Use sys.modules.get() in the "with _ModuleLockManager(name):" block
to protect the dictionary key with the module lock and use an atomic
get to prevent race condition.

Remove also _bootstrap._POPULATE since it was unused
(_bootstrap_external now has its own _POPULATE object), add a new
_SENTINEL object instead.

instead of failing with SystemError.

Relative import from non-package now fails with ImportError rather than
SystemError.

* Rewrite importlib _get_module_lock(): it is now responsible to hold
  the imp lock directly.
* _find_and_load() now holds the module lock to check if name is in
  sys.modules to prevent a race condition

bpo-30436: Raise ModuleNotFoundError for importlib.util.find_spec() when parent isn't a package (GH-1899)

Previously AttributeError was raised, but that's not very reflective of the fact that the requested module can't be found since the specified parent isn't actually a package.

PEP 432 specifies a number of large changes to interpreter startup code, including exposing a cleaner C-API. The major changes depend on a number of smaller changes. This patch includes all those smaller changes.

Special thanks to INADA Naoki for pushing the patch through
the last mile, Serhiy Storchaka for reviewing the code, and to
Victor Stinner for suggesting the idea (originally implemented
in the PyPy project).

Handling zero-argument super() in __init_subclass__ and
__set_name__ involved moving __class__ initialisation to
type.__new__. This requires cooperation from custom
metaclasses to ensure that the new __classcell__ entry
is passed along appropriately.

The initial implementation of that change resulted in abruptly
broken zero-argument super() support in metaclasses that didn't
adhere to the new requirements (such as Django's metaclass for
Model definitions).

The updated approach adopted here instead emits a deprecation
warning for those cases, and makes them work the same way they
did in Python 3.5.

This patch also improves the related class machinery documentation
to cover these details and to include more reader-friendly
cross-references and index entries.

a var-positional argument.  Added opcode BUILD_TUPLE_UNPACK_WITH_CALL.

The __class__ cell used by zero-argument super() is now initialized
from type.__new__ rather than __build_class__, so class methods
relying on that will now work correctly when called from metaclass
methods during class creation.

Patch by Martin Teichmann.

Issue #27213: Rework CALL_FUNCTION* opcodes to produce shorter and more
efficient bytecode:

* CALL_FUNCTION now only accepts position arguments
* CALL_FUNCTION_KW accepts position arguments and keyword arguments, but keys
  of keyword arguments are packed into a constant tuple.
* CALL_FUNCTION_EX is the most generic, it expects a tuple and a dict for
  positional and keyword arguments.

CALL_FUNCTION_VAR and CALL_FUNCTION_VAR_KW opcodes have been removed.

2 tests of test_traceback are currently broken: skip test, the issue #28050 was
created to track the issue.

Patch by Demur Rumed, design by Serhiy Storchaka, reviewed by Serhiy Storchaka
and Victor Stinner.

Patch by Ivan Levkivskyi.

Class was dead code. Thanks to Xiang Zhang for the patch.

Windows.

Originally only b'PYTHONCASEOK' was being checked for in os.environ,
but that won't work under Windows where all environment variables are
strings (on OS X they are bytes).

Thanks to Eryk Sun for the bug report.

Thanks to Oren Milman for the patch.

modules  can't be lazily loaded.

Thanks to Python 3.6 allowing for types.ModuleType to have its
__class__ mutated, the restriction can be lifted by calling
create_module() on the wrapped loader.

lazy-loaded module was already in sys.modules.

Patch by Demur Rumed.

function with generalized unpacking (PEP 448) and conflicting keyword names
could cause undefined behavior.

Most fixes to Doc/ and Lib/ directories by Ville Skyttä.

Patch by Demur Rumed.

Issue #26073: Updates magic number comment in _bootstrap_external.py and changes numbers in launcher.py to decimal to match official table.

Issue #26637: The importlib module now emits an ImportError rather than a
TypeError if __import__() is tried during the Python shutdown process but
sys.path is already cleared (set to None).

Issue #26538: libregrtest: Fix setup_tests() to keep module.__path__ type
(_NamespacePath), don't convert to a list.

Add _NamespacePath.__setitem__() method to importlib._bootstrap_external.

importlib.util.LazyLoader.

The class was checking its argument as to whether its implementation
of create_module() came directly from importlib.abc.Loader. The
problem is that the classes coming from imoprtlib.machinery do not
directly inherit from the ABC as they come from _frozen_importlib.
Because the documentation has always said that create_module() was
ignored, the check has simply been removed.