get variable from config_setting in bazel

Question

I have the following config_setting defined:

config_setting(
    name = "perception_env",
    values = {"perception": "true"},
)

print(perception_env)

However, I can't seem to print the variable, it says it doesn't exist.

Answer 1

config_setting is only used for selecting the different possible values in a select(). A config_setting doesn't really have a value, it's more an association of a variable (a Bazel flag, a Starlark-defined flag, platform constraints) and its value. The values attribute is basically for flag values ("perception" would have to be a bazel flag).

For example,

config_setting(
    name = "my_config_setting_opt",
    values = {"compilation_mode": "opt"}
)

config_setting(
    name = "config_setting_dbg",
    values = {"compilation_mode": "dbg"}
)

config_setting(
    name = "config_setting_fastbuild",
    values = {"compilation_mode": "fastbuild"}
)

genrule(
  name = "gen_out",
  outs = ["out"],
  cmd = select({
    ":my_config_setting_opt": "echo Opt mode > $@",
    ":config_setting_dbg": "echo Dbg mode > $@",
    ":config_setting_fastbuild": "echo Fastbuild mode > $@",
  }),
)

The 3 config_settings declare 3 different associations of the --compilation_mode flag, one for each of its possible values (see https://bazel.build/docs/user-manual#compilation-mode)

Then the select() declares 3 different possible values for the cmd attribute of the genrule gen_out. Then setting the --compilation_mode flag to different values changes which value for cmd is selected:

$ bazel build out --compilation_mode=dbg && cat bazel-bin/out
INFO: Build option --compilation_mode has changed, discarding analysis cache.
INFO: Analyzed target //:out (0 packages loaded, 11 targets configured).
INFO: Found 1 target...
Target //:out up-to-date:
  bazel-bin/out
INFO: Elapsed time: 0.145s, Critical Path: 0.01s
INFO: 2 processes: 1 internal, 1 linux-sandbox.
INFO: Build completed successfully, 2 total actions
Dbg mode

$ bazel build out --compilation_mode=opt && cat bazel-bin/out
INFO: Build option --compilation_mode has changed, discarding analysis cache.
INFO: Analyzed target //:out (0 packages loaded, 11 targets configured).
INFO: Found 1 target...
Target //:out up-to-date:
  bazel-bin/out
INFO: Elapsed time: 0.111s, Critical Path: 0.01s
INFO: 2 processes: 1 internal, 1 linux-sandbox.
INFO: Build completed successfully, 2 total actions
Opt mode

$ bazel build out --compilation_mode=fastbuild && cat bazel-bin/out
INFO: Build option --compilation_mode has changed, discarding analysis cache.
INFO: Analyzed target //:out (0 packages loaded, 11 targets configured).
INFO: Found 1 target...
Target //:out up-to-date:
  bazel-bin/out
INFO: Elapsed time: 0.145s, Critical Path: 0.01s
INFO: 1 process: 1 internal.
INFO: Build completed successfully, 1 total action
Fastbuild mode