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matplotlib's experiences porting to Python 3.

- Easy things (that 2to3 handles pretty well):

  - print function

  - "in" vs. "has_key"

  - "except Exception as exc" syntax

  - stdlib stuff

- Unicode

  - Unicode literals by default

  - no implicit conversion between Unicode and bytes (encoding must be explicit)

  - files must have explicit Unicode encoding, or be binary -- hard to do both at same time

- Exceptions

{{{
  exceptions_found = []
  try:
      do_something()
  except Exception as e:
      exceptions_found.append(e)
}}}
  
Since e now has a __traceback__ member, this creates a cycle e -> traceback -> current stack frame -> e. This cycle will keep alive all local variables in all frames beneath the source of the exception.

- C/C++ issues

  - For us, mostly hidden by using CXX

  - PyCObject was dropped in Python 3.2 -- had to port CXX to use its replacement PyCapsule