Python 3

This is a status page for keeping track of what needs to be done to make progress on Mercurial on Python 3. Our current aim is to support Python 3.5+.

1. Status

We have been testing by installing default mercurial on our system using Python 3. Most of the things work correctly. Things which don't work can be found at BetaBugs section below.

We are planning to mark hg 5.0 which is scheduled for May 1 as Python 3 beta release.

If you are an extension author and want to port the extension, https://www.mercurial-scm.org/repo/hg-committed/file/tip/mercurial/pycompat.py contains most of our compatibility hacks. If you need help or guidance, you can message on IRC or devel mailing list. We will be happy to help you.

2. Things need to be investigated

• Windows encoding changes

  https://docs.python.org/3/whatsnew/3.6.html#pep-529-change-windows-filesystem-encoding-to-utf-8

• Lazy importer performance overhead. Our custom importer on Python 2 always returns a stub module during import. Python 3's does I/O to verify the module exists then returns a lazy module that is loaded when first accessed. In addition to behavior differences, the I/O may contribute sufficient performance overhead to constitute a problem.

• A mechanism for extensions to advertise that they are Python 3 compatible. Nearly every extension will break in Python 3. We may want a mechanism that requires extensions to self-declare that they are Python 3 compatible - possibly via special syntax in their source code or the presence of a well-named variable. It might have to be at the source level because Python 3 would need to evaluate code in order to obtain the value of a module-level variable.

3. Beta bugs

Following are things which don't work right now:

• ~1% of tests fail
• The phabricator extension
• curses interface
• out of core extensions

If you find anything apart from this not working, definitely go ahead and edit this page and we will fix it.

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