Extensions > API reference > attr

attr

This is a top-level module for defining the attribute schemas of a rule or aspect. Each function returns an object representing the schema of a single attribute. These objects are used as the values of the attrs dictionary argument of rule() and aspect().

See the Rules page for more on defining and using attributes.

bool

Attribute attr.bool(default=False, doc='', mandatory=False)

Creates a schema for a boolean attribute.

Parameters

Parameter Description

Parameter	Description
`default`	`bool` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).

default

bool

A default value to use if no value for this attribute is given when instantiating the rule.

doc

string

A description of the attribute that can be extracted by documentation generating tools.

mandatory

bool

If true, the value must be specified explicitly (even if it has a default).

int

Attribute attr.int(default=0, doc='', mandatory=False, values=[])

Creates a schema for an integer attribute.

Parameters

Parameter	Description
`default`	`int` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`values`	`sequence of ints` The list of allowed values for the attribute. An error is raised if any other value is given.

int_list

Attribute attr.int_list(mandatory=False, non_empty=False, allow_empty=True, *, default=[], doc='')

Creates a schema for a list-of-integers attribute.

Parameters

Parameter	Description
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.
`allow_empty`	`bool` True if the attribute can be empty.
`default`	`sequence of ints` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.

label

Attribute attr.label(default=None, doc='', executable=False, allow_files=None, allow_single_file=None, mandatory=False, providers=[], allow_rules=None, single_file=False, cfg=None, aspects=[])

Creates a schema for a label attribute. This is a dependency attribute.

This attribute contains Label values. If a string is supplied in place of a Label, it will be converted using the label constructor. The relative parts of the label path, including the (possibly renamed) repository, are resolved with respect to the instantiated target's package.

At analysis time (within the rule's implementation function), when retrieving the attribute value from ctx.attr, labels are replaced by the corresponding Targets. This allows you to access the providers of the currrent target's dependencies.

In addition to ordinary source files, this kind of attribute is often used to refer to a tool -- for example, a compiler. Such tools are considered to be dependencies, just like source files. To avoid requiring users to specify the tool's label every time they use the rule in their BUILD files, you can hard-code the label of a canonical tool as the default value of this attribute. If you also want to prevent users from overriding this default, you can make the attribute private by giving it a name that starts with an underscore. See the Rules page for more information.

Parameters

Parameter	Description
`default`	`Label; or string; or LateBoundDefault; or function` A default value to use if no value for this attribute is given when instantiating the rule.Use a string or the `Label` function to specify a default value, for example, `attr.label(default = "//a:b")`.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`executable`	`bool` True if the dependency has to be executable. This means the label must refer to an executable file, or to a rule that outputs an executable file. Access the label with `ctx.executable.<attribute_name>`.
`allow_files`	Whether `File` targets are allowed. Can be `True`, `False` (default), or a list of file extensions that are allowed (for example, `[".cc", ".cpp"]`).
`allow_single_file`	This is similar to `allow_files`, with the restriction that the label must correspond to a single File. Access it through `ctx.file.<attribute_name>`.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`providers`	`sequence` The providers that must be given by any dependency appearing in this attribute. The format of this argument is a list of lists of providers -- `*Info` objects returned by `provider()` (or in the case of a legacy provider, its string name). The dependency must return ALL providers mentioned in at least ONE of the inner lists. As a convenience, this argument may also be a single-level list of providers, in which case it is wrapped in an outer list with one element.
`allow_rules`	`sequence of strings` Which rule targets (name of the classes) are allowed. This is deprecated (kept only for compatibility), use providers instead.
`single_file`	`bool` Deprecated: Use `allow_single_file` instead. If True, the label must correspond to a single File. Access it through `ctx.file.<attribute_name>`.
`cfg`	Configuration of the attribute. It can be either `"host"` or `"target"`. This parameter is required if `executable` is True to guard against accidentally building host tools in the target configuration. `"target"` has no semantic effect, so don't set it when `executable` is False unless it really helps clarify your intentions.
`aspects`	`sequence of Aspects` Aspects that should be applied to the dependency or dependencies specified by this attribute.

label_keyed_string_dict

Attribute attr.label_keyed_string_dict(allow_empty=True, *, default={}, doc='', allow_files=None, allow_rules=None, providers=[], flags=[], mandatory=False, non_empty=False, cfg=None, aspects=[])

Creates a schema for an attribute holding a dictionary, where the keys are labels and the values are strings. This is a dependency attribute.

