# Buggy code
def _add_calibration_datasets(
    self, ch, datasets, reverse_offset, reverse_scale, decimals
):
    _reverse_offset = reverse_offset
    _reverse_scale = reverse_scale
    _decimals = decimals
    _table_calibration = ""
    found_calibration = False
    skip_calibration = False
    for i, ds in enumerate(datasets):
        if isinstance(ds.attrs["prerequisites"][i], DatasetID):
            if ds.attrs["prerequisites"][i][0] == ch:
                if ds.attrs["prerequisites"][i][4] == "RADIANCE":
                    raise NotImplementedError(
                        "Mitiff radiance calibration not implemented."
                    )
                # _table_calibration += ', Radiance, '
                # _table_calibration += '[W/m²/µm/sr]'
                # _decimals = 8
                elif ds.attrs["prerequisites"][i][4] == "brightness_temperature":
                    found_calibration = True
                    _table_calibration += ", BT, "
                    _table_calibration += "\u00b0"  # '\u2103'
                    _table_calibration += "[C]"

                    _reverse_offset = 255.0
                    _reverse_scale = -1.0
                    _decimals = 2
                elif ds.attrs["prerequisites"][i][4] == "reflectance":
                    found_calibration = True
                    _table_calibration += ", Reflectance(Albedo), "
                    _table_calibration += "[%]"
                    _decimals = 2
                else:
                    LOG.warning(
                        "Unknown calib type. Must be Radiance, Reflectance or BT."
                    )

                    break
            else:
                continue
        else:
            _table_calibration = ""
            skip_calibration = True
            break
    if not found_calibration:
        _table_calibration = ""
        skip_calibration = True
        # How to format string by passing the format
        # http://stackoverflow.com/questions/1598579/rounding-decimals-with-new-python-format-function
    return (
        skip_calibration,
        _table_calibration,
        _reverse_offset,
        _reverse_scale,
        _decimals,
    )


# Traceback (KeyError)
scn.save_dataset('natural', writer='mitiff', filename='/tmp/mitiff.tif')
[DEBUG: 2018-07-16 12:29:43 : satpy.writers] Reading ['/home/lahtinep/Software/miniconda3/lib/python3.6/site-packages/satpy-0.9.1a0.dev0-py3.6.egg/satpy/etc/writers/mitiff.yaml']
[DEBUG: 2018-07-16 12:29:43 : satpy.writers.mitiff] Starting in mitiff save_dataset ...
[WARNING: 2018-07-16 12:29:43 : satpy.writers.mitiff] Unset save_dir. Use: ./
/home/lahtinep/Software/miniconda3/lib/python3.6/site-packages/pyresample/kd_tree.py:924: RuntimeWarning: invalid value encountered in sqrt
mask=mask)
[DEBUG: 2018-07-16 12:29:45 : satpy.writers.mitiff] create_opts: ./
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-48-762780f9f91a> in <module>()
----> 1 lcl.save_dataset('natural', writer='mitiff', filename='/tmp/mitiff.tif')

~/Software/miniconda3/lib/python3.6/site-packages/satpy-0.9.1a0.dev0-py3.6.egg/satpy/scene.py in save_dataset(self, dataset_id, filename, writer, overlay, compute, **kwargs)
1008         return writer.save_dataset(self[dataset_id], filename=filename,
1009                                    overlay=overlay, compute=compute,
-> 1010                                    **save_kwargs)
1011
1012     def save_datasets(self, writer="geotiff", datasets=None, compute=True,

~/Software/miniconda3/lib/python3.6/site-packages/satpy-0.9.1a0.dev0-py3.6.egg/satpy/writers/mitiff.py in save_dataset(self, dataset, filename, fill_value, compute, base_dir, **kwargs)
116
117         if compute:
--> 118             return delayed.compute()
119         return delayed
120

~/Software/miniconda3/lib/python3.6/site-packages/dask/base.py in compute(self, **kwargs)
154         dask.base.compute
155         """
--> 156         (result,) = compute(self, traverse=False, **kwargs)
157         return result
158

~/Software/miniconda3/lib/python3.6/site-packages/dask/base.py in compute(*args, **kwargs)
400     keys = [x.__dask_keys__() for x in collections]
401     postcomputes = [x.__dask_postcompute__() for x in collections]
--> 402     results = schedule(dsk, keys, **kwargs)
403     return repack([f(r, *a) for r, (f, a) in zip(results, postcomputes)])
404

