PEP 648 – Extensible customizations of the interpreter at startup
- Mario Corchero <mariocj89 at gmail.com>
- Pablo Galindo
- Discourse thread
- Standards Track
- 16-Dec-2020, 18-Dec-2020
Table of Contents
- PEP Rejection
- Discovering the new
- Time of
- Order of execution within
- Interaction with
- Execution of files within
- Failure handling
- Interaction with virtual environments
- Interaction with
- Identifying all installed files
- Files naming convention
- Disabling start files
- Support in build backends
- Impact on startup time
- Audit Event
- Security implications
- How to teach this
- Backward compatibility
- Reference Implementation
- Rejected Ideas
This PEP proposes supporting extensible customization of the interpreter by allowing users to install files that will be executed at startup.
PEP 648 was rejected by the steering council as it has a limited number of use cases and further complicates the startup sequence.
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
sitecustomize.py for system administrators
whilst libraries rely on exploiting
pth files. This PEP proposes a way of
achieving the same functionality in a more user-friendly and structured way.
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
pth files, which are loaded at startup and implemented via the
site module , can include Python code that will be executed when the
pth file is evaluated.
pth files were originally developed to just add additional
sys.path, but they may also contain lines which start
with “import”, which will be passed to
exec(). Users have exploited this
feature to allow the customizations that they needed. See setuptools
 or betterexceptions  as examples.
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
Additionally, it is also non-ideal for users of the interpreter if they want
to inspect what is being executed at Python startup as they need to review
pth files for potential code execution which can be spread across
all site paths. Most of those
pth files will be “legitimate”
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  and .
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. Should the change
happen, it will also improve the situation for these users, as rather than
sitecustomize.py 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
sitecustomize.py 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.
This PEP proposes supporting extensible customization of the interpreter at
startup by executing all files discovered in directories named
__sitecustomize__ in sitepackages  or
usersitepackages  at startup time.
The name aims to follow the already existing concept of
As the directory will be within
sys.path, given that it is located in
site paths, we choose to use double underscore around its name, to prevent
colliding with the already existing
Discovering the new
The Python interpreter will look at startup for directory named
__sitecustomize__ within any of the standard site-packages path.
These are commonly the Python system location and the user location, but are ultimately defined by the site module logic.
Users can use
site.sitepackages  and
site.usersitepackages  to know the paths where
the interpreter can discover
__sitecustomize__ directories will be discovered exactly after
files are discovered in a site-packages path as part of
These is repeated for each of the site-packages path in the exact same order
that is being followed today for
Order of execution within
The implementation will execute the files within
sorting them by name when discovering each of the
directories. We discourage users to rely on the order of execution though.
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. We chose to run the files after the
pth files in
case a user needs to add items to the path before running a files.
pth files can be used to add paths into
sys.path, but this should not
__sitecustomize__ discovery process, as those directories are
looked up exclusively in site-packages paths.
Execution of files within
__sitecustomize__ directory is discovered, all of the files that
.py extension within it will be read with
executed by using
An empty dictionary will be passed as
globals to the
to prevent unexpected interactions between different files.
Any error on the execution of any of the files will not be logged unless the
interpreter is run in verbose mode and it should not stop the evaluation of
other files. The user will receive a message in stderr saying that the file
failed to be executed and that verbose mode can be used to get more
information. This behaviour mimics the one existing for
Interaction with virtual environments
The customizations applied to an interpreter via the new
__sitecustomize__ solutions will continue to work when a user creates a
virtual environment the same way that
interact with virtual environments.
This is a difference when compared to
pth files, which are not propagated
into virtual environments unless
include-system-site-packages is enabled.
If library maintainers have features installed via
they do not want to propagate into virtual environments, they should detect
if they are running within a virtual environment by checking
sys.base_prefix. This behavior is similar to packages that modify the global
usercustomize will be executed after
__sitecustomize__ similar to pth files. See the Backward compatibility
section for information on removal plans for
Identifying all installed files
To facilitate debugging of the Python startup, if the site module is invoked
it will print the
__sitecustomize__ directories that will be discovered
Files naming convention
Packages will be encouraged to include the name of the package within the
name of the file to avoid collisions between packages. But the only
requirement on the filename is that it ends in
.py for the interpreter to
Disabling start files
In some scenarios, like when the startup time is key, it might be desired to
disable this option altogether. The already existing flag
will disable all
site-related manipulation, including this new feature.
If the flag is passed in,
__sitecustomize__ directories will not be
Additionally, to allow for starting the interpreter disabling only this new
feature a new option will be added under
which will disable the discovery of
Lastly, the user can disable the discovery of
directories only in the user site by disabling the user site via any of the
multiple options in the
Support in build backends
Whilst build backends can choose to provide an option to facilitate the
installation of these files into a
__sitecustomize__ directory, this
PEP does not address that directly. Similar to
pth files, build backends
can choose to not provide an easy-to-configure mechanism for
__sitecustomize__ files and let users hook into the installation
process to include such files. We do not think build backends enhanced
support as a requirement for this PEP.
Impact on startup time
A concern in this implementation is how Python interpreter startup time can be affected by this addition. We expect the performance impact to be highly coupled to the logic in the files that a user or sysadmin installs in the Python environment being tested.
If the interpreter has any files in their
the file execution time plus a call reading the code will be added to the
startup time. This is similar to how code execution is impacting startup time
usercustomize.py and code in
We will therefore focus here on comparing this solution against those three,
as otherwise the actual time added to startup is highly dependent on the code
that is being executed in those files.
Results were gathered by running “./python.exe -c pass” with perf on 50 iterations, repeating 50 times the command on each iteration and getting the geometric mean of all the results. The file used to run those benchmarks is checked in in the reference implementation .
