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Python Code Parsing and pyc Files

This guide elaborates on how Python files are parsed on the GraalVM Python runtime.

Creating and Managing pyc Files #

.pyc files are created automatically by the GraalVM Python runtime when no or an invalid .pyc file is found matching the desired .py file.

When a Python source file (module) is imported during an execution for the first time, the appropriate .pyc file is created automatically. If the same module is imported again, then the already created .pyc file is used. That means that there are no .pyc files for source files that were not executed (imported) yet. The creation of .pyc files is done entirely through the FileSystem API, so that embedders can manage the file system access.

Every subsequent execution of a script will reuse the already existing .pyc files, or will generate a new one. A .pyc file is regenerated if the timestamp or hashcode of the original source file is changed. The hashcode is generated only based on the Python source by calling source.hashCode(), which is the JDK hash code over the array of source file bytes, calculated with java.util.Arrays.hashCode(byte[]).

The .pyc files are also regenerated if a magic number in the Python parser is changed. The magic number is hard-coded in the Python source and can not be changed by the user (unless of course that user has access to the bytecode of Python).

The developers of GraalVM’s Python runtime change the magic number when the bytecode format changes. This is an implementation detail, so the magic number does not have to correspond to the version of GraalVM’s Python runtime (just like in CPython). The magic number of pyc is a function of the concrete Python runtime Java code that is running.

Note that if you use .pyc files, you will need to allow write-access to GraalVM’s Python runtime at least when switching versions or changing the original source code. Otherwise, the regeneration of source files will fail and every import will have the overhead of accessing the old .pyc file, parsing the code, serializing it, and trying (and failing) to write out a new .pyc file.

A *.pyc file is never deleted by GraalVM’s Python runtime, only regenerated. It is regenerated when the appropriate source file is changed (timestamp of last modification or hashcode of the content) or the magic number of the Python imnplementation parser changes. Magic number changes will be communicated in the release notes so that embedders or system administrators can delete old .pyc files when upgrading.

The folder structure created for .pyc files looks like this:


By default the __pycache__ directory is created on the same directory level as a source code file and in this directory all .pyc files from the same directory are stored. This folder may store .pyc files created with different versions of Python (including, e.g., CPython), so the user may see files ending in *.cpython3-6.pyc for example.

.pyc files are largely managed automatically by the runtime in a manner compatible to CPython. Like on CPython there are options to specify their location, and if they should be written at all, and both of these options can be changed by guest code.

The creation of *.pyc files can be controlled in the same ways as on CPython (c.f.

  • GraalVM’s Python launcher (graalpy) reads the PYTHONDONTWRITEBYTECODE environment variable. If this is set to a non-empty string, Python will not try to write .pyc files when importing modules.
  • The launcher command line option -B, if given, has the same effect as the above.
  • A guest language code can change the attribute dont_write_bytecode of the sys built-in module at runtime to change the behavior for subsequent imports.
  • The launcher reads the PYTHONPYCACHEPREFIX environment variable. If set, the __pycache__ directory will be created at the path pointed to by the prefix, and a mirror of the directory structure of the source tree will be created on-demand to house the .pyc files.
  • A guest language code can change the attribute pycache_prefix of the sys module at runtime to change the location for subsequent imports.

Since the embedder cannot use environment variables or CPython options to communicate these options to GraalVM’s implementation of Python, these options are made available as these language options:

  • python.DontWriteBytecodeFlag - equivalent to -B or PYTHONDONTWRITEBYTECODE
  • python.PyCachePrefix - equivalent to PYTHONPYCACHEPREFIX

Note that a Python context will not enable writing .pyc files by default. The graalpy launcher enables it by default, but if this is desired in the embedding use case, care should be taken to ensure that the __pycache__ location is properly managed and the files in that location are secured against manipulation just like the source .py files they were derived from.

Note also that to upgrade the application sources to GraalVM Enteprise’s Python runtime, old .pyc files must be removed by the embedder as required.

Security Considerations #

All file operations (obtaining the data, timestamps, and writing pyc files) are done through the FileSystem API. Embedders can modify all of these operations by means of custom (e.g., read-only) FileSystem implementations. The embedder can also effectively disable the creation of .pyc files by disabling I/O permissions for GraalVM’s Python runtime.

If the .pyc files are not readable, their location is not writable. If the .pyc files’ serialization data or magic numbers are corrupted in any way, the deserialization fails and we just parse the .py file again. This comes with a minor performance hit only for the parsing of modules, which should not be significant for most applications (provided the application does actual work besides loading Python code).

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