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Options

A deep dive into how options may be configured.

Option scopes

Options are partitioned into named scopes.

Some systemwide options belong in the global scope. For example, the --level option, which controls the logging level, is in the global scope.

Other options belong to a subsystem scope. A subsystem is simply a collection of related options, in a scope. For example, the pytest subsystem contains options related to Python's test framework pytest.

Setting options

Every option can be set in the following ways, in order of precedence:

  1. Via a command line flag.
  2. In an environment variable.
  3. In a config file (pants.toml).

If an option isn't set in one of these ways, it will take on a default value.

You can inspect both the current value and the default value by using ./pants help $scope or ./pants help-advanced $scope, e.g. ./pants help global.

Command-line flags

Global options are set using an unqualified flag:

./pants --level=debug ...

Subsystem options are set by providing the flag, with the name prefixed with the lower-case scope name and a dash. So for the option --root-patterns in the scope source:

./pants --source-root-patterns="['^ext']"

Environment variables

Global options are set using the environment variable PANTS_{OPTION_NAME}:

PANTS_LEVEL=debug ./pants ...

Subsystem options are set using the environment variable
PANTS_{SCOPE}_{OPTION_NAME}:

PANTS_SOURCE_ROOT_PATTERNS="['^ext']" ./pants ...

Note that the scope and option name are upper-cased, and any dashes in the option flag name are converted to underscores: --multiword-name becomes MULTIWORD_NAME.

Config file entries

Global options are set in the GLOBAL section of the config file:

[GLOBAL]
level = "debug"

Subsystem options are set in the section named for their scope:

[source]
root_patterns = ["/src/python"]

Note that any dashes in the option flag name are converted to underscores: --multiword-name becomes multiword_name.

Additionally, a few different variables may be interpolated into strings in config files via a %(var)s syntax. For example, this expands to the absolute path of a file in the root of your repository:

[GLOBAL]
pythonpath = ["%(buildroot)s/examples"]

Option types

Every option has a type, and any values you set must be of that type.

The option types are:

  • string
  • integer
  • bool
  • list
  • dict

A list-valued option may also declare a specific type for its members (e.g., a list of strings, or a list of integers).

String and integer values

Standalone string and integer values are written without quotes. Any quotes will be considered part of the value, after shell escaping.

Command-line flags:

./pants --scope-intopt=42
./pants --scope-stropt=qux

Environment variables:

PANTS_SCOPE_INTOPT=42
PANTS_SCOPE_STROPT=qux

Config file entries:

[scope]
intopt = 42
stropt = "qux"

Boolean values

Boolean values can be specified using the special strings true and false. When specifying them via command-line flags you can also use the --boolopt/--no-boolopt syntax.

Command-line flags:

./pants --scope-boolopt=true
./pants --scope-boolopt
./pants --no-scope-boolopt

Environment variables:

PANTS_SCOPE_BOOLOPT=true

Config file entries:

[scope]
boolopt = true

List values

List values are parsed as Python list literals, so you must quote string values, and you may need to apply shell-level quoting and/or escaping, as required.

Command-line flags:

./pants --scope-listopt="['foo','bar']"

You can also leave off the [] to append elements. So we can rewrite the above to:

./pants --scope-listopt=foo --scope-listopt=bar

Environment variables:

PANTS_SCOPE_LISTOPT="['foo','bar']"

Like with command-line flags, you can leave off the [] to append elements:

PANTS_SCOPE_LISTOPT=foo

Config file entries:

[scope]
listopt = [
  'foo', 
  'bar'
]

Add/remove semantics

List values have some extra semantics:

  • A value can be preceded by +, which will append the elements to the value obtained from lower-precedence sources.
  • A value can be preceded by -, which will remove the elements from the value obtained from lower-precedence sources.
  • Multiple + and - values can be provided, separated by commas.
  • Otherwise, the value replaces the one obtained from lower-precedence sources.

For example, if the value of --listopt in scope is set to [1, 2] in a config file, then

./pants --scope-listopt="+[3,4]"

will set the value to [1, 2, 3, 4].

./pants --scope-listopt="-[1],+[3,4]"

will set the value to [2, 3, 4], and

./pants --scope-listopt="[3,4]"

will set the value to [3, 4].

📘

Add/remove syntax in .toml files

The +/- syntax works in .toml files, but the entire value must be quoted:

[scope]
listopt = "+[1,2],-[3,4]"

This means that TOML treats the value as a string, instead of a TOML list.

Alternatively, you can use this syntactic sugar, which allows the values to be regular TOML lists:

[scope]
listopt.add = [1, 2]
listopt.remove = [3, 4]

But note that this only works in Pants's .toml config files, not in environment variables or command-line flags.

Dict values

Dict values are parsed as Python dict literals on the command-line and environment variables, so you must quote string keys and values, and you may need to apply shell-level quoting and/or escaping, as required.

Command-line flags:

./pants --scope-dictopt="{'foo':1,'bar':2}"

Environment variables:

PANTS_SCOPE_DICTOPT="{'foo':1,'bar':2}"

Config file entries:

You can use TOML's nested table features. These are equivalent:

[scope]
dictopt = { foo = 1, bar = 2}
[scope.dictopt]
foo = 1
bar = 2

You can also use a string literal. Note the quotes:

[scope]
dictopt = """{
 'foo': 1,
 'bar': 2,
}"""

Add/replace semantics

  • A value can be preceded by +, which will update the value obtained from lower-precedence sources with the entries.
  • Otherwise, the value replaces the one obtained from lower-precendence sources.

For example, if the value of --dictopt in scope is set to {'foo', 1, 'bar': 2} in a config file, then

./pants --scope-dictopt="+{'foo':42,'baz':3}"

will set the value to {'foo': 42, 'bar': 2, 'baz': 3}, and

./pants --scope-dictopt="{'foo':42,'baz':3}"

will set the value to {'foo': 42, 'baz': 3}.

.pants.rc file

You can set up personal Pants config files, using the same TOML syntax as pants.toml. By default, Pants looks for the paths /etc/pantsrc, ~/.pants.rc, and .pants.rc in the repository root.

For example:

[python]
# Even though our repository uses 3.8+, because I have an M1, 
# I must use Python 3.9+.
interpreter_constraints = ["==3.9.*"]

If you want to ban this feature, set [GLOBAL].pantsrc = false in pants.toml.


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