This package will install Python $FULL_VERSION for macOS $MACOSX_DEPLOYMENT_TARGET for the following architecture(s): $ARCHITECTURES.

Certificate verification and OpenSSL

This package includes its own private copy of OpenSSL 1.1.1.   The trust certificates in system and user keychains managed by the Keychain Access application and the security command line utility are not used as defaults by the Python ssl module.  A sample command script is included in /Applications/Python 3.9 to install a curated bundle of default root certificates from the third-party certifi package (https://pypi.org/project/certifi/).  Double-click on Install Certificates to run it.

The bundled pip has its own default certificate store for verifying download connections.

Using IDLE or other Tk applications 

This package includes its own private version of Tcl/Tk 8.6. It does not use any system-supplied or third-party supplied versions of Tcl/Tk.

Due to new security checks on macOS 10.15 Catalina, when launching IDLE macOS may open a window with a message "Python" would like to access files in your Documents folder.  This is normal as IDLE uses your Documents folder as its default when opening and saving files; you can still choose other locations in the Open and Save file dialog windows.  Click on the OK button to proceed.

macOS 11.0 (Big Sur) and Apple Silicon Mac support [new in 3.9.1]

As of 2020-11, macOS 11.0 (Big Sur) is the latest release of macOS and one of its major features is the support of new Apple Silicon Macs that are based on the ARM64 CPU architecture specification rather than the Intel 64 (x86_64) architecture used previously. There are other changes in Big Sur that affect Python operation regardless of CPU architecture. As of 3.9.1, Python binaries from python.org fully support Big Sur. 

python.org binaries for macOS have been provided via a downloadable installer that supports the Intel 64 architecture on macOS 10.9 and newer.  This installer variant remains the default download for 3.9.1;  it will install and run on all Macs that run macOS 10.9 or later, including 11.0 (Big Sur). This variant should run transparently on new Apple Silicon Macs using Apple's Rosetta 2 emulation. 

Beginning with 3.9.1, we also provide a new "universal2" installer variant that provides universal binaries for both ARM64 and Intel 64 architectures and is also supported on all Macs that support macOS 10.9 or later.  Some of the advantages of the new installer variant: native ARM64 code on Apple Silicon Macs should run significantly faster than Rosetta2-emulated code; some operating system functions and options introduced in macOS releases since 10.9 are now exposed when available (primarily in the os module); binary wheels built for use with the current 10.9 variant *should* also work with the new variant; the new installer variant includes Tcl/Tk 8.6.10 rather than 8.6.8.

Because of the scope of changes needed to fully support 11.0 and Apple Silicon Macs, the new "universal2" variant should be considered experimental in the 3.9.1 release. You may need to upgrade third-party components, like pip, to later versions once they are released. You may experience differences in behavior in IDLE and other Tk-based applications due to using the newer version of Tk.  As always, if you encounter problems when using this installer variant, please check https://bugs.python.org for existing reports and for opening new issues.

Other changes

For other changes in this release, see the What's new section in the Documentation Set for this release and its Release Notes link at https://www.python.org/downloads/.