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Last active Dec 23, 2020
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This post proposes supporting extensible customization of the interpreter, by allowing users to install scripts that will be executed at startup.


System administrators, tools that repackage the interpreter and some libraries need to customize aspects of the interpreter at startup time.

This is usually achieved via for system administrators whilst libraries rely on exploiting pth files. This post proposes a way of achieving the same in a more user-friendly and structured way.

Limitations of pth files

If a library needs to perform any customization before an import or that relates to the general working of the interpreter, they often rely on the fact that pth files, which are loaded at startup, can include Python code that will be executed when the pth file is evaluated.

Note that pth files were originally developed to just add additional directories to sys.path, but it also allowed to contain lines that started with "import", which will be execed. Users have exploited this feature to allow the customizations that they needed. See setuptools [link] or betterexceptions [link] as examples.

Using pth files for this purpose is far from ideal for library developers, as they need to inject code into a single line preceded by an import, making it rather unreadable. Library developers following that practice will usually create a module that performs all actions on import, as done by betterexceptions, but the approach is still not really really user friendly. Aditionally, it is also non-ideal for users of the interpreter as if they want to inspect what is being executed at Python startup they need to review all the pth files for potential code execution which can be spread across all site paths. Most of those pth will be "legit" pth files that just modify the path, answering the question of "what is changing my interpreter at startup" a rather complex one.

Lastly, there have been multiple suggestions for removing code execution from pth files, see [1] and [2].

Limitations of

Whilst sitecustomize is an acceptable solution, it assumes a single person is in charge of the system and the interpreter. If both the system administrator and the responsibility of provisioning the interpreter want to add customizations at the interpreter startup they need to agree on the contents of the file and combine all the changes. This is not a major limitation though, and it is not the main driver of this change, but should the change happen, it will also improve the situation for this users, as rather than having a which performs all those actions, they can have custom isolated files named after the features they want to enhance. As an example, ubuntu could change their current to just be ubuntu_apport_python_hook. This not only better represents its intent but also gives users of the interpreter a better understanding of the modifications happening on their interpreter.

Benefits of __sitecustomize__

Having a structured way of injecting custom startup scripts, will allow supporting in a better way the cases presented above. It will result in both maintainers and users better experience as detailed, and allow CPython to deprecate and eventually remove code execution from pth files, as desired in the previously mentioned bpos. Additionally, these solutions provide a unique way to support all use-cases that before have been fulfilled via the misuse of pth files, and The use of a __sitecustomize__ will allow for packages, tools and system admins to inject scripts that will be loaded at startups through an easy to understand mechanism.


This post proposes supporting extensible customization of the interpreter at startup by allowing users to install scripts into a namespace package named __sitecustomze__ that will be executed a startup. The implementation will attempt to import __sitecustomize__ and fail silently if not present. Should the namespace package be present, Python will import all scripts within it.

The site module will expose an option on its main function that allows listing all scripts found in that namespace package, which will allow users to quickly see all customizations that affect an interpreter.

We will also work with build backends on facilitating the installation of these files.

Why __sitecustomize__

The name aims to follow the already existing concept of To prevent conflict with the existing package and to make explicit it is not a module that is intended to be imported by the user, it is surrounded with double _.

Namespace package

Having __sitecustomize__ be a namespace package allows developers to install customizations in any path present in their Python path. This allows for easy installation in usersite, at the interpreter site or any other path the user chooses to add. We choose to use a standard namespace package rather than just looking for the folder name in the different site paths to support tools that might modify the path in any way and to make it easier to reason about. Python will just "import" __sitecustomize__ and execute all scripts in that namespace package.

The risk of using a namespace package for it is that a tool might by mistake choose to install an in the folder, breaking the mechanism to resolve the namespace package. Given that we are documenting this as a "folder to drop scripts" and the fact that it is a namespace package is an implementation detail, we do not expect this to be an issue.

