Add support for 'functools.partial' like functionality. (#34)

* Add support for 'functools.partial' like functionality.

https://docs.python.org/3/library/functools.html#functools.partial

BUILD

@@ -24,6 +24,7 @@ skylark_library(
     deps = [
         "//lib:collections",
         "//lib:dicts",
+        "//lib:partial",
         "//lib:paths",
         "//lib:selects",
         "//lib:sets",

CONTRIBUTORS

@@ -15,3 +15,4 @@ Nathan Herring <nherring@google.com>
 Laurent Le Brun <laurentlb@google.com>
 Dmitry Lomov <dslomov@google.com>
 Jingwen Chen <jingwen@google.com>
+Dave MacLachlan <dmaclach@google.com>

lib.bzl

@@ -16,6 +16,7 @@
 
 load("//lib:collections.bzl", "collections")
 load("//lib:dicts.bzl", "dicts")
+load("//lib:partial.bzl", "partial")
 load("//lib:paths.bzl", "paths")
 load("//lib:selects.bzl", "selects")
 load("//lib:sets.bzl", "sets")

lib/BUILD

@@ -21,6 +21,11 @@ skylark_library(
     srcs = ["dicts.bzl"],
 )
 
+skylark_library(
+    name = "partial",
+    srcs = ["partial.bzl"],
+)
+
 skylark_library(
     name = "paths",
     srcs = ["paths.bzl"],

lib/partial.bzl

@@ -0,0 +1,130 @@
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Skylark module for working with function objects where some parameters are
+   bound before the call.
+
+Similar to https://docs.python.org/3/library/functools.html#functools.partial.
+"""
+
+def _call(partial, *args, **kwargs):
+  """Calls a partial created using `make`.
+
+  Args:
+    partial: The partial to be called.
+    *args: Additional positional arguments to be appended to the ones given to
+           make.
+    **kwargs: Additional keyword arguments to augment and override the ones
+              given to make.
+
+  Returns:
+    Whatever the function in the partial returns.
+  """
+  function_args = partial.args + args
+  function_kwargs = dict(partial.kwargs)
+  function_kwargs.update(kwargs)
+  return partial.function(*function_args, **function_kwargs)
+
+def _make(func, *args, **kwargs):
+  """Creates a partial that can be called using `call`.
+
+  A partial can have args assigned to it at the make site, and can have args
+  passed to it at the call sites.
+
+  A partial 'function' can be defined with positional args and kwargs:
+
+    # function with no args
+    def function1():
+      ...
+
+    # function with 2 args
+    def function2(arg1, arg2):
+      ...
+
+    # function with 2 args and keyword args
+    def function3(arg1, arg2, x, y):
+      ...
+
+  The positional args passed to the function are the args passed into make
+  followed by any additional positional args given to call. The below example
+  illustrates a function with two positional arguments where one is supplied by
+  make and the other by call:
+
+    # function demonstrating 1 arg at make site, and 1 arg at call site
+    def _foo(make_arg1, func_arg1):
+	  print(make_arg1 + " " + func_arg1 + "!")
+
+  For example:
+
+    hi_func = partial.make(_foo, "Hello")
+    bye_func = partial.make(_foo, "Goodbye")
+    partial.call(hi_func, "Jennifer")
+    partial.call(hi_func, "Dave")
+    partial.call(bye_func, "Jennifer")
+    partial.call(bye_func, "Dave")
+
+  prints:
+  
+    "Hello, Jennifer!"
+    "Hello, Dave!"
+    "Goodbye, Jennifer!"
+    "Goodbye, Dave!"
+
+  The keyword args given to the function are the kwargs passed into make
+  unioned with the keyword args given to call. In case of a conflict, the
+  keyword args given to call take precedence. This allows you to set a default
+  value for keyword arguments and override it at the call site.
+
+  Example with a make site arg, a call site arg, a make site kwarg and a
+  call site kwarg:
+  
+    def _foo(make_arg1, call_arg1, make_location, call_location):
+      print(make_arg1 + " is from " + make_location + " and " +
+            call_arg1 + " is from " + call_location + "!")
+
+    func = partial.make(_foo, "Ben", make_location="Hollywood")
+    partial.call(func, "Jennifer", call_location="Denver")
+
+  Prints "Ben is from Hollywood and Jennifer is from Denver!".
+
+    partial.call(func, "Jennifer", make_location="LA", call_location="Denver")
+
+  Prints "Ben is from LA and Jennifer is from Denver!". 
+  
+  Note that keyword args may not overlap with positional args, regardless of
+  whether they are given during the make or call step. For instance, you can't
+  do:
+
+	def foo(x):
+	  pass
+
+	func = partial.make(foo, 1)
+	partial.call(func, x=2)
+
+
+  Args:
+    func: The function to be called.
+    *args: Positional arguments to be passed to function.
+    **kwargs: Keyword arguments to be passed to function. Note that these can
+              be overridden at the call sites.
+
+  Returns:
+    A new `partial` that can be called using `call`
+  """
+  return struct(function=func, args=args, kwargs=kwargs)
+
+partial = struct(
+    make=_make,
+    call=_call,
+)