The benchmark was run with 3.10 alpha 7 compiled with PGO and LTO with the following parameters and system state:
- Perf event: Max sample rate set to 1 per second
- CPU Frequency: Minimum frequency of CPU 17,35 set to the maximum frequency
- Turbo Boost (MSR): Turbo Boost disabled on CPU 17: MSR 0x1a0 set to 0x4000850089
- IRQ affinity: Set default affinity to CPU 0-16,18-34
- IRQ affinity: Set affinity of IRQ 1,3-16,21,25-31,56-59,68-85,87,89-90,92-93,95-104 to CPU 0-16,18-34
- CPU: use 2 logical CPUs: 17,35
- Perf event: Maximum sample rate: 1 per second
- ASLR: Full randomization
- Linux scheduler: Isolated CPUs (2/36): 17,35
- Linux scheduler: RCU disabled on CPUs (2/36): 17,35
- CPU Frequency: 0-16,18-34=min=1200 MHz, max=3600 MHz; 17,35=min=max=3600 MHz
- Turbo Boost (MSR): CPU 17,35: disabled
The code placed to be executed in
usercustomize.py and files within
__sitecustomize__ is the following:
import time; x = time.time() ** 5
The file is aimed at execution a simple operation but still expected to be
negligible. This is to put the experiment in a situation where we make
visible any hit on performance due to the mechanism whilst still making it
relatively realistic. Additionally, it starts with an import and is a single
line to be able to be used in
|Test||# of files||Time (us)|
||Run 1||Run 2|
|1||0||0||0||Dir not created||13884||13897|
|6||0||0||0||2 (system + user)||14063||14040|
Results can be reproduced with
run-benchmark.py script provided in the
reference implementation .
We interpret the following from these results:
- Using two
__sitecustomize__scripts compared to
usercustomize.pyslows down the interpreter by 0.3%. We expect this slowdown until
usercustomize.pyare removed in a future release as even if the user does not create the files, the interpreter will still attempt to import them.
- With the arbitrary 50 pth files with code tested, moving those to
__sitecustomize__produces a speedup of ~3.5% in startup. Which is likely related to the simpler logic to evaluate
__sitecustomize__files compared to
- In general all measurements show that there is a low impact on startup time with this addition.
A new audit event will be added and triggered on
execution to facilitate security inspection by calling
 with “sitecustimze.exec_file” as name and the filename as
This PEP aims to move all code execution from
pth files to files within a
__sitecustomize__ directory. We think this is an improvement to system admins
for the following reasons:
- Allows to quickly identify the code being executed at startup time by the
interpreter by looking into a single directory rather than having to scan
- Allows to track usage of this feature through the new proposed audit event.
- Gives finer grain control by allowing to tune permissions on the
__sitecustomize__directory, potentially allowing users to install only packages that does not change the interpreter startup.
In short, whilst this allows for a malicious users to drop a file that will
be executed at startup, it’s an improvement compared to the existing
How to teach this
This can be documented and taught as simple as saying that the interpreter
will try to look for the
__sitecustomize__ directory at startup in its
site paths and if it finds any files with
.py extension, it will then
execute it one by one.
For system administrators and tools that package the interpreter, we can now
recommend placing files in
__sitecustomize__ as they used to place
sitecustomize.py. 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_files=["scripts/betterexceptions.py"], 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 file’s name.
This PEP adds a deprecation warning on
pth code execution in 3.11, 3.12 and 3.13. With
plans on removing those features by 3.14. The migration from those solutions
__sitecustomize__ should ideally be just moving the logic into a
Whilst the existing
sitecustomize.py mechanism was created targeting
System Administrators that placed it in a site path, the file could be
actually placed anywhere in the path at the time that the interpreter was
starting up. The new mechanism does not allow for users to place
__sitecustomize__ directories anywhere in the path, but only in site
paths. System administrators can recover a similar behavior to
sitecustomize.py by adding a custom file in
sitecustomize as a migration path.
An initial implementation that passes the CPython test suite is available for evaluation .
This implementation is just for the reviewer to play with and check potential issues that this PEP could generate.
Whilst the current status “works” it presents the issues listed in the motivation. After analyzing the impact of this change, we believe it is worth it, given the enhanced experience it brings.
Another option would be to just glorify and document the usage of
to inject code at startup code, but that is a suboptimal experience for users
as listed in the motivation.
__sitecustomize__ a namespace package
We considered making the directory a namespace package and just import all
the modules within it, which allowed searching across all paths in
sys.path at initialization time and provided a way to declare
dependencies between files by importing each other. This was rejected for
- This was unnecessarily broadening the list of paths where arbitrary files are executed.
- The logic brought additional complexity, like what to do if a package were
to install an
__init__.pyfile in one of the locations.
- It’s cheaper to search for
__sitecustomize__as we are looking for
pthfiles already in the site paths compared to performing an actual import of a namespace package.
Support for shutdown customization
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
We considered extending the use of entry points to allow specifying files
that should be executed at startup but we discarded that solution due to two
main reasons. The first one being impact on startup time. This approach will
require scanning all packages distribution information to just execute a
handful of files. This has an impact on performance even if the user is not
using the feature and such impact growths linearly with the number of packages
installed in the environment. The second reason was that the proposed
implementation in this PEP offers a single solution for startup customization
for packages and system administrators. Additionally, if the main objective of
entry points is to make it easy for libraries to install files at startup,
that can still be added and make the build backends just install the files
This document is placed in the public domain or under the CC0-1.0-Universal license, whichever is more permissive.
Thanks Pablo Galindo for contributing to this PEP and offering his PC to run the benchmark.
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Last modified: 2022-04-20 09:53:08 GMT