Extensions > API reference > Args

Args

An object that encapsulates, in a memory-efficient way, the data needed to build part or all of a command line.

It often happens that an action requires a large command line containing values accumulated from transitive dependencies. For example, a linker command line might list every object file needed by all of the libraries being linked. It is best practice to store such transitive data in depsets, so that they can be shared by multiple targets. However, if the rule author had to convert these depsets into lists of strings in order to construct an action command line, it would defeat this memory-sharing optimization.

For this reason, the action-constructing functions accept Args objects in addition to strings. Each Args object represents a concatenation of strings and depsets, with optional transformations for manipulating the data. Args objects do not process the depsets they encapsulate until the execution phase, when it comes time to calculate the command line. This helps defer any expensive copying until after the analysis phase is complete. See the Optimizing Performance page for more information.

Args are constructed by calling ctx.actions.args(). They can be passed as the arguments parameter of ctx.actions.run() or ctx.actions.run_shell(). Each mutation of an Args object appends values to the eventual command line.

The map_each feature allows you to customize how items are transformed into strings. If you do not provide a map_each function, the standard conversion is as follows:

  • Values that are already strings are left as-is.
  • File objects are turned into their File.path values.
  • All other types are turned into strings in an unspecified manner. For this reason, you should avoid passing values that are not of string or File type to add(), and if you pass them to add_all() or add_joined() then you should provide a map_each function.

When using string formatting (format, format_each, and format_joined params of the add*() methods), the format template is interpreted in the same way as %-substitution on strings, except that the template must have exactly one substitution placeholder and it must be %s. Literal percents may be escaped as %%. Formatting is applied after the value is converted to a string as per the above.

Each of the add*() methods have an alternate form that accepts an extra positional parameter, an "arg name" string to insert before the rest of the arguments. For add_all and add_joined the extra string will not be added if the sequence turns out to be empty. For instance, the same usage can add either --foo val1 val2 val3 --bar or just --bar to the command line, depending on whether the given sequence contains val1..val3 or is empty.

If the size of the command line can grow longer than the maximum size allowed by the system, the arguments can be spilled over into parameter files. See use_param_file() and set_param_file_format().

Example: Suppose we wanted to generate the command line:

--foo foo1.txt foo2.txt ... fooN.txt --bar bar1.txt,bar2.txt,...,barM.txt --baz
We could use the following Args object:
# foo_deps and bar_deps are depsets containing
# File objects for the foo and bar .txt files.
args = ctx.actions.args()
args.add_all("--foo", foo_deps)
args.add_joined("--bar", bar_deps, join_with=",")
args.add("--baz")
ctx.actions.run(
  ...
  arguments = [args],
  ...
)

add

Args Args.add(arg_name_or_value, value=unbound, *, format=None)

Appends an argument to this command line.

Parameters

Parameter Description
arg_name_or_value required
If two positional parameters are passed this is interpreted as the arg name. The arg name is added before the value without any processing. If only one positional parameter is passed, it is interpreted as value (see below).
value default = unbound
The object to append. It will be converted to a string using the standard conversion mentioned above. Since there is no map_each parameter for this function, value should be either a string or a File. A list, tuple, depset, or directory File must be passed to add_all() or add_joined() instead of this method.
format string; or None; default = None
A format string pattern, to be applied to the stringified version of value.

add_all

Args Args.add_all(arg_name_or_values, values=unbound, *, map_each=None, format_each=None, before_each=None, omit_if_empty=True, uniquify=False, expand_directories=True, terminate_with=None)

Appends multiple arguments to this command line. The items are processed lazily during the execution phase.

Most of the processing occurs over a list of arguments to be appended, as per the following steps:

  1. Each directory File item is replaced by all Files recursively contained in that directory.
  2. If map_each is given, it is applied to each item, and the resulting lists of strings are concatenated to form the initial argument list. Otherwise, the initial argument list is the result of applying the standard conversion to each item.
  3. Each argument in the list is formatted with format_each, if present.
  4. If uniquify is true, duplicate arguments are removed. The first occurrence is the one that remains.
  5. If a before_each string is given, it is inserted as a new argument before each existing argument in the list. This effectively doubles the number of arguments to be appended by this point.
  6. Except in the case that the list is empty and omit_if_empty is true (the default), the arg name and terminate_with are inserted as the first and last arguments, respectively, if they are given.
Note that empty strings are valid arguments that are subject to all these processing steps.

Parameters

Parameter Description
arg_name_or_values required
If two positional parameters are passed this is interpreted as the arg name. The arg name is added before the values without any processing. This arg name will not be added if omit_if_empty is true (the default) and no other items are appended (as happens if values is empty or all of its items are filtered). If only one positional parameter is passed, it is interpreted as values (see below).
values sequence; or depset; default = unbound
The list, tuple, or depset whose items will be appended.
map_each callable; or None; default = None
A function that converts each item to zero or more strings, which may be further processed before appending. If this param is not provided, the standard conversion is used.

The function is passed either one or two positional arguments: the item to convert, followed by an optional DirectoryExpander. The second argument will be passed only if the supplied function is user-defined (not built-in) and declares more than one parameter.

