_frontend: Allow printing dependencies using `bst show`

At present, there isn't an easy way to print anything from `bst show`
that would give the users an idea of what the dependency graph looks
like. One could use things like `--deps build`, but that will just print
a list, without any information about the dependency edges.

Add `%{deps}`, `%{build-deps}` and `%{runtime-deps}` format strings to
`bst show` that would simply print the list of all dependencies, build
dependencies and runtime dependencies respectively.

Summary of changes:

  * buildstream/_frontend/cli.py: Add help for new format symbols.
  * buildstream/_frontend/widget.py: Add support for new format symbols
                                     for dependencies.
  * tests/frontend/show.py: Add tests for new format symbols.

3 files changed, 50 insertions, 1 deletions

diff --git a/buildstream/_frontend/cli.py b/buildstream/_frontend/cli.py
index 49d5717b4..b3c4634a9 100644
--- a/buildstream/_frontend/cli.py
+++ b/buildstream/_frontend/cli.py
@@ -440,6 +440,9 @@ def show(app, elements, deps, except_, order, format_):
         %{public}         Public domain data
         %{workspaced}     If the element is workspaced
         %{workspace-dirs} A list of workspace directories
+        %{deps}           A list of all dependencies
+        %{build-deps}     A list of build dependencies
+        %{runtime-deps}   A list of runtime dependencies
 
     The value of the %{symbol} without the leading '%' character is understood
     as a pythonic formatting string, so python formatting features apply,
diff --git a/buildstream/_frontend/widget.py b/buildstream/_frontend/widget.py
index de122f8a3..b8e3d920a 100644
--- a/buildstream/_frontend/widget.py
+++ b/buildstream/_frontend/widget.py
@@ -27,7 +27,7 @@ from ruamel import yaml
 import click
 
 from . import Profile
-from .. import Element, Consistency
+from .. import Element, Consistency, Scope
 from .. import _yaml
 from .. import __version__ as bst_version
 from .._exceptions import ImplError
@@ -435,6 +435,27 @@ class LogLine(Widget):
                     line = p.fmt_subst(
                         line, 'workspace-dirs', '')
 
+            # Dependencies
+            if "%{deps" in format_:
+                deps = [e.name for e in element.dependencies(Scope.ALL, recurse=False)]
+                line = p.fmt_subst(
+                    line, 'deps',
+                    yaml.safe_dump(deps, default_style=None).rstrip('\n'))
+
+            # Build Dependencies
+            if "%{build-deps" in format_:
+                build_deps = [e.name for e in element.dependencies(Scope.BUILD, recurse=False)]
+                line = p.fmt_subst(
+                    line, 'build-deps',
+                    yaml.safe_dump(build_deps, default_style=False).rstrip('\n'))
+
+            # Runtime Dependencies
+            if "%{runtime-deps" in format_:
+                runtime_deps = [e.name for e in element.dependencies(Scope.RUN, recurse=False)]
+                line = p.fmt_subst(
+                    line, 'runtime-deps',
+                    yaml.safe_dump(runtime_deps, default_style=False).rstrip('\n'))
+
             report += line + '\n'
 
         return report.rstrip('\n')
diff --git a/tests/frontend/show.py b/tests/frontend/show.py
index ad3ae3591..ac1edebd6 100644
--- a/tests/frontend/show.py
+++ b/tests/frontend/show.py
@@ -400,3 +400,28 @@ def test_exceed_max_recursion_depth(cli, tmpdir, dependency_depth):
         assert result.exit_code == -1
 
     shutil.rmtree(project_path)
+
+
+###############################################################
+#                   Testing format symbols                    #
+###############################################################
+@pytest.mark.datafiles(os.path.join(DATA_DIR, 'project'))
+@pytest.mark.parametrize("dep_kind, expected_deps", [
+    ('%{deps}', '[import-dev.bst, import-bin.bst]'),
+    ('%{build-deps}', '[import-dev.bst]'),
+    ('%{runtime-deps}', '[import-bin.bst]')
+])
+def test_format_deps(cli, datafiles, dep_kind, expected_deps):
+    project = os.path.join(datafiles.dirname, datafiles.basename)
+    target = 'checkout-deps.bst'
+    result = cli.run(project=project, silent=True, args=[
+        'show',
+        '--deps', 'none',
+        '--format', '%{name}: ' + dep_kind,
+        target])
+    result.assert_success()
+
+    expected = '{name}: {deps}'.format(name=target, deps=expected_deps)
+    if result.output.strip() != expected:
+        raise AssertionError("Expected output:\n{}\nInstead received output:\n{}"
+                             .format(expected, result.output))