Let’s fix Python’s Windows distribution
Python NSIS Visual Basic
Latest commit 3598500 Oct 19, 2017 @uranusjr uranusjr Guard against copy failure
Only copy files to scripts, not directories, and don't stop at errors --
print a warning and march one.


SNAFU the Python Manager

Build status

SNAFU is a Python installer manager for Windows. It fetches and installs Python of a version you specify, and manages the Path environment variable for you.



Find installers in Releases. Install, and restart your shell.

Installing Pythons

Install Python 3.6:

snafu install 3.6

This should install the latest patch version of CPython 3.6. We don’t support installing patch versions. This is because there is no good way to deal with patch versions on Windows (the py launcher only supports major and minor versioning), and you should always use the latest patch anyway.

After installing a version, pythonX.Y will be available as a command.

If you’re on a 64-bit OS, the 64-bit version is installed by default. To install a 32-bit version on a 64-bit machine, use the -32 suffix:

snafu install 3.6-32

If you install both 32- and 64-bit Pythons, pythonX.Y will point to the 64-bit version, and pythonX.Y-32 32-bit.

(Note: Due to restrictions in the standard Python installer, versions up to 3.4 are available in either 64- or 32-bit, not both.)

On 32-bit hosts, only 32-bit Pythons are available. Not suffixes are needed.

Set Default Python Version

Set Python 3.5 as default:

snafu use 3.5

This does two things:

  1. python3 is mapped to Python 3.5.
  2. Scripts in Python35\Scripts are linked into PATH.

Special case: python, pip and easy_install are never linked, to avoid ambiguity between Python 2 and 3. Use python3, pip3, pip3.5, etc. instead.

You can activate multiple versions together:

snafu use 3.5 2.7

The version used first (3.5 in this example) takes precedence if there are conflicting commands, e.g. if both 2.7 and 3.5 have virtualenv installed, the 3.5 version will be linked.

Check the current using order:

> snafu use
3.5 2.7


To uninstall a version:

snafu uninstall 3.5

Listing Versions

List installed versions:

snafu list

List all versions available, including those not installed:

snafu list --all

This results in something like this:

o 2.7
o 3.4
* 3.6
  • The o prefix means the version is installed.
  • * signifies an active version.
  • No prefix if the version is not installed.

The installation status is detected through the Windows registry.

Development Guide


Optional Dependencies

  • NSIS 3.x if you want to build the installer. makensis needs to be available in your shell.

Project Setup

Download and enter the project:

git clone https://github.com/uranusjr/snafu.git
cd snafu

Set up environment:

pipenv install --dev

Run Tests

pipenv run -- pytest tests

Unfortunately there are only very limited tests right now.


pipenv run -- python -m snafu [COMMAND] ...

This should have the same behaviour as an installed SNAFU command, but run inside the Pipenv-managed virtual environment.

Build the Installer

pipenv run -- python installers\build.py

You can only build installers of your host’s architecture. Cross compilation is certainly possible, but I haven’t found the need to set it up.

After the command finishes you should get an EXE in the installers directory. There are some other options available in build.py you can check them out yourself.

Development Guideline

Try to follow the code style. For Python code, run the linter to check for issues before submitting:

pipenv run -- flake8 .

Frequently Asked Questions

Why Not Just Use the Option “Add Python to PATH”?

CPython’s standard Windows build, unlike on UNIX-like systems, does not provide the “altinstall” option. This means every Python distribution on Windows only has one Python executable called python.exe, not versioned names such as python3.6.exe.

Adding Python to PATH stops being a good idea the moment you need a second installtion. You can only access one Python at a time, and installed scripts from different versions start to mix, which is a bad thing.[#]_ The PATH environment variable is also very tedious and delicate to manipulate.

[1]This is not a Windows-only problem, but also exactly why tutorials these days don’t recommand installing Python via python.org, but with platform-specific tools instead.

Why Not Use the Py Launcher?

Python introduced PEP 397 partly to solve the python.exe problem (also to interpret the shebang line on Windows). It installs a py.exe to your PATH, and instead of invoking python.exe directly, you should use, for example:

py -3.5 foo.py

to run foo.py with Python 3.5.

This is such a good idea SNAFU installs the Py Launcher during setup, and I encourage you to use it. But SNAFU also solves a few additional use cases that py.exe doesn’t:

  • Availability of versioned Python executables, e.g. python3.6.exe.
  • Managing commands other than python.exe.

SNAFU’s implementation also relies on a lot of the same values read by py.exe, so you can view SNAFU as an extension to it, not a replacement.

Architecture (Implementation Details)

How are Pythons installed?

The official CPython installers are downloaded, and executed in a non-interactive manner. Check out the relevant documentation for more details:

Where are Pythons installed?

%LOCALAPPDATA%\Programs\Python\<version>. This is the standard “only-for-me” installation location for Python 3.5+, and we retrofit this rule to older versions as well for consistency.

How are Executables linked?

Script executables are copied. .py files works as well because they have appropriate shebang lines, and can be handled by the py launcher, as specified in PEP 397.

The python.exe programs cannot be copied as-is because they require additional DLL files. SNAFU creates Windows shortcuts (.lnk) and makes them executable, so you can run them like recular commands. So what you get is actually python3.6.lnk, not .exe, but that’s good enough most of the time.

Why the Name?

Because Python is hard, Windows is harder, and setting up Windows for Python development is SNAFU. Or it’s Supernatrual Administration for You. Mosky says it sounds kind of like snake, so there’s that.