.

Parameters

Parameter Description
extension_proxy required
A module extension proxy object returned by a get_module_extension call.
args required
The names of the repos to import.
kwargs required
Specifies certain repos to import into the scope of the current module with different names. The keys should be the name to use in the current scope, whereas the values should be the original names exported by the module extension.

workspace

None workspace(name, managed_directories={})

This function can only be used in a WORKSPACE file and must be declared before all other functions in the WORKSPACE file. Each WORKSPACE file should have a workspace function.

Sets the name for this workspace. Workspace names should be a Java-package-style description of the project, using underscores as separators, e.g., github.com/bazelbuild/bazel should use com_github_bazelbuild_bazel.

This name is used for the directory that the repository's runfiles are stored in. For example, if there is a runfile foo/bar in the local repository and the WORKSPACE file contains workspace(name = 'baz'), then the runfile will be available under mytarget.runfiles/baz/foo/bar. If no workspace name is specified, then the runfile will be symlinked to bar.runfiles/foo/bar.

Remote repository rule names must be valid workspace names. For example, you could have maven_jar(name = 'foo'), but not maven_jar(name = 'foo%bar'), as Bazel would attempt to write a WORKSPACE file for the maven_jar containing workspace(name = 'foo%bar').

Parameters

Parameter Description
name required
the name of the workspace. Names must start with a letter and can only contain letters, numbers, underscores, dashes, and dots.
managed_directories default = {}
Dict (strings to list of strings) for defining the mappings between external repositories and relative (to the workspace root) paths to directories they incrementally update. Managed directories must be excluded from the source tree by listing them (or their parent directories) in the .bazelignore file.

zip

list zip(*args)

Returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. The list has the size of the shortest input. With a single iterable argument, it returns a list of 1-tuples. With no arguments, it returns an empty list. Examples:
zip()  # == []
zip([1, 2])  # == [(1,), (2,)]
zip([1, 2], [3, 4])  # == [(1, 3), (2, 4)]
zip([1, 2], [3, 4, 5])  # == [(1, 3), (2, 4)]

Parameters

Parameter Description
args required
lists to zip.