[Python-Dev] Mysterious Python pyc file corruption problems (original) (raw)

Terry Jan Reedy tjreedy at udel.edu
Fri May 17 00:00:49 CEST 2013

On 5/16/2013 5:30 PM, Brett Cannon wrote:

On Thu, May 16, 2013 at 5:19 PM, Guido van Rossum <guido at python.org> wrote:

This reminds me of the following bug, which can happen when two processes are both writing the .pyc file and a third is reading it. First some background.

When writing a .pyc file, we use the following strategy: - open the file for writing - write a dummy header (four null bytes) - write the .py file's mtime - write the marshalled code object - replace the dummy heaer with the correct magic word Just so people know, this is how we used to do it. In importlib we write the entire file to a temp file and then to an atomic rename. Even pycompile.py (used by compileall.py) uses this strategy. pycompile as of Python 3.4 now just uses importlib directly, so it matches its semantics.

But in 3.3, it still is as Guido describes, even though importlib is improved.

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