Parameters

Parameter	Description
`allow_empty`	`bool` True if the attribute can be empty.
`default`	`dict; or function` A default value to use if no value for this attribute is given when instantiating the rule.Use strings or the `Label` function to specify default values, for example, `attr.label_keyed_string_dict(default = {"//a:b": "value", "//a:c": "string"})`.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`allow_files`	Whether `File` targets are allowed. Can be `True`, `False` (default), or a list of file extensions that are allowed (for example, `[".cc", ".cpp"]`).
`allow_rules`	`sequence of strings` Which rule targets (name of the classes) are allowed. This is deprecated (kept only for compatibility), use providers instead.
`providers`	`sequence` The providers that must be given by any dependency appearing in this attribute. The format of this argument is a list of lists of providers -- `*Info` objects returned by `provider()` (or in the case of a legacy provider, its string name). The dependency must return ALL providers mentioned in at least ONE of the inner lists. As a convenience, this argument may also be a single-level list of providers, in which case it is wrapped in an outer list with one element.
`flags`	`sequence of strings` Deprecated, will be removed.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.
`cfg`	Configuration of the attribute. It can be either `"host"` or `"target"`.
`aspects`	`sequence of Aspects` Aspects that should be applied to the dependency or dependencies specified by this attribute.

label_list

Attribute attr.label_list(allow_empty=True, *, default=[], doc='', allow_files=None, allow_rules=None, providers=[], flags=[], mandatory=False, non_empty=False, cfg=None, aspects=[])

Creates a schema for a list-of-labels attribute. This is a dependency attribute.

Parameters

Parameter	Description
`allow_empty`	`bool` True if the attribute can be empty.
`default`	`sequence of Labels; or function` A default value to use if no value for this attribute is given when instantiating the rule.Use strings or the `Label` function to specify default values, for example, `attr.label_list(default = ["//a:b", "//a:c"])`.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`allow_files`	Whether `File` targets are allowed. Can be `True`, `False` (default), or a list of file extensions that are allowed (for example, `[".cc", ".cpp"]`).
`allow_rules`	`sequence of strings` Which rule targets (name of the classes) are allowed. This is deprecated (kept only for compatibility), use providers instead.
`providers`	`sequence` The providers that must be given by any dependency appearing in this attribute. The format of this argument is a list of lists of providers -- `*Info` objects returned by `provider()` (or in the case of a legacy provider, its string name). The dependency must return ALL providers mentioned in at least ONE of the inner lists. As a convenience, this argument may also be a single-level list of providers, in which case it is wrapped in an outer list with one element.
`flags`	`sequence of strings` Deprecated, will be removed.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.
`cfg`	Configuration of the attribute. It can be either `"host"` or `"target"`.
`aspects`	`sequence of Aspects` Aspects that should be applied to the dependency or dependencies specified by this attribute.

output

Attribute attr.output(doc='', mandatory=False)

Creates a schema for an output (label) attribute.

At analysis time, the corresponding File can be retrieved using ctx.outputs.

Parameters

Parameter Description

Parameter	Description
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).

doc

string

A description of the attribute that can be extracted by documentation generating tools.

mandatory

bool

If true, the value must be specified explicitly (even if it has a default).

output_list

Attribute attr.output_list(allow_empty=True, *, doc='', mandatory=False, non_empty=False)

Creates a schema for a list-of-outputs attribute.

At analysis time, the corresponding File can be retrieved using ctx.outputs.

Parameters

Parameter	Description
`allow_empty`	`bool` True if the attribute can be empty.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.

string

Attribute attr.string(default='', doc='', mandatory=False, values=[])

Creates a schema for a string attribute.

Parameters

Parameter	Description
`default`	`string` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`values`	`sequence of strings` The list of allowed values for the attribute. An error is raised if any other value is given.

string_dict

Attribute attr.string_dict(allow_empty=True, *, default={}, doc='', mandatory=False, non_empty=False)

Creates a schema for an attribute holding a dictionary, where the keys and values are strings.

Parameters

Parameter	Description
`allow_empty`	`bool` True if the attribute can be empty.
`default`	`dict` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.

string_list

Attribute attr.string_list(mandatory=False, non_empty=False, allow_empty=True, *, default=[], doc='')

Creates a schema for a list-of-strings attribute.

Parameters

Parameter	Description
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.
`allow_empty`	`bool` True if the attribute can be empty.
`default`	`sequence of strings` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.

string_list_dict

Attribute attr.string_list_dict(allow_empty=True, *, default={}, doc='', mandatory=False, non_empty=False)

Creates a schema for an attribute holding a dictionary, where the keys are strings and the values are lists of strings.

Parameters

Parameter	Description
`allow_empty`	`bool` True if the attribute can be empty.
`default`	`dict` A default value to use if no value for this attribute is given when instantiating the rule.
`doc`	`string` A description of the attribute that can be extracted by documentation generating tools.
`mandatory`	`bool` If true, the value must be specified explicitly (even if it has a `default`).
`non_empty`	`bool` True if the attribute must not be empty. Deprecated: Use `allow_empty` instead.