# ctx

A context object that is passed to the implementation function for a rule or aspect. It provides access to the information and methods needed to analyze the current target.

In particular, it lets the implementation function access the current target's label, attributes, configuration, and the providers of its dependencies. It has methods for declaring output files and the actions that produce them.

Context objects essentially live for the duration of the call to the implementation function. It is not useful to access these objects outside of their associated function. See the Rules page for more information.

## Members

- actions
- aspect_ids
- attr
- bin_dir
- build_file_path
- build_setting_value
- configuration
- coverage_instrumented
- created_actions
- default_provider
- disabled_features
- exec_groups
- executable
- expand_location
- expand_make_variables
- features
- file
- files
- fragments
- genfiles_dir
- host_configuration
- host_fragments
- info_file
- label
- new_file
- outputs
- resolve_command
- resolve_tools
- rule
- runfiles
- split_attr
- target_platform_has_constraint
- toolchains
- var
- version_file
- workspace_name

## actions

    actions ctx.actions

Contains methods for declaring output files and the actions that produce them.

## aspect_ids

    list ctx.aspect_ids

A list of ids for all aspects applied to the target. Only available in aspect implementation functions.

## attr

    struct ctx.attr

A struct to access the values of the attributes. The values are provided by the user (if not, a default value is used). The attributes of the struct and the types of their values correspond to the keys and values of the `attrs` dict provided to the `rule` function. See example of use.

## bin_dir

    root ctx.bin_dir

The root corresponding to bin directory.

## build_file_path

    string ctx.build_file_path

The path to the BUILD file for this rule, relative to the source root.

## build_setting_value

    unknown ctx.build_setting_value

Experimental. This field is experimental and subject to change at any time. Do not depend on it.

The value of the build setting that is represented by the current target. It is an error to access this field for rules that do not set the `build_setting` attribute in their rule definition.

## configuration

    configuration ctx.configuration

The default configuration. See the configuration type for more details.

## coverage_instrumented

    bool ctx.coverage_instrumented(target=None)

Returns whether code coverage instrumentation should be generated when performing compilation actions for this rule or, if `target` is provided, the rule specified by that Target. (If a non-rule or a Starlark rule Target is provided, this returns False.) Checks if the sources of the current rule (if no Target is provided) or the sources of Target should be instrumented based on the --instrumentation_filter and --instrument_test_targets config settings. This differs from `coverage_enabled` in the configuration, which notes whether coverage data collection is enabled for the entire run, but not whether a specific target should be instrumented.

### Parameters

Parameter | Description
--- | ---
`target` | `Target; or None`; default = None A Target specifying a rule. If not provided, defaults to the current rule.

## created_actions

    StarlarkValue ctx.created_actions()

For rules with _skylark_testable set to `True`, this returns an Actions provider representing all actions created so far for the current rule. For all other rules, returns `None`. Note that the provider is not updated when subsequent actions are created, so you will have to call this function again if you wish to inspect them.
This is intended to help write tests for rule-implementation helper functions, which may take in a `ctx` object and create actions on it.

## default_provider

    Provider ctx.default_provider

Deprecated. Use DefaultInfo instead.

## disabled_features

    list ctx.disabled_features

The set of features that are explicitly disabled by the user for this rule.

## exec_groups

    ExecGroupCollection ctx.exec_groups

A collection of the execution groups available for this rule, indexed by their name. Access with `ctx.exec_groups[name_of_group]`.

## executable

    struct ctx.executable

A `struct` containing executable files defined in label type attributes marked as `executable=True`. The struct fields correspond to the attribute names. Each value in the struct is either a `File` or `None`. If an optional attribute is not specified in the rule then the corresponding struct value is `None`. If a label type is not marked as `executable=True`, no corresponding struct field is generated. See example of use.

## expand_location

    string ctx.expand_location(input, targets=[], short_paths=False)

Expands all `$(location ...)` templates in the given string by replacing `$(location //x)` with the path of the output file of target //x. Expansion only works for labels that point to direct dependencies of this rule or that are explicitly listed in the optional argument `targets`.
`$(location ...)` will cause an error if the referenced target has multiple outputs. In this case, please use `$(locations ...)` since it produces a space-separated list of output paths. It can be safely used for a single output file, too.
This function is useful to let the user specify a command in a BUILD file (like for `genrule`). In other cases, it is often better to manipulate labels directly.