~/Software/miniconda3/lib/python3.6/site-packages/dask/threaded.py in get(dsk, result, cache, num_workers, **kwargs)
73     results = get_async(pool.apply_async, len(pool._pool), dsk, result,
74                         cache=cache, get_id=_thread_get_id,
---> 75                         pack_exception=pack_exception, **kwargs)
76
77     # Cleanup pools associated to dead threads

~/Software/miniconda3/lib/python3.6/site-packages/dask/local.py in get_async(apply_async, num_workers, dsk, result, cache, get_id, rerun_exceptions_locally, pack_exception, raise_exception, callbacks, dumps, loads, **kwargs)
519                         _execute_task(task, data)  # Re-execute locally
520                     else:
--> 521                         raise_exception(exc, tb)
522                 res, worker_id = loads(res_info)
523                 state['cache'][key] = res

~/Software/miniconda3/lib/python3.6/site-packages/dask/compatibility.py in reraise(exc, tb)
67         if exc.__traceback__ is not tb:
68             raise exc.with_traceback(tb)
---> 69         raise exc
70
71 else:

~/Software/miniconda3/lib/python3.6/site-packages/dask/local.py in execute_task(key, task_info, dumps, loads, get_id, pack_exception)
288     try:
289         task, data = loads(task_info)
--> 290         result = _execute_task(task, data)
291         id = get_id()
292         result = dumps((result, id))

~/Software/miniconda3/lib/python3.6/site-packages/dask/local.py in _execute_task(arg, cache, dsk)
269         func, args = arg[0], arg[1:]
270         args2 = [_execute_task(a, cache) for a in args]
--> 271         return func(*args2)
272     elif not ishashable(arg):
273         return arg

~/Software/miniconda3/lib/python3.6/site-packages/satpy-0.9.1a0.dev0-py3.6.egg/satpy/writers/mitiff.py in _delayed_create(create_opts, dataset)
76                     kwargs['name'] = dataset.attrs['name']
77                 if 'start_time' not in kwargs:
---> 78                     kwargs['start_time'] = dataset.attrs['start_time']
79                 if 'sensor' not in kwargs:
80                     kwargs['sensor'] = dataset.attrs['sensor']

KeyError: 'start_time'

# Fixed code
def _add_calibration_datasets(
    self, ch, datasets, reverse_offset, reverse_scale, decimals
):
    _reverse_offset = reverse_offset
    _reverse_scale = reverse_scale
    _decimals = decimals
    _table_calibration = ""
    found_calibration = False
    skip_calibration = False
    for i, ds in enumerate(datasets):
        if "prerequisites" in ds.attrs and isinstance(
            ds.attrs["prerequisites"][i], DatasetID
        ):
            if ds.attrs["prerequisites"][i][0] == ch:
                if ds.attrs["prerequisites"][i][4] == "RADIANCE":
                    raise NotImplementedError(
                        "Mitiff radiance calibration not implemented."
                    )
                # _table_calibration += ', Radiance, '
                # _table_calibration += '[W/m²/µm/sr]'
                # _decimals = 8
                elif ds.attrs["prerequisites"][i][4] == "brightness_temperature":
                    found_calibration = True
                    _table_calibration += ", BT, "
                    _table_calibration += "\u00b0"  # '\u2103'
                    _table_calibration += "[C]"

                    _reverse_offset = 255.0
                    _reverse_scale = -1.0
                    _decimals = 2
                elif ds.attrs["prerequisites"][i][4] == "reflectance":
                    found_calibration = True
                    _table_calibration += ", Reflectance(Albedo), "
                    _table_calibration += "[%]"
                    _decimals = 2
                else:
                    LOG.warning(
                        "Unknown calib type. Must be Radiance, Reflectance or BT."
                    )

                    break
            else:
                continue
        else:
            _table_calibration = ""
            skip_calibration = True
            break
    if not found_calibration:
        _table_calibration = ""
        skip_calibration = True
        # How to format string by passing the format
        # http://stackoverflow.com/questions/1598579/rounding-decimals-with-new-python-format-function
    return (
        skip_calibration,
        _table_calibration,
        _reverse_offset,
        _reverse_scale,
        _decimals,
    )