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+# Porting to Python 3
+
+This is still a proposal at this stage.
+
+## Motivation
+
+* Make code behave the same on Python 2 and Python 3, insofar as one can, so e.g. `map()` is the same on Python 2 and Python 3 (i.e. lazy).
+* Reduce errors by relying on Python 2 behavior and tests as well as manual review.
+* Try to reduce grunt work.
+
+## How to choose a module to port
+
+TBD, something involving core abstractions first, then dependency graph topological traversal.
+
+Assume for now we've picked a module.
+
+## The porting process, big picture
+
+For a module M, there is also a corresponding module T, the unittests for M.
+If the tests for M are embedded into a module that tests multiple modules, step one is to split off the tests so there's T that only tests M.
+
+Then:
+
+1. Update T to run on both 2+3 (see below for what that looks like).
+2. Run T's tests on Python 2. They should still pass! If they don’t, something broke.
+3. Port the code module M.
+4. Now run T's tests on Python 3.
+5. Fix any problems caught by the tests.
+6. Add both M and T to `allmydata/util/_python3.py`.
+7. Submit for code review.
+
+
+### Porting a specific Python file
+
+**First**, add explicit byte or unicode annotations for strings where needed.
+
+**Second**, run `futurize --write --both-stages --all-imports path/to/file.py`.
+
+**Third**, replace the `from builtins import *` variant, if any, with:
+
+```#!python
+from future.utils import PY2
+if PY2:
+    from builtins import filter, map, zip, ascii, chr, hex, input, next, oct, open, pow, round, super, bytes, dict, int, list, object, range, str, max, min  # noqa: F401
+```
+
+This adds builtins that match Python 3's semantics. The `#noqa: F401` keeps flake8/pyflakes from complaining about unused imports. We do unused imports so that people changing code later don't have to manually check if `map()` is old style or new style.
+
+**Fourth**, manually review the code. Futureize is nice, but it very definitely doesn't catch everything, or it makes wrong decisions.
+
+In particular:
+
+* `map()`, `filter()`, etc. are now lazy.
+* `dict.keys()` and friends now return a view of the underlying data, rather than a list with a copy.
+
+**Fifth**, add a note to the module docstring saying it was ported to Python 3.
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