Exception catching failure with Python 3.7a3

While testing Astropy with Python 3.7a3 and Numpy 1.14pre I get failures with lead me to this piece of Cython code:

        for key, val in fill_values.items():
            try:
                self.fill_values[int(key)] = val
                self.fill_values[float(key)] = val
            except (ValueError, np.ma.MaskError):
                pass

where key can be a numpy.ma.core.MaskedConstant subclass (with a __hash__ value). And I get this traceback:

Traceback (most recent call last):
  File "test.py", line 11, in <module>
    t.write('test.csv', overwrite=True)
  File "/home/simon/dev/astropy/astropy/table/table.py", line 2536, in write
    io_registry.write(self, *args, **kwargs)
  File "/home/simon/dev/astropy/astropy/io/registry.py", line 564, in write
    writer(data, *args, **kwargs)
  File "/home/simon/dev/astropy/astropy/io/ascii/connect.py", line 86, in write_csv
    return write(table, filename, **kwargs)
  File "/home/simon/dev/astropy/astropy/io/ascii/ui.py", line 880, in write
    writer.write(table, output)
  File "/home/simon/dev/astropy/astropy/io/ascii/fastbasic.py", line 194, in write
    self._write(table, output, {'fill_values': [(core.masked, '')]})
  File "/home/simon/dev/astropy/astropy/io/ascii/fastbasic.py", line 170, in _write
    writer = cparser.FastWriter(table, **write_kwargs)
SystemError: <class 'astropy.io.ascii.cparser.FastWriter'> returned NULL without setting an error

I found a number of workarounds that avoid the crash, like printing the fill_values dict before the loop, or printing a random number or str in the except clause (before pass), or maybe more interesting getting a local reference for MaskError before the loop and using except (ValueError, MaskError).

Is there a known reason for this error, which happen only with Python 3.7 ?

[scoder]

We are currently preparing the release of Cython 0.29, which is recommended for use with Python 3.7. Just wait another week or two.

[pllim]

Hello. Exactly which commit to master fixed this? I am curious on why this error still happens on Astropy for Cython 0.28.5. Thanks!