diff --git a/Lib/compileall.py b/Lib/compileall.py
@@ -206,11 +206,11 @@ def compile_file(fullname, ddir=None, force=False, rx=None, quiet=0,
            if not force:
                try:
                    mtime = int(os.stat(fullname).st_mtime)
-                    expect = struct.pack('<4sll', importlib.util.MAGIC_NUMBER,
                                         0, mtime)
                    for cfile in opt_cfiles.values():
                        with open(cfile, 'rb') as chandle:
-                            actual = chandle.read(12)
                        if expect != actual:
                            break
                    else:

diff --git a/Lib/importlib/__init__.py b/Lib/importlib/__init__.py
@@ -49,6 +49,7 @@

# To simplify imports in test code
_pack_uint32 = _bootstrap_external._pack_uint32
_unpack_uint32 = _bootstrap_external._unpack_uint32

# Fully bootstrapped at this point, import whatever you like, circular

diff --git a/Lib/importlib/_bootstrap_external.py b/Lib/importlib/_bootstrap_external.py
@@ -42,12 +42,20 @@ def _relax_case():
            return False
    return _relax_case


def _pack_uint32(x):
    """Convert a 32-bit integer to little-endian."""
    return (int(x) & 0xFFFFFFFF).to_bytes(4, 'little')


def _unpack_uint32(data):
    """Convert 4 bytes in little-endian to an integer."""
    assert len(data) == 4
@@ -277,6 +285,7 @@ def _write_atomic(path, data, mode=0o666):
#     Python 3.9a2  3423 (add IS_OP, CONTAINS_OP and JUMP_IF_NOT_EXC_MATCH bytecodes #39156)
#     Python 3.9a2  3424 (simplify bytecodes for *value unpacking)
#     Python 3.9a2  3425 (simplify bytecodes for **value unpacking)

#
# MAGIC must change whenever the bytecode emitted by the compiler may no
@@ -286,7 +295,7 @@ def _write_atomic(path, data, mode=0o666):
# Whenever MAGIC_NUMBER is changed, the ranges in the magic_values array
# in PC/launcher.c must also be updated.

-MAGIC_NUMBER = (3425).to_bytes(2, 'little') + b'\r\n'
_RAW_MAGIC_NUMBER = int.from_bytes(MAGIC_NUMBER, 'little')  # For import.c

_PYCACHE = '__pycache__'
@@ -501,7 +510,7 @@ def _classify_pyc(data, name, exc_details):
    """Perform basic validity checking of a pyc header and return the flags field,
    which determines how the pyc should be further validated against the source.

-    *data* is the contents of the pyc file. (Only the first 16 bytes are
    required, though.)

    *name* is the name of the module being imported. It is used for logging.
@@ -518,7 +527,7 @@ def _classify_pyc(data, name, exc_details):
        message = f'bad magic number in {name!r}: {magic!r}'
        _bootstrap._verbose_message('{}', message)
        raise ImportError(message, **exc_details)
-    if len(data) < 16:
        message = f'reached EOF while reading pyc header of {name!r}'
        _bootstrap._verbose_message('{}', message)
        raise EOFError(message)
@@ -534,7 +543,7 @@ def _validate_timestamp_pyc(data, source_mtime, source_size, name,
                            exc_details):
    """Validate a pyc against the source last-modified time.

-    *data* is the contents of the pyc file. (Only the first 16 bytes are
    required.)

    *source_mtime* is the last modified timestamp of the source file.
@@ -549,12 +558,12 @@ def _validate_timestamp_pyc(data, source_mtime, source_size, name,
    An ImportError is raised if the bytecode is stale.

