TL;DR

Update python3 references with vpython3 for consistent, reproducible builds.

Root cause

Instructions for setting up the Engine development environment guide users to install both Chromium's depot_tools and Python.

• Chromium's depot_tools (make sure it's in your path). We use the gclient tool from depot_tools.
  Make sure to install gclient by running ./gclient in the command line.
• Python (used by many of our tools, including gclient).

Hence, developers correctly assume that they may rely on python3 when developing scripts, but a user installed python3 can lead to unexpected behavior as seen in #108122 and confusion.

The history shows that we intended to use vpython3 for consistent, reproducible builds, but due the following upstream issue, #88719, those references were reverted back to python3.

WIP Todo List:

1. Update python3 references to vpython3

• Re-land vpython3 in .gn from depot_tools  buildroot#590
• Use vpython3 in windows tools/gn wrapper.
  Update reference to python3 with vpython3 engine#34950
• Use vpython3 in tools/gn.
  Update python3 reference to vpython3 engine#34952

2. Update Setting up the Engine development environment

• https://github.com/flutter/flutter/wiki/Setting-up-the-Engine-development-environment