Rely on always_iterable to conditionally extend the lib_opts.

distutils/_itertools.py

@@ -0,0 +1,52 @@
+# from more_itertools 10.2
+def always_iterable(obj, base_type=(str, bytes)):
+    """If *obj* is iterable, return an iterator over its items::
+
+        >>> obj = (1, 2, 3)
+        >>> list(always_iterable(obj))
+        [1, 2, 3]
+
+    If *obj* is not iterable, return a one-item iterable containing *obj*::
+
+        >>> obj = 1
+        >>> list(always_iterable(obj))
+        [1]
+
+    If *obj* is ``None``, return an empty iterable:
+
+        >>> obj = None
+        >>> list(always_iterable(None))
+        []
+
+    By default, binary and text strings are not considered iterable::
+
+        >>> obj = 'foo'
+        >>> list(always_iterable(obj))
+        ['foo']
+
+    If *base_type* is set, objects for which ``isinstance(obj, base_type)``
+    returns ``True`` won't be considered iterable.
+
+        >>> obj = {'a': 1}
+        >>> list(always_iterable(obj))  # Iterate over the dict's keys
+        ['a']
+        >>> list(always_iterable(obj, base_type=dict))  # Treat dicts as a unit
+        [{'a': 1}]
+
+    Set *base_type* to ``None`` to avoid any special handling and treat objects
+    Python considers iterable as iterable:
+
+        >>> obj = 'foo'
+        >>> list(always_iterable(obj, base_type=None))
+        ['f', 'o', 'o']
+    """
+    if obj is None:
+        return iter(())
+
+    if (base_type is not None) and isinstance(obj, base_type):
+        return iter((obj,))
+
+    try:
+        return iter(obj)
+    except TypeError:
+        return iter((obj,))

distutils/ccompiler.py

@@ -21,6 +21,7 @@
 from ._modified import newer_group
 from .util import split_quoted, execute
 from ._log import log
+from ._itertools import always_iterable
 
 
 class CCompiler:
@@ -1233,11 +1234,7 @@ def gen_lib_options(compiler, library_dirs, runtime_library_dirs, libraries):
         lib_opts.append(compiler.library_dir_option(dir))
 
     for dir in runtime_library_dirs:
-        opt = compiler.runtime_library_dir_option(dir)
-        if isinstance(opt, list):
-            lib_opts = lib_opts + opt
-        else:
-            lib_opts.append(opt)
+        lib_opts.extend(always_iterable(compiler.runtime_library_dir_option(dir)))
 
     # XXX it's important that we *not* remove redundant library mentions!
     # sometimes you really do have to say "-lfoo -lbar -lfoo" in order to