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Diffstat (limited to 'Doc/reference/import.rst')
| -rw-r--r-- | Doc/reference/import.rst | 27 |
1 files changed, 27 insertions, 0 deletions
diff --git a/Doc/reference/import.rst b/Doc/reference/import.rst index 881e0ae..45d4172 100644 --- a/Doc/reference/import.rst +++ b/Doc/reference/import.rst @@ -675,6 +675,33 @@ Here are the exact rules used: :meth:`~importlib.abc.Loader.module_repr` method, if defined, before trying either approach described above. However, the method is deprecated. +.. _pyc-invalidation: + +Cached bytecode invalidation +---------------------------- + +Before Python loads cached bytecode from ``.pyc`` file, it checks whether the +cache is up-to-date with the source ``.py`` file. By default, Python does this +by storing the source's last-modified timestamp and size in the cache file when +writing it. At runtime, the import system then validates the cache file by +checking the stored metadata in the cache file against at source's +metadata. + +Python also supports "hash-based" cache files, which store a hash of the source +file's contents rather than its metadata. There are two variants of hash-based +``.pyc`` files: checked and unchecked. For checked hash-based ``.pyc`` files, +Python validates the cache file by hashing the source file and comparing the +resulting hash with the hash in the cache file. If a checked hash-based cache +file is found to be invalid, Python regenerates it and writes a new checked +hash-based cache file. For unchecked hash-based ``.pyc`` files, Python simply +assumes the cache file is valid if it exists. Hash-based ``.pyc`` files +validation behavior may be overridden with the :option:`--check-hash-based-pycs` +flag. + +.. versionchanged:: 3.7 + Added hash-based ``.pyc`` files. Previously, Python only supported + timestamp-based invalidation of bytecode caches. + The Path Based Finder ===================== |