Refactoring for parameter extensions (#44)

defs.bzl

diff --git a/defs.bzl b/defs.bzl
index 45bb84b..c956339 100644
--- a/defs.bzl
+++ b/defs.bzl
@@ -2,7 +2,6 @@
 
 DEFAULT_REPOSITORY_NAME = "pip"
 
-
 def _pip_repository_impl(rctx):
     python_interpreter = rctx.attr.python_interpreter
     if rctx.attr.python_interpreter_target != None:
@@ -25,33 +24,34 @@
     ]
     pypath = ":".join([str(p) for p in [rules_root] + thirdparty_roots])
 
+    args = [
+        python_interpreter,
+        "-m",
+        "extract_wheels",
+        "--requirements",
+        rctx.path(rctx.attr.requirements),
+        "--repo",
+        "@%s" % rctx.attr.name,
+    ]
+
     result = rctx.execute(
-        [
-            python_interpreter,
-            "-m",
-            "extract_wheels",
-            "--requirements",
-            rctx.path(rctx.attr.requirements),
-            "--repo",
-            "@%s" % rctx.attr.name,
-        ],
-        environment={
+        args,
+        environment = {
             # Manually construct the PYTHONPATH since we cannot use the toolchain here
-            "PYTHONPATH": pypath
+            "PYTHONPATH": pypath,
         },
-        timeout=rctx.attr.timeout,
+        timeout = rctx.attr.timeout,
     )
     if result.return_code:
         fail("rules_python_external failed: %s (%s)" % (result.stdout, result.stderr))
 
     return
 
-
 pip_repository = repository_rule(
-    attrs={
-        "requirements": attr.label(allow_single_file=True, mandatory=True,),
+    attrs = {
+        "requirements": attr.label(allow_single_file = True, mandatory = True),
         "wheel_env": attr.string_dict(),
-        "python_interpreter": attr.string(default="python3"),
+        "python_interpreter": attr.string(default = "python3"),
         "python_interpreter_target": attr.label(allow_single_file = True, doc = """
 If you are using a custom python interpreter built by another repository rule,
 use this attribute to specify its BUILD target. This allows pip_repository to invoke
@@ -61,11 +61,12 @@
         # 600 is documented as default here: https://docs.bazel.build/versions/master/skylark/lib/repository_ctx.html#execute
         "timeout": attr.int(default = 600),
     },
-    implementation=_pip_repository_impl,
+    implementation = _pip_repository_impl,
 )
 
-
-def pip_install(requirements, name=DEFAULT_REPOSITORY_NAME, **kwargs):
+def pip_install(requirements, name = DEFAULT_REPOSITORY_NAME, **kwargs):
     pip_repository(
-        name=name, requirements=requirements, **kwargs
+        name = name,
+        requirements = requirements,
+        **kwargs
     )

extract_wheels/__init__.py

diff --git a/extract_wheels/__init__.py b/extract_wheels/__init__.py
index 390dcd0..89c7c59 100644
--- a/extract_wheels/__init__.py
+++ b/extract_wheels/__init__.py
@@ -65,9 +65,10 @@
     )
     args = parser.parse_args()
 
+    pip_args = [sys.executable, "-m", "pip", "wheel", "-r", args.requirements]
     # Assumes any errors are logged by pip so do nothing. This command will fail if pip fails
     subprocess.check_output(
-        [sys.executable, "-m", "pip", "wheel", "-r", args.requirements]
+        pip_args
     )
 
     extras = requirements.parse_extras(args.requirements)