### Parameters

Parameter | Description
--- | ---
`input` | required String to be expanded.
`targets` | `sequence of Targets`; default = [] List of targets for additional lookup information.
`short_paths` | default = False Use root relative paths instead of full exec paths

May return `None`.

## expand_make_variables

    string ctx.expand_make_variables(attribute_name, command, additional_substitutions)

Deprecated. Use ctx.var to access the variables instead.
Returns a string after expanding all references to "Make variables". The variables must have the following format: `$(VAR_NAME)`. Also, `$$VAR_NAME` expands to `$VAR_NAME`. Examples:

    ctx.expand_make_variables("cmd", "$(MY_VAR)", {"MY_VAR": "Hi"})  # == "Hi"
    ctx.expand_make_variables("cmd", "$$PWD", {})  # == "$PWD"

Additional variables may come from other places, such as configurations. Note that this function is experimental.

### Parameters

Parameter | Description
--- | ---
`attribute_name` | required The attribute name. Used for error reporting.
`command` | required The expression to expand. It can contain references to "Make variables".
`additional_substitutions` | required Additional substitutions to make beyond the default make variables.

## features

    list ctx.features

The set of features that are explicitly enabled by the user for this rule. See example of use.

## file

    struct ctx.file

A `struct` containing files defined in label type attributes marked as `allow_single_file`. The struct fields correspond to the attribute names. The struct value is always a `File` or `None`. If an optional attribute is not specified in the rule then the corresponding struct value is `None`. If a label type is not marked as `allow_single_file`, no corresponding struct field is generated. It is a shortcut for:

    list(ctx.attr.<ATTR>.files)[0]

In other words, use `file` to access the (singular) default output of a dependency. See example of use.

## files

    struct ctx.files

A `struct` containing files defined in label or label list type attributes. The struct fields correspond to the attribute names. The struct values are `list` of `File`s. It is a shortcut for:

    [f for t in ctx.attr.<ATTR> for f in t.files]

In other words, use `files` to access the default outputs of a dependency. See example of use.

## fragments

    fragments ctx.fragments

Allows access to configuration fragments in target configuration.

## genfiles_dir

    root ctx.genfiles_dir

The root corresponding to genfiles directory.

## host_configuration

    configuration ctx.host_configuration

The host configuration. See the configuration type for more details.

## host_fragments

    fragments ctx.host_fragments

Allows access to configuration fragments in host configuration.

## info_file

    File ctx.info_file

The file that is used to hold the non-volatile workspace status for the current build request. See documentation for --workspace_status_command for more information.

## label

    Label ctx.label

The label of the target currently being analyzed.

## new_file

    File ctx.new_file(var1, var2=unbound, var3=unbound)

DEPRECATED. Use ctx.actions.declare_file.
Creates a file object. There are four possible signatures to this method:

- new_file(filename): Creates a file object with the given filename in the current package.
- new_file(file_root, filename): Creates a file object with the given filename under the given file root.
- new_file(sibling_file, filename): Creates a file object in the same directory as the given sibling file.
- new_file(file_root, sibling_file, suffix): Creates a file object with same base name of the sibling_file but with different given suffix, under the given file root.

Does not actually create a file on the file system, just declares that some action will do so. You must create an action that generates the file. If the file should be visible to other rules, declare a rule output instead when possible. Doing so enables Blaze to associate a label with the file that rules can refer to (allowing finer dependency control) instead of referencing the whole rule.

### Parameters

Parameter | Description
--- | ---
`var1` | `string; or root; or File`; required
`var2` | `string; or File`; default = unbound
`var3` | `string`; default = unbound

## outputs

    structure ctx.outputs

A pseudo-struct containing all the predeclared output files, represented by `File` objects. See the Rules page for more information and examples.

This field does not exist on aspect contexts, since aspects do not have predeclared outputs.

The fields of this object are defined as follows. It is an error if two outputs produce the same field name or have the same label.

- If the rule declares an `outputs` dict, then for every entry in the dict, there is a field whose name is the key and whose value is the corresponding `File`.
- For every attribute of type `attr.output` that the rule declares, there is a field whose name is the attribute's name. If the target specified a label for that attribute, then the field value is the corresponding `File`; otherwise the field value is `None`.
- For every attribute of type `attr.output_list` that the rule declares, there is a field whose name is the attribute's name. The field value is a list of `File` objects corresponding to the labels given for that attribute in the target, or an empty list if the attribute was not specified in the target.
- (Deprecated) If the rule is marked `executable` or `test`, there is a field named `"executable"`, which is the default executable. It is recommended that instead of using this, you pass another file (either predeclared or not) to the `executable` arg of `DefaultInfo`.