The return value's type depends on how many arguments are to be produced for the item:

  • In the common case when each item turns into one string, the function should return that string.
  • If the item is to be filtered out entirely, the function should return None.
  • If the item turns into multiple strings, the function returns a list of those strings.
Returning a single string or None has the same effect as returning a list of length 1 or length 0 respectively. However, it is more efficient and readable to avoid creating a list where it is not needed.

Ordinarily, items that are directories are automatically expanded to their contents when expand_directories=True is set. However, this will not expand directories contained inside other values -- for instance, when the items are structs that have directories as fields. In this situation, the DirectoryExpander argument can be applied to manually obtain the files of a given directory.

To avoid unintended retention of large analysis-phase data structures into the execution phase, the map_each function must be declared by a top-level def statement; it may not be a nested function closure.

Warning: print() statements that are executed during the call to map_each will not produce any visible output.

format_each string; or None; default = None
An optional format string pattern, applied to each string returned by the map_each function. The format string must have exactly one '%s' placeholder.
before_each string; or None; default = None
An optional string to append before each argument derived from values is appended.
omit_if_empty default = True
If true, if there are no arguments derived from values to be appended, then all further processing is suppressed and the command line will be unchanged. If false, the arg name and terminate_with, if provided, will still be appended regardless of whether or not there are other arguments.
uniquify default = False
If true, duplicate arguments that are derived from values will be omitted. Only the first occurrence of each argument will remain. Usually this feature is not needed because depsets already omit duplicates, but it can be useful if map_each emits the same string for multiple items.
expand_directories default = True
If true, any directories in values will be expanded to a flat list of files. This happens before map_each is applied.
terminate_with string; or None; default = None
An optional string to append after all other arguments. This string will not be added if omit_if_empty is true (the default) and no other items are appended (as happens if values is empty or all of its items are filtered).

add_joined

Args Args.add_joined(arg_name_or_values, values=unbound, *, join_with, map_each=None, format_each=None, format_joined=None, omit_if_empty=True, uniquify=False, expand_directories=True)

Appends an argument to this command line by concatenating together multiple values using a separator. The items are processed lazily during the execution phase.

Processing is similar to add_all(), but the list of arguments derived from values is combined into a single argument as if by join_with.join(...), and then formatted using the given format_joined string template. Unlike add_all(), there is no before_each or terminate_with parameter since these are not generally useful when the items are combined into a single argument.

If after filtering there are no strings to join into an argument, and if omit_if_empty is true (the default), no processing is done. Otherwise if there are no strings to join but omit_if_empty is false, the joined string will be an empty string.

Parameters

Parameter Description
arg_name_or_values required
If two positional parameters are passed this is interpreted as the arg name. The arg name is added before values without any processing. This arg will not be added if omit_if_empty is true (the default) and there are no strings derived from values to join together (which can happen if values is empty or all of its items are filtered).If only one positional parameter is passed, it is interpreted as values (see below).
values sequence; or depset; default = unbound
The list, tuple, or depset whose items will be joined.
join_with required
A delimiter string used to join together the strings obtained from applying map_each and format_each, in the same manner as string.join().
map_each callable; or None; default = None
Same as for add_all.
format_each string; or None; default = None
Same as for add_all.
format_joined string; or None; default = None
An optional format string pattern applied to the joined string. The format string must have exactly one '%s' placeholder.
omit_if_empty default = True
If true, if there are no strings to join together (either because values is empty or all its items are filtered), then all further processing is suppressed and the command line will be unchanged. If false, then even if there are no strings to join together, two arguments will be appended: the arg name followed by an empty string (which is the logical join of zero strings).
uniquify default = False
Same as for add_all.
expand_directories default = True
Same as for add_all.

set_param_file_format

Args Args.set_param_file_format(format)

Sets the format of the param file, if one is used

Parameters

Parameter Description
format required
Must be one of:
  • "multiline": Each item (argument name or value) is written verbatim to the param file with a newline character following it.
  • "shell": Same as "multiline", but the items are shell-quoted
  • "flag_per_line": Same as "multiline", but (1) only flags (beginning with '--') are written to the param file, and (2) the values of the flags, if any, are written on the same line with a '=' separator. This is the format expected by the Abseil flags library.

The format defaults to "shell" if not called.

use_param_file

Args Args.use_param_file(param_file_arg, *, use_always=False)

Spills the args to a params file, replacing them with a pointer to the param file. Use when your args may be too large for the system's command length limits.

Bazel may choose to elide writing the params file to the output tree during execution for efficiency.If you are debugging actions and want to inspect the param file, pass --materialize_param_files to your build.

Parameters

Parameter Description
param_file_arg required
A format string with a single "%s". If the args are spilled to a params file then they are replaced with an argument consisting of this string formatted with the path of the params file.

For example, if the args are spilled to a params file "params.txt", then specifying "--file=%s" would cause the action command line to contain "--file=params.txt".

use_always default = False
Whether to always spill the args to a params file. If false, bazel will decide whether the arguments need to be spilled based on your system and arg length.