8000 closes bpo-31650: PEP 552 (Deterministic pycs) implementation (#4575) · python/cpython@42aa93b · GitHub
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closes bpo-31650: PEP 552 (Deterministic pycs) implementation (#4575)
Python now supports checking bytecode cache up-to-dateness with a hash of the source contents rather than volatile source metadata. See the PEP for details. While a fairly straightforward idea, quite a lot of code had to be modified due to the pervasiveness of pyc implementation details in the codebase. Changes in this commit include: - The core changes to importlib to understand how to read, validate, and regenerate hash-based pycs. - Support for generating hash-based pycs in py_compile and compileall. - Modifications to our siphash implementation to support passing a custom key. We then expose it to importlib through _imp. - Updates to all places in the interpreter, standard library, and tests that manually generate or parse pyc files to grok the new format. - Support in the interpreter command line code for long options like --check-hash-based-pycs. - Tests and documentation for all of the above.
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‎Doc/glossary.rst

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@@ -458,6 +458,12 @@ Glossary
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is believed that overcoming this performance issue would make the
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implementation much more complicated and therefore costlier to maintain.
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hash-based pyc
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A bytecode cache file that uses the the hash rather than the last-modified
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time of the corresponding source file to determine its validity. See
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:ref:`pyc-invalidation`.
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hashable
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An object is *hashable* if it has a hash value which never changes during
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its lifetime (it needs a :meth:`__hash__` method), and can be compared to

‎Doc/library/compileall.rst

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If ``0`` is used, then the result of :func:`os.cpu_count()`
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will be used.
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.. cmdoption:: --invalidation-mode [timestamp|checked-hash|unchecked-hash]
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Control how the generated pycs will be invalidated at runtime. The default
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setting, ``timestamp``, means that ``.pyc`` files with the source timestamp
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and size embedded will be generated. The ``checked-hash`` and
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``unchecked-hash`` values cause hash-based pycs to be generated. Hash-based
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pycs embed a hash of the source file contents rather than a timestamp. See
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:ref:`pyc-invalidation` for more information on how Python validates bytecode
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cache files at runtime.
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.. versionchanged:: 3.2
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Added the ``-i``, ``-b`` and ``-h`` options.
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was changed to a multilevel value. ``-b`` will always produce a
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byte-code file ending in ``.pyc``, never ``.pyo``.
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.. versionchanged:: 3.7
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Added the ``--invalidation-mode`` parameter.
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There is no command-line option to control the optimization level used by the
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:func:`compile` function, because the Python interpreter itself already
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Public functions
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----------------
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.. function:: compile_dir(dir, maxlevels=10, ddir=None, force=False, rx=None, quiet=0, legacy=False, optimize=-1, workers=1)
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.. function:: compile_dir(dir, maxlevels=10, ddir=None, force=False, rx=None, quiet=0, legacy=False, optimize=-1, workers=1, invalidation_mode=py_compile.PycInvalidationMode.TIMESTAMP)
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Recursively descend the directory tree named by *dir*, compiling all :file:`.py`
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files along the way. Return a true value if all the files compiled successfully,
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then sequential compilation will be used as a fallback. If *workers* is
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lower than ``0``, a :exc:`ValueError` will be raised.
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*invalidation_mode* should be a member of the
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:class:`py_compile.PycInvalidationMode` enum and controls how the generated
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pycs are invalidated at runtime.
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.. versionchanged:: 3.2
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Added the *legacy* and *optimize* parameter.
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.. versionchanged:: 3.6
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Accepts a :term:`path-like object`.
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.. function:: compile_file(fullname, ddir=None, force=False, rx=None, quiet=0, legacy=False, optimize=-1)
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.. versionchanged:: 3.7
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The *invalidation_mode* parameter was added.
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.. function:: compile_file(fullname, ddir=None, force=False, rx=None, quiet=0, legacy=False, optimize=-1, invalidation_mode=py_compile.PycInvalidationMode.TIMESTAMP)
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Compile the file with path *fullname*. Return a true value if the file
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compiled successfully, and a false value otherwise.
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*optimize* specifies the optimization level for the compiler. It is passed to
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the built-in :func:`compile` function.
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*invalidation_mode* should be a member of the
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:class:`py_compile.PycInvalidationMode` enum and controls how the generated
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pycs are invalidated at runtime.
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.. versionadded:: 3.2
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.. versionchanged:: 3.5
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The *legacy* parameter only writes out ``.pyc`` files, not ``.pyo`` files
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no matter what the value of *optimize* is.
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.. function:: compile_path(skip_curdir=True, maxlevels=0, force=False, quiet=0, legacy=False, optimize=-1)
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.. versionchanged:: 3.7
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The *invalidation_mode* parameter was added.
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.. function:: compile_path(skip_curdir=True, maxlevels=0, force=False, quiet=0, legacy=False, optimize=-1, invalidation_mode=py_compile.PycInvalidationMode.TIMESTAMP)
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Byte-compile all the :file:`.py` files found along ``sys.path``. Return a
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true value if all the files compiled successfully, and a false value otherwise.
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The *legacy* parameter only writes out ``.pyc`` files, not ``.pyo`` files
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no matter what the value of *optimize* is.
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.. versionchanged:: 3.7
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The *invalidation_mode* parameter was added.
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To force a recompile of all the :file:`.py` files in the :file:`Lib/`
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subdirectory and all its subdirectories::
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‎Doc/library/importlib.rst