Disabling start scripts

In some scenarios, like when the startup time is key, it might be desired to disable this option altogether. Whilst we could have added a new flag to do so, we think that the already existing flag -S is already good enough, as it disables all site related manipulation. If the flag is passed in, __sitecustomze__ will not be used.

Order of execution

The scripts in __sitecustomize__ will be executed in alphabetic order after all other site-dependent manipulations have been executed. This means after the evaluation of all pth files and the execution of and We considered executing them in random order, but that could result in different results depending on how the interpreter chooses to pick up those files. So even if it won't be a good practice to rely on other files being executed, we think that is better than having randomly different results on interpreter startup.

Additionally, if needed, packages can declare dependencies between their startup scripts by importing them from __sitecustomize__. As all these files are just python scripts that are executed by importing them, if one of them imports another (thanks to __sitecustomize__ being a namespace package), it will make sure that it is executed before the other.

Impact on startup time

If an interpreter is not using the tool, the impact on performance is expected to be that of an import that fails and the exception being ignored. This impact will be reduced in the future as we will remove two other imports: "" and "".

If the user has custom scripts, we think that the impact on the performance of importing the namespace package and walking it's acceptable, as the user wants to use this feature. If they need to run a time-sensitive application, they can always use -S to disable this entirely.

Running "./python -c pass" with perf on 50 iterations, repeating 50 times the command on each and getting the geometric mean on a commodity laptop did not reveal any substantial raise on CPU time beyond nanoseconds with this implementation, which is expected given the additional import.

Failure handling

Any error on any of the scripts will not be logged unless the interpreter is run in verbose mode and it should not stop the evaluation of other scripts. The user will just receive a message saying that the script failed to be executed, that verbose mode can be used to get more information. This behaviour follows the one already existing for

Scripts naming convention

Packages will be encouraged to include the name of the package within the name of the script to avoid collisions between packages, even if they might likely.

Relationship with sitecustomize and usercustomize

The existing logic for and could be left for a release and later deprecated and scheduled for removal. Once __sitecustomize__ is supported, it will provide better integration for all existing users, and even if it will indeed require a migration for System administrators, we expect the effort required to be minimal, it will just require moving and renaming the current into the new provided folder.

Identifying all installed scripts

To facilitate debugging of the Python startup, a new option will be added to the main of the site module to list all scripts that will be executed as part of the __sitecustomze__ initialization.

How to teach this

This can be documented and taught as simple as saying that the interpreter will try to import the __sitecustomize__ package at startup and it if finds any modules within it, it will then execute all of them.

For system administrators and tools that package the interpreter, we can now recommend placing files in __sitecustomze__ as they used to place Being more comfortable on that their content won't be overridden by the next person, as they can provide with specific files to handle the logic they want to customize.

Library developers should be able to specify a new argument on tools like setuptools that will inject those new files. Something like sitecustomize_scripts=["scripts/"], which allows them to add those. Should the build backend not support that, they can manually install them as they used to do with pth files. We will recommend them to include the name of the package as part of the scripts name.

Backward compatibility

We propose to add support for __sitecustomize__ in the next release of Python, add a warning on the three next releases on the deprecation and future removal of, and code execution in pth files, and remove it after maintainers have had 4 releases to migrate. Ignoring those lines in pth files.

Rejected Ideas

Do nothing

Whilst the current status "works" it presents the issues listed in the motivation. After analysing the impact of this change, we believe it is worth given the enhanced experience it brings.

Formalize using pth files

Another option would be to just glorify and document the usage of pth files to inject code at startup code, but that is a suboptimal experience for users as listed in the motivation.

Searching files within a folder rather than a namespace package

Similarly to how pth files are looked up, we could have implemented the __sitecustomize__ logic. We preferred to use a namespace package as it brings other benefits like being able to declare dependencies easily and we consider it is easier to teach.

Support for shutdown custom scripts

init.d users might be tempted to implement this feature in a way that users could also add code at shutdown, but extra support for that is not needed, as Python users can already do that via atexit.

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