## resolve_command

    tuple ctx.resolve_command(command='', attribute=None, expand_locations=False, make_variables=None, tools=[], label_dict={}, execution_requirements={})

(Experimental) Returns a tuple `(inputs, command, input_manifests)` of the list of resolved inputs, the argv list for the resolved command, and the runfiles metadata required to run the command, all of them suitable for passing as the same-named arguments of the `ctx.action` method.
Note for Windows users: this method requires Bash (MSYS2). Consider using `resolve_tools()` instead (if that fits your needs).

### Parameters

Parameter | Description
--- | ---
`command` | default = '' Command to resolve.
`attribute` | `string; or None`; default = None Name of the associated attribute for which to issue an error, or None.
`expand_locations` | default = False Shall we expand $(location) variables? See ctx.expand_location() for more details.
`make_variables` | `dict; or None`; default = None Make variables to expand, or None.
`tools` | `sequence of Targets`; default = [] List of tools (list of targets).
`label_dict` | default = {} Dictionary of resolved labels and the corresponding list of Files (a dict of Label : list of Files).
`execution_requirements` | default = {} Information for scheduling the action to resolve this command. See tags for useful keys.

## resolve_tools

    tuple ctx.resolve_tools(tools=[])

Returns a tuple `(inputs, input_manifests)` of the depset of resolved inputs and the runfiles metadata required to run the tools, both of them suitable for passing as the same-named arguments of the `ctx.actions.run` method.
In contrast to `ctx.resolve_command`, this method does not require that Bash be installed on the machine, so it's suitable for rules built on Windows.

### Parameters

Parameter | Description
--- | ---
`tools` | `sequence of Targets`; default = [] List of tools (list of targets).

## rule

    rule_attributes ctx.rule

Rule attributes descriptor for the rule that the aspect is applied to. Only available in aspect implementation functions.

## runfiles

    runfiles ctx.runfiles(files=[], transitive_files=None, collect_data=False, collect_default=False, symlinks={}, root_symlinks={})

Creates a runfiles object.

### Parameters

Parameter | Description
--- | ---
`files` | `sequence of Files`; default = [] The list of files to be added to the runfiles.
`transitive_files` | `depset of Files; or None`; default = None The (transitive) set of files to be added to the runfiles. The depset should use the `default` order (which, as the name implies, is the default).
`collect_data` | default = False Use of this parameter is not recommended. See runfiles guide. Whether to collect the data runfiles from the dependencies in srcs, data and deps attributes.
`collect_default` | default = False Use of this parameter is not recommended. See runfiles guide. Whether to collect the default runfiles from the dependencies in srcs, data and deps attributes.
`symlinks` | `dict; or depset of SymlinkEntrys`; default = {} Either a SymlinkEntry depset or the map of symlinks, prefixed by workspace name, to be added to the runfiles. See Runfiles symlinks in the rules guide.
`root_symlinks` | `dict; or depset of SymlinkEntrys`; default = {} Either a SymlinkEntry depset or a map of symlinks to be added to the runfiles. See Runfiles symlinks in the rules guide.

## split_attr

    struct ctx.split_attr

A struct to access the values of attributes with split configurations. If the attribute is a label list, the value of split_attr is a dict of the keys of the split (as strings) to lists of the ConfiguredTargets in that branch of the split. If the attribute is a label, then the value of split_attr is a dict of the keys of the split (as strings) to single ConfiguredTargets. Attributes with split configurations still appear in the attr struct, but their values will be single lists with all the branches of the split merged together.

## target_platform_has_constraint

    bool ctx.target_platform_has_constraint(constraintValue)

Returns true if the given constraint value is part of the current target platform.

### Parameters

Parameter | Description
--- | ---
`constraintValue` | required The constraint value to check the target platform against.

## toolchains

    ToolchainContext ctx.toolchains

Toolchains for the default exec group of this rule.

## var

    dict ctx.var

Dictionary (String to String) of configuration variables.

## version_file

    File ctx.version_file

The file that is used to hold the volatile workspace status for the current build request. See documentation for --workspace_status_command for more information.

## workspace_name

    string ctx.workspace_name

The workspace name as defined in the WORKSPACE file.