tests/BUILD

@@ -1,5 +1,6 @@
 load(":collections_tests.bzl", "collections_test_suite")
 load(":dicts_tests.bzl", "dicts_test_suite")
+load(":partial_tests.bzl", "partial_test_suite")
 load(":paths_tests.bzl", "paths_test_suite")
 load(":selects_tests.bzl", "selects_test_suite")
 load(":sets_tests.bzl", "sets_test_suite")
@@ -13,6 +14,8 @@ collections_test_suite()
 
 dicts_test_suite()
 
+partial_test_suite()
+
 paths_test_suite()
 
 selects_test_suite()

tests/partial_tests.bzl

@@ -0,0 +1,85 @@
+# Copyright 2018 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Unit tests for partial.bzl."""
+
+load("//:lib.bzl", "partial", "asserts", "unittest")
+
+
+def _make_noargs_nokwargs():
+  """Test utility for no args no kwargs case"""
+  return 1
+
+def _make_args_nokwargs(arg1, arg2, arg3):
+  """Test utility for args no kwargs case"""
+  return arg1 + arg2 + arg3
+
+def _make_args_kwargs(arg1, arg2, arg3, **kwargs):
+  """Test utility for args and kwargs case"""
+  return arg1 + arg2 + arg3 + kwargs["x"] + kwargs["y"]
+
+def _call_noargs_nokwargs(call_arg1):
+  """Test utility no args no kwargs case where values passed from call site"""
+  return call_arg1;
+
+def _call_args_nokwargs(func_arg1, call_arg1):
+  """Test utility for args no kwargs case where values passed from call site"""
+  return func_arg1 + call_arg1;
+
+def _call_args_kwargs(func_arg1, call_arg1, func_mult, call_mult):
+  """Test utility for args and kwargs case where values passed from call site"""
+  return (func_arg1 + call_arg1) * func_mult * call_mult;
+
+def _make_call_test(ctx):
+  """Unit tests for partial.make and partial.call."""
+  env = unittest.begin(ctx)
+
+  # Test cases where there are no args (or kwargs) at the make site, only
+  # at the call site.
+  foo = partial.make(_make_noargs_nokwargs)
+  asserts.equals(env, 1, partial.call(foo))
+
+  foo = partial.make(_make_args_nokwargs)
+  asserts.equals(env, 6, partial.call(foo, 1, 2, 3))
+
+  foo = partial.make(_make_args_kwargs)
+  asserts.equals(env, 15, partial.call(foo, 1, 2, 3, x=4, y=5))
+
+  # Test cases where there are args (and/or kwargs) at the make site and the
+  # call site.
+  foo = partial.make(_call_noargs_nokwargs, 100)
+  asserts.equals(env, 100, partial.call(foo))
+
+  foo = partial.make(_call_args_nokwargs, 100)
+  asserts.equals(env, 112, partial.call(foo, 12))
+
+  foo = partial.make(_call_args_kwargs, 100, func_mult=10)
+  asserts.equals(env, 2240, partial.call(foo, 12, call_mult=2))
+
+  # Test case where there are args and kwargs ath the make site, and the call
+  # site overrides some make site args.
+  foo = partial.make(_call_args_kwargs, 100, func_mult=10)
+  asserts.equals(env, 1120, partial.call(foo, 12, func_mult=5, call_mult=2))
+
+  unittest.end(env)
+
+make_call_test = unittest.make(_make_call_test)
+
+
+def partial_test_suite():
+  """Creates the test targets and test suite for partial.bzl tests."""
+  unittest.suite(
+      "partial_tests",
+      make_call_test,
+  )