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. What Works

Most of the basic and daily usage commands work with out of tree extensions being disabled. More than 98% of test suite works with Python 3. The lists of passing tests can be found at python3-whitelist

It is very likely that an alpha or beta Python 3 release will be there in first half of 2019.

Out of tree hg-extensions still don't work. There would be several unicode issues.

2. Contributing

We will be happy to review patches and speed up the work related to Python 3. Before you start there are few things related to current porting and how things work currently. Most of our efforts are to make sure have Python 2 compatibility intact while making Python 3 run.

• We have a source transformer which does following things on Python 3.

  1. It adds b'' in front of string starting with ' or " and not having any u'', r'' or b'' in front.

  2. Adds this line from mercurial.pycompat import delattr, getattr, hasattr, setattr, xrange, open on every python file.

  3. Converts every occurrence of iteritems to items on Python 3.

  4. Converts argument of *attr and encode, decode to unicodes by adding u''.

  5. The transformer currently works on mercurial/, hgext/ and hgext3rd/.

  6. The transformer code lies here and you can also use transformer on your .py files by adding them in the transformer.

• We deal with bytes internally, we have pycompat.py which contains hacks related to various functions of os module on Python.

• We also have encoding.environ which helps us using a bytes version of os.environ on both Python 2 and 3.

• We are also adding r'' at some places to make it a raw string.

• Encoding issues are generally uncovered by our tests (as everything was byte string on Python 2.)

3. How to contribute

• Pick a failing tests, run python3 ./run-tests.py <test-name> and try to fix the exceptions raised.

Pure-python tests are sometimes easier to port, but often need to be ported to use unittest first instead of our legacy testing system. The first step in migrating such tests to Python 3 involves porting to unittest, followed by any necessary followups to fix issues on Python 3. A list of tests that probably still need this work done can be obtained by running comm -23 <(hg files 'set:tests/test*py - grep(unittest)' | sed 's$tests/$$') contrib/python3-whitelist.

The practice we follow now is run commands which are not yet fixed and try to fix the exceptions raised. So our current approach is exception based.

4. 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.

CategoryAudit