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:pep:`489`
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Multi-phase extension module initialization
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:pep:`552`
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Deterministic pycs
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:pep:`3120`
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Using UTF-8 as the Default Source Encoding
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.. versionchanged:: 3.6
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Accepts a :term:`path-like object`.
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.. function:: source_hash(source_bytes)
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Return the hash of *source_bytes* as bytes. A hash-based ``.pyc`` file embeds
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the :func:`source_hash` of the corresponding source file's contents in its
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header.
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.. versionadded:: 3.7
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.. class:: LazyLoader(loader)
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A class which postpones the execution of the loader of a module until the

‎Doc/library/py_compile.rst

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Exception raised when an error occurs while attempting to compile the file.
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.. function:: compile(file, cfile=None, dfile=None, doraise=False, optimize=-1)
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.. function:: compile(file, cfile=None, dfile=None, doraise=False, optimize=-1, invalidation_mode=PycInvalidationMode.TIMESTAMP)
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Compile a source file to byte-code and write out the byte-code cache file.
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The source code is loaded from the file named *file*. The byte-code is
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:func:`compile` function. The default of ``-1`` selects the optimization
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level of the current interpreter.
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*invalidation_mode* should be a member of the :class:`PycInvalidationMode`
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enum and controls how the generated ``.pyc`` files are invalidated at
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runtime.
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.. versionchanged:: 3.2
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Changed default value of *cfile* to be :PEP:`3147`-compliant. Previous
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default was *file* + ``'c'`` (``'o'`` if optimization was enabled).
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caveat that :exc:`FileExistsError` is raised if *cfile* is a symlink or
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non-regular file.
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.. versionchanged:: 3.7
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The *invalidation_mode* parameter was added as specified in :pep:`552`.
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.. class:: PycInvalidationMode
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A enumeration of possible methods the interpreter can use to determine
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whether a bytecode file is up to date with a source file. The ``.pyc`` file
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indicates the desired invalidation mode in its header. See
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:ref:`pyc-invalidation` for more information on how Python invalidates
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``.pyc`` files at runtime.
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.. versionadded:: 3.7
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.. attribute:: TIMESTAMP
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The ``.pyc`` file includes the timestamp and size of the source file,
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which Python will compare against the metadata of the source file at
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runtime to determine if the ``.pyc`` file needs to be regenerated.
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.. attribute:: CHECKED_HASH
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The ``.pyc`` file includes a hash of the source file content, which Python
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will compare against the source at runtime to determine if the ``.pyc``
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file needs to be regenerated.
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.. attribute:: UNCHECKED_HASH
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Like :attr:`CHECKED_HASH`, the ``.pyc`` file includes a hash of the source
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file content. However, Python will at runtime assume the ``.pyc`` file is
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up to date and not validate the ``.pyc`` against the source file at all.
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This option is useful when the ``.pycs`` are kept up to date by some
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system external to Python like a build system.
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.. function:: main(args=None)
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‎Doc/reference/import.rst

