Allow for requirements files to differ per platform

As a common example, we need a compiled requirements file for linux that differs from mac os

Author: Alex Eagle

python/pip.bzl

@@ -16,7 +16,7 @@
 load("//python/pip_install:pip_repository.bzl", "pip_repository")
 load("//python/pip_install:repositories.bzl", "pip_install_dependencies")
 
-def pip_install(requirements, name = "pip", **kwargs):
+def pip_install(requirements = None, name = "pip", **kwargs):
     """Imports a `requirements.txt` file and generates a new `requirements.bzl` file.
 
     This is used via the `WORKSPACE` pattern:

python/pip_install/pip_repository.bzl

@@ -101,6 +101,15 @@ def _pip_repository_impl(rctx):
 
     pypath = _construct_pypath(rctx)
 
+    os = rctx.os.name.lower()
+    requirements_txt = rctx.attr.requirements
+
+    if os in rctx.attr.platform_requirements:
+        requirements_txt = Label(rctx.attr.platform_requirements[os])
+
+    if requirements_txt == None:
+        fail("Expected one of requirements attribute, or platform_requirements attribute to have key " + os)
+
     if rctx.attr.incremental:
         args = [
             python_interpreter,
@@ -120,7 +129,7 @@ def _pip_repository_impl(rctx):
             "-m",
             "python.pip_install.extract_wheels",
             "--requirements",
-            rctx.path(rctx.attr.requirements),
+            rctx.path(requirements_txt),
         ]
 
     args += ["--repo", rctx.attr.name]
@@ -208,6 +217,9 @@ pip_repository_attrs = {
         allow_single_file = True,
         doc = "A 'requirements.txt' pip requirements file.",
     ),
+    "platform_requirements": attr.string_dict(
+        doc = "Override the requirements attribute when running on a matching host platform",
+    ),
     "requirements_lock": attr.label(
         allow_single_file = True,
         doc = """