    """
-    if _unpack_uint32(data[8:12]) != (source_mtime & 0xFFFFFFFF):
        message = f'bytecode is stale for {name!r}'
        _bootstrap._verbose_message('{}', message)
        raise ImportError(message, **exc_details)
    if (source_size is not None and
-        _unpack_uint32(data[12:16]) != (source_size & 0xFFFFFFFF)):
        raise ImportError(f'bytecode is stale for {name!r}', **exc_details)


@@ -599,8 +608,8 @@ def _code_to_timestamp_pyc(code, mtime=0, source_size=0):
    "Produce the data for a timestamp-based pyc."
    data = bytearray(MAGIC_NUMBER)
    data.extend(_pack_uint32(0))
-    data.extend(_pack_uint32(mtime))
-    data.extend(_pack_uint32(source_size))
    data.extend(marshal.dumps(code))
    return data

@@ -612,6 +621,8 @@ def _code_to_hash_pyc(code, source_hash, checked=True):
    data.extend(_pack_uint32(flags))
    assert len(source_hash) == 8
    data.extend(source_hash)
    data.extend(marshal.dumps(code))
    return data

@@ -888,7 +899,7 @@ def get_code(self, fullname):
                    }
                    try:
                        flags = _classify_pyc(data, fullname, exc_details)
-                        bytes_data = memoryview(data)[16:]
                        hash_based = flags & 0b1 != 0
                        if hash_based:
                            check_source = flags & 0b10 != 0
@@ -1070,7 +1081,7 @@ def get_code(self, fullname):
        }
        _classify_pyc(data, fullname, exc_details)
        return _compile_bytecode(
-            memoryview(data)[16:],
            name=fullname,
            bytecode_path=path,
        )

diff --git a/Lib/modulefinder.py b/Lib/modulefinder.py
@@ -347,7 +347,7 @@ def load_module(self, fqname, fp, pathname, file_info):
            except ImportError as exc:
                self.msgout(2, "raise ImportError: " + str(exc), pathname)
                raise
-            co = marshal.loads(memoryview(data)[16:])
        else:
            co = None
        m = self.add_module(fqname)

diff --git a/Lib/pkgutil.py b/Lib/pkgutil.py
@@ -47,7 +47,7 @@ def read_code(stream):
    if magic != importlib.util.MAGIC_NUMBER:
        return None

-    stream.read(12) # Skip rest of the header
    return marshal.load(stream)



diff --git a/Lib/test/test_compileall.py b/Lib/test/test_compileall.py
@@ -53,9 +53,9 @@ def add_bad_source_file(self):

    def timestamp_metadata(self):
        with open(self.bc_path, 'rb') as file:
-            data = file.read(12)
        mtime = int(os.stat(self.source_path).st_mtime)
-        compare = struct.pack('<4sll', importlib.util.MAGIC_NUMBER, 0, mtime)
        return data, compare

    def recreation_check(self, metadata):
@@ -76,7 +76,7 @@ def recreation_check(self, metadata):

    def test_mtime(self):
        # Test a change in mtime leads to a new .pyc.
-        self.recreation_check(struct.pack('<4sll', importlib.util.MAGIC_NUMBER,
                                          0, 1))

    def test_magic_number(self):

diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
@@ -674,7 +674,7 @@ def test_module_without_source(self):
    def test_foreign_code(self):
        py_compile.compile(self.file_name)
        with open(self.compiled_name, "rb") as f:
-            header = f.read(16)
            code = marshal.load(f)
        constants = list(code.co_consts)
        foreign_code = importlib.import_module.__code__

diff --git a/Lib/test/test_importlib/source/test_file_loader.py b/Lib/test/test_importlib/source/test_file_loader.py
@@ -464,14 +464,14 @@ def _test_partial_timestamp(self, test, *, del_source=False):
    def _test_partial_size(self, test, *, del_source=False):
        with util.create_modules('_temp') as mapping:
            bc_path = self.manipulate_bytecode('_temp', mapping,
-                                                lambda bc: bc[:15],
                                                del_source=del_source)
            test('_temp', mapping, bc_path)