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:meth:`~importlib.abc.Loader.module_repr` method, if defined, before
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trying either approach described above. However, the method is deprecated.
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.. _pyc-invalidation:
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Cached bytecode invalidation
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----------------------------
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Before Python loads cached bytecode from ``.pyc`` file, it checks whether the
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cache is up-to-date with the source ``.py`` file. By default, Python does this
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by storing the source's last-modified timestamp and size in the cache file when
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writing it. At runtime, the import system then validates the cache file by
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checking the stored metadata in the cache file against at source's
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metadata.
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Python also supports "hash-based" cache files, which store a hash of the source
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file's contents rather than its metadata. There are two variants of hash-based
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``.pyc`` files: checked and unchecked. For checked hash-based ``.pyc`` files,
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Python validates the cache file by hashing the source file and comparing the
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resulting hash with the hash in the cache file. If a checked hash-based cache
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file is found to be invalid, Python regenerates it and writes a new checked
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hash-based cache file. For unchecked hash-based ``.pyc`` files, Python simply
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assumes the cache file is valid if it exists. Hash-based ``.pyc`` files
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validation behavior may be overridden with the :option:`--check-hash-based-pycs`
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flag.
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.. versionchanged:: 3.7
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Added hash-based ``.pyc`` files. Previously, Python only supported
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timestamp-based invalidation of bytecode caches.
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The Path Based Finder
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=====================

‎Doc/using/cmdline.rst

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import of source modules. See also :envvar:`PYTHONDONTWRITEBYTECODE`.
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.. cmdoption:: --check-hash-based-pycs default|always|never
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Control the validation behavior of hash-based ``.pyc`` files. See
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:ref:`pyc-invalidation`. When set to ``default``, checked and unchecked
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hash-based bytecode cache files are validated according to their default
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semantics. When set to ``always``, all hash-based ``.pyc`` files, whether
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checked or unchecked, are validated against their corresponding source
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file. When set to ``never``, hash-based ``.pyc`` files are not validated
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against their corresponding source files.
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The semantics of timestamp-based ``.pyc`` files are unaffected by this
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option.
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.. cmdoption:: -d
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Turn on parser debugging output (for expert only, depending on compilation

‎Doc/whatsnew/3.7.rst

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See :option:`-X` ``dev`` for the details.
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Hash-based pycs
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---------------
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Python has traditionally checked the up-to-dateness of bytecode cache files
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(i.e., ``.pyc`` files) by comparing the source metadata (last-modified timestamp
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and size) with source metadata saved in the cache file header when it was
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generated. While effective, this invalidation method has its drawbacks. When
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filesystem timestamps are too coarse, Python can miss source updates, leading to
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user confusion. Additionally, having a timestamp in the cache file is
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problematic for `build reproduciblity <https://reproducible-builds.org/>`_ and
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content-based build systems.
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:pep:`552` extends the pyc format to allow the hash of the source file to be
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used for invalidation instead of the source timestamp. Such ``.pyc`` files are
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called "hash-based". By default, Python still uses timestamp-based invalidation
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and does not generate hash-based ``.pyc`` files at runtime. Hash-based ``.pyc``
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files may be generated with :mod:`py_compile` or :mod:`compileall`.
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Hash-based ``.pyc`` files come in two variants: checked and unchecked. Python
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validates checked hash-based ``.pyc`` files against the corresponding source
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files at runtime but doesn't do so for unchecked hash-based pycs. Unchecked
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hash-based ``.pyc`` files are a useful performance optimization for environments
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where a system external to Python (e.g., the build system) is responsible for
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keeping ``.pyc`` files up-to-date.
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See :ref:`pyc-invalidation` for more information.
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Other Language Changes
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======================

‎Include/internal/hash.h

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#ifndef Py_INTERNAL_HASH_H
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#define Py_INTERNAL_HASH_H
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uint64_t _Py_KeyedHash(uint64_t, const char *, Py_ssize_t);
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#endif

‎Include/internal/import.h

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#ifndef Py_INTERNAL_IMPORT_H
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#define Py_INTERNAL_IMPORT_H
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extern const char *_Py_CheckHashBasedPycsMode;
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#endif

‎Include/pygetopt.h

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PyAPI_FUNC(void) _PyOS_ResetGetOpt(void);
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PyAPI_FUNC(int) _PyOS_GetOpt(int argc, wchar_t **argv, wchar_t *optstring);
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typedef struct {
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const wchar_t *name;
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int has_arg;
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int val;
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} _PyOS_LongOption;
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PyAPI_FUNC(int) _PyOS_GetOpt(int argc, wchar_t **argv, wchar_t *optstring,
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const _PyOS_LongOption *longopts, int *longindex);
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#endif /* !Py_LIMITED_API */
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#ifdef __cplusplus

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