feat: create a linter aspect for flake8

Second commit in "how to add a new linter".

example/.aspect/cli/config.yaml

@@ -7,3 +7,4 @@ plugins:
       lint_aspects:
         - //:lint.bzl%eslint
         - //:lint.bzl%buf
+        - //:lint.bzl%flake8

example/lint.bzl

@@ -2,6 +2,7 @@
 
 load("@aspect_rules_lint//lint:eslint.bzl", "eslint_aspect")
 load("@aspect_rules_lint//lint:buf.bzl", "buf_lint_aspect")
+load("@aspect_rules_lint//lint:flake8.bzl", "flake8_aspect")
 
 buf = buf_lint_aspect(
     config = "@@//:buf.yaml",
@@ -11,3 +12,8 @@ eslint = eslint_aspect(
     binary = "@@//:eslint",
     config = "@@//:eslintrc",
 )
+
+flake8 = flake8_aspect(
+    binary = "@@//:flake8",
+    config = "@@//:.flake8",
+)

example/lint.sh

@@ -13,7 +13,7 @@ if [ "$#" -eq 0 ]; then
 fi
 
 # Produce report files
-bazel build --aspects //:lint.bzl%eslint,//:lint.bzl%buf --output_groups=report $@
+bazel build --aspects //:lint.bzl%eslint,//:lint.bzl%buf,//:lint.bzl%flake8 --output_groups=report $@
 
 # Show the results.
 # `-mtime -1`: only look at files modified in the last day, to mitigate showing stale results of old bazel runs.

lint/flake8.bzl

@@ -0,0 +1,93 @@
+"""API for declaring a flake8 lint aspect that visits py_library rules.
+
+Typical usage:
+
+```
+load("@aspect_rules_lint//lint:flake8.bzl", "flake8_aspect")
+
+flake8 = flake8_aspect(
+    binary = "@@//:flake8",
+    config = "@@//:.flake8",
+)
+```
+"""
+
+def flake8_action(ctx, executable, srcs, config, report, use_exit_code = False):
+    """Run flake8 as an action under Bazel.
+    
+    Based on https://flake8.pycqa.org/en/latest/user/invocation.html
+
+    Args:
+        ctx: Bazel Rule or Aspect evaluation context
+        executable: label of the the flake8 program
+        srcs: python files to be linted
+        config: label of the flake8 config file (setup.cfg, tox.ini, or .flake8)
+        report: output file to generate
+        use_exit_code: whether to fail the build when a lint violation is reported
+    """
+    inputs = srcs + [config]
+    outputs = [report]
+
+    # Wire command-line options, see
+    # https://flake8.pycqa.org/en/latest/user/options.html
+    args = ctx.actions.args()
+    args.add_all(srcs)
+    args.add(report, format="--output-file=%s")
+    args.add(config, format="--config=%s")
+    if not use_exit_code:
+        args.add("--exit-zero")
+
+    ctx.actions.run(
+        inputs = inputs,
+        outputs = outputs,
+        executable = executable,
+        arguments = [args],
+        mnemonic = "flake8",
+    )
+
+# buildifier: disable=function-docstring
+def _flake8_aspect_impl(target, ctx):
+    if ctx.rule.kind in ["py_library"]:
+        report = ctx.actions.declare_file(target.label.name + ".flake8-report.txt")
+        flake8_action(ctx, ctx.executable._flake8, ctx.rule.files.srcs, ctx.file._config_file, report)
+        results = depset([report])
+    else:
+        results = depset()
+
+    return [
+        OutputGroupInfo(report = results),
+    ]
+
+def flake8_aspect(binary, config):
+    """A factory function to create a linter aspect.
+
+    Attrs:
+        binary: a flake8 executable. Can be obtained from rules_python like so:
+
+            ```
+            load("@rules_python//python/entry_points:py_console_script_binary.bzl", "py_console_script_binary")
+
+            py_console_script_binary(
+                name = "flake8",
+                pkg = "@pip//flake8:pkg",
+            )
+            ```
+        config: the flake8 config file (`setup.cfg`, `tox.ini`, or `.flake8`)
+    """
+    return aspect(
+        implementation = _flake8_aspect_impl,
+        # Edges we need to walk up the graph from the selected targets.
+        # Needed for linters that need semantic information like transitive type declarations.
+        # attr_aspects = ["deps"],
+        attrs = {
+            "_flake8": attr.label(
+                default = binary,
+                executable = True,
+                cfg = "exec",
+            ),
+            "_config_file": attr.label(
+                default = config,
+                allow_single_file = True,
+            ),
+        },
+    )