    def _test_no_marshal(self, *, del_source=False):
        with util.create_modules('_temp') as mapping:
            bc_path = self.manipulate_bytecode('_temp', mapping,
-                                                lambda bc: bc[:16],
                                                del_source=del_source)
            file_path = mapping['_temp'] if not del_source else bc_path
            with self.assertRaises(EOFError):
@@ -480,7 +480,7 @@ def _test_no_marshal(self, *, del_source=False):
    def _test_non_code_marshal(self, *, del_source=False):
        with util.create_modules('_temp') as mapping:
            bytecode_path = self.manipulate_bytecode('_temp', mapping,
-                                    lambda bc: bc[:16] + marshal.dumps(b'abcd'),
                                    del_source=del_source)
            file_path = mapping['_temp'] if not del_source else bytecode_path
            with self.assertRaises(ImportError) as cm:
@@ -491,7 +491,7 @@ def _test_non_code_marshal(self, *, del_source=False):
    def _test_bad_marshal(self, *, del_source=False):
        with util.create_modules('_temp') as mapping:
            bytecode_path = self.manipulate_bytecode('_temp', mapping,
-                                                lambda bc: bc[:16] + b'<test>',
                                                del_source=del_source)
            file_path = mapping['_temp'] if not del_source else bytecode_path
            with self.assertRaises(EOFError):
@@ -635,7 +635,7 @@ def test_bad_marshal(self):
    def test_old_timestamp(self):
        # When the timestamp is older than the source, bytecode should be
        # regenerated.
-        zeros = b'\x00\x00\x00\x00'
        with util.create_modules('_temp') as mapping:
            py_compile.compile(mapping['_temp'])
            bytecode_path = self.util.cache_from_source(mapping['_temp'])
@@ -644,10 +644,10 @@ def test_old_timestamp(self):
                bytecode_file.write(zeros)
            self.import_(mapping['_temp'], '_temp')
            source_mtime = os.path.getmtime(mapping['_temp'])
-            source_timestamp = self.importlib._pack_uint32(source_mtime)
            with open(bytecode_path, 'rb') as bytecode_file:
                bytecode_file.seek(8)
-                self.assertEqual(bytecode_file.read(4), source_timestamp)

    # [bytecode read-only]
    @util.writes_bytecode_files

diff --git a/Lib/test/test_importlib/test_abc.py b/Lib/test/test_importlib/test_abc.py
@@ -890,8 +890,8 @@ def verify_code(self, code_object, *, bytecode_written=False):
            self.assertIn(self.cached, self.loader.written)
            data = bytearray(self.util.MAGIC_NUMBER)
            data.extend(self.init._pack_uint32(0))
-            data.extend(self.init._pack_uint32(self.loader.source_mtime))
-            data.extend(self.init._pack_uint32(self.loader.source_size))
            data.extend(marshal.dumps(code_object))
            self.assertEqual(self.loader.written[self.cached], bytes(data))


diff --git a/Lib/test/test_importlib/util.py b/Lib/test/test_importlib/util.py
@@ -125,7 +125,7 @@ def get_code_from_pyc(pyc_path):
    No header validation is performed.
    """
    with open(pyc_path, 'rb') as pyc_f:
-        pyc_f.seek(16)
        return marshal.load(pyc_f)



diff --git a/Lib/test/test_py_compile.py b/Lib/test/test_py_compile.py
@@ -150,6 +150,14 @@ def test_source_date_epoch(self):

        self.assertEqual(flags, expected_flags)

    @unittest.skipIf(sys.flags.optimize > 0, 'test does not work with -O')
    def test_double_dot_no_clobber(self):
        # http://bugs.python.org/issue22966

diff --git a/Misc/NEWS.d/next/Core and Builtins/2020-04-22-02-39-20.bpo-34990.txu75f.rst b/Misc/NEWS.d/next/Core and Builtins/2020-04-22-02-39-20.bpo-34990.txu75f.rst
@@ -0,0 +1,3 @@