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Axle Runtime is the runtime part of the Python Wheel enhancement library

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Axle-Runtime - Python Wheel enhancement library

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This is a companion project to Wheel Axle/bdist_axle


  1. Python wheels do not support symlinks.
  2. PIP installation procedure is not locally extensible and does not allow adding post-install hooks.



Wheel Axle Runtime library utilizes a little-known trick used in's .pth files that allows executing arbitrary code while the site packages are being added. Thus, specially-crafted wheels can silently execute installed code on Python interpreter startup, facilitating the "post-install hook" functionality.

Python Invariants

The core functionality relies on the following Python behaviors:

These invariants have not changed for 18 years.


Once the distribution-specific .pth is executed by the Python interpreter, the Wheel Axle Runtime behaves as follows:

  1. The library checks whether a file .dist-info/axle.done exists. If it does it is the indication that the post-install hook has executed successfully and nothing more is to be done, terminating all further processing.
  2. A process-wide inter-thread lock is acquired.
  3. An OS-wide inter-process file lock is acquired on a file .dist-info/axle.lck.
  4. Once the locks are acquired the .dist-info/axle.done existence is rechecked (double-checked locking optimization).
  5. Now that in-process and inter-process race conditions are excluded the post-install work can begin.
  6. Registered installers are run in sequence. Installers should be idempotent. The following installers are currently implemented:
    1. Symlinks installer processes .dist-info/symlinks.txt, if any.
      1. Based on the location of the .pth file being executed the current installation schema and its paths are determined. Currently, installation into a virtual environment or user location is supported and tested.
      2. For each symlink the target path is resolved and realpath is used to determine the final target path.
      3. If the symlink path and symlink target path are within one of the permitted schema locations the symlink is created. Otherwise, an exception is raised and the processing is aborted.
      4. After all symlinks are created, the .dist-info/RECORD file is updated to reflect the created symlinks.
    2. Axle installer finalizes the installation. This installer is always executed last.
      1. The .dist-info/RECORD is updated with .dist-info/axle.done file record.
      2. .dist-info/axle.done is created.
      3. <distribution name and version>.pth is then removed. If the file cannot be removed it is left in place. This can happen on Windows, since the .pth file in question is likely opened for exclusive reading on Windows.
  7. Any failure anywhere in the above process will result in an abort, an error message, and a retry the next time the .pth will be activated.


There are several security requirements and implications of having post-install hooks implemented this way.

  1. The installation requires write permissions to the distribution. This will be a problem if the package is installed as root in locations such as /usr or /usr/local, or is otherwise not write-permitted, unless the post-install hook is also ran with the sufficient privileges. This is generally acceptable as the primary use is considered to be installation into virtual envs and user locations. That said, simply running python -c pass or any other python invocation that does activate under the required privileges will finalize post-install procedures.
  2. There is an attempt to ensure that that axle wheels symlinks and targets don't extend beyond the allowed schema locations. Those attempts are superficial and have not been formally verified. For example, it may be possible to escape the path validation/confinement by:
    • hacking symlink creation order
    • hacking symlink directory targets
    • exploiting OS-specific realpath implementation idiosyncrasies (i.e. strict vs not, and what is considered strict)


  • Support schema detection for prefix installations.
  • Validate and verify Windows support.

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