New method in_stdlib to efficiently check if a module name is part of std lib.

It relies on functools' `@lru_cache` to cache the stdlib list and query results for similar calls.
Therefore it is much more efficient than `module_name in stdlib_list()` especially if you wish to perform multiple checks.

Author: Sylvain MARIE

stdlib_list/__init__.py

@@ -3,4 +3,4 @@
 del get_versions
 
 # Import all the things that used to be in here for backwards-compatibility reasons
-from .base import stdlib_list, get_canonical_version, short_versions, long_versions, base_dir, list_dir
+from .base import stdlib_list, in_stdlib, get_canonical_version, short_versions, long_versions, base_dir, list_dir

stdlib_list/base.py

@@ -3,6 +3,11 @@
 import os
 import sys
 
+try:
+    from functools import lru_cache
+except ImportError:
+    from functools32 import lru_cache
+
 base_dir = os.path.dirname(os.path.realpath(__file__))
 
 list_dir = os.path.join(base_dir, "lists")
@@ -45,3 +50,36 @@ def stdlib_list(version=None):
         result = [y for y in [x.strip() for x in f.readlines()] if y]
 
     return result
+
+
+@lru_cache(maxsize=16)
+def _stdlib_list_with_cache(version=None):
+    """Internal cached version of `stdlib_list`"""
+    return stdlib_list(version=version)
+
+
+@lru_cache(maxsize=256)
+def in_stdlib(module_name, version=None):
+    """
+    Return a ``bool`` indicating if module ``module_name`` is in the list of stdlib
+    symbols for python version ``version``. If ``version`` is ``None`` (default), the
+    version of current python interpreter is used.
+
+    Note that ``True`` will be returned for built-in modules too, since this project
+    considers they are part of stdlib. See :issue:21.
+
+    It relies on ``@lru_cache`` to cache the stdlib list and query results for similar
+    calls. Therefore it is much more efficient than ``module_name in stdlib_list()``
+    especially if you wish to perform multiple checks.
+
+    :param str|None module_name: The module name (as a string) to query for.
+    :param str|None version: The version (as a string) whose list of libraries you want
+    (one of ``"2.6"``, ``"2.7"``, ``"3.2"``, ``"3.3"``, ``"3.4"``, or ``"3.5"``).
+    If not specified, the current version of Python will be used.
+
+    :return: A bool indicating if the given module name is part of standard libraries
+    for the specified version of Python.
+    :rtype: list
+    """
+    ref_list = _stdlib_list_with_cache(version=version)
+    return module_name in ref_list