我想使用Dask将一些数据从单列扩展到多列。我这样做是通过使用Dask的DataFrame apply方法和一个自定义函数返回一个新的Pandas DataFrame来实现的。但是，当我尝试这样做时，我得到ValueError: If using all scalar values, you must pass an index。这是一个最小的复制案例：
import dask.dataframe as dd
import pandas as pd
pd_df = pd.DataFrame({'a': [1, 2, 3, 4, 5]}, dtype=float)
df = dd.from_pandas(pd_df, npartitions=2)
def custom_fn(row):
num = row['a']
frame = pd.DataFrame({
'squared': [num * num],
'x2': [num * 2],
}, dtype=float, index=[0])
return frame
new_frame = df.apply(custom_fn, axis=1, meta={
'squared': float,
'x2': float,
}, result_type='expand')
new_frame.head()
和堆栈跟踪：
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-38-6aaf3a5d32b2> in <module>()
12 }, result_type='expand')
13
---> 14 new_frame.head()
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/dataframe/core.pyc in head(self, n, npartitions, compute)
896
897 if compute:
--> 898 result = result.compute()
899 return result
900
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/base.pyc in compute(self, **kwargs)
154 dask.base.compute
155 """
--> 156 (result,) = compute(self, traverse=False, **kwargs)
157 return result
158
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/base.pyc in compute(*args, **kwargs)
396 keys = [x.__dask_keys__() for x in collections]
397 postcomputes = [x.__dask_postcompute__() for x in collections]
--> 398 results = schedule(dsk, keys, **kwargs)
399 return repack([f(r, *a) for r, (f, a) in zip(results, postcomputes)])
400
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/threaded.pyc in get(dsk, result, cache, num_workers, pool, **kwargs)
74 results = get_async(pool.apply_async, len(pool._pool), dsk, result,
75 cache=cache, get_id=_thread_get_id,
---> 76 pack_exception=pack_exception, **kwargs)
77
78 # Cleanup pools associated to dead threads
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/local.pyc in get_async(apply_async, num_workers, dsk, result, cache, get_id, rerun_exceptions_locally, pack_exception, raise_exception, callbacks, dumps, loads, **kwargs)
458 _execute_task(task, data) # Re-execute locally
459 else:
--> 460 raise_exception(exc, tb)
461 res, worker_id = loads(res_info)
462 state['cache'][key] = res
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/local.pyc in execute_task(key, task_info, dumps, loads, get_id, pack_exception)
228 try:
229 task, data = loads(task_info)
--> 230 result = _execute_task(task, data)
231 id = get_id()
232 result = dumps((result, id))
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/core.pyc in _execute_task(arg, cache, dsk)
116 elif istask(arg):
117 func, args = arg[0], arg[1:]
--> 118 args2 = [_execute_task(a, cache) for a in args]
119 return func(*args2)
120 elif not ishashable(arg):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/core.pyc in _execute_task(arg, cache, dsk)
117 func, args = arg[0], arg[1:]
118 args2 = [_execute_task(a, cache) for a in args]
--> 119 return func(*args2)
120 elif not ishashable(arg):
121 return arg
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/optimization.pyc in __call__(self, *args)
940 % (len(self.inkeys), len(args)))
941 return core.get(self.dsk, self.outkey,
--> 942 dict(zip(self.inkeys, args)))
943
944 def __reduce__(self):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/core.pyc in get(dsk, out, cache)
147 for key in toposort(dsk):
148 task = dsk[key]
--> 149 result = _execute_task(task, cache)
150 cache[key] = result
151 result = _execute_task(out, cache)
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/core.pyc in _execute_task(arg, cache, dsk)
117 func, args = arg[0], arg[1:]
118 args2 = [_execute_task(a, cache) for a in args]
--> 119 return func(*args2)
120 elif not ishashable(arg):
121 return arg
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/dataframe/core.pyc in apply_and_enforce(*args, **kwargs)
3792 func = kwargs.pop('_func')
3793 meta = kwargs.pop('_meta')
-> 3794 df = func(*args, **kwargs)
3795 if is_dataframe_like(df) or is_series_like(df) or is_index_like(df):
3796 if not len(df):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/dask/utils.pyc in __call__(self, obj, *args, **kwargs)
714
715 def __call__(self, obj, *args, **kwargs):
--> 716 return getattr(obj, self.method)(*args, **kwargs)
717
718 def __reduce__(self):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/frame.pyc in apply(self, func, axis, broadcast, raw, reduce, result_type, args, **kwds)
6485 args=args,
6486 kwds=kwds)
-> 6487 return op.get_result()
6488
6489 def applymap(self, func):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/apply.pyc in get_result(self)
149 return self.apply_raw()
150
--> 151 return self.apply_standard()
152
153 def apply_empty_result(self):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/apply.pyc in apply_standard(self)
258
259 # wrap results
--> 260 return self.wrap_results()
261
262 def apply_series_generator(self):
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/apply.pyc in wrap_results(self)
306 if len(results) > 0 and is_sequence(results[0]):
307
--> 308 return self.wrap_results_for_axis()
309
310 # dict of scalars
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/apply.pyc in wrap_results_for_axis(self)
382 # we have requested to expand
383 if self.result_type == 'expand':
--> 384 result = self.infer_to_same_shape()
385
386 # we have a non-series and don't want inference
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/apply.pyc in infer_to_same_shape(self)
400 results = self.results
401
--> 402 result = self.obj._constructor(data=results)
403 result = result.T
404
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/frame.pyc in __init__(self, data, index, columns, dtype, copy)
390 dtype=dtype, copy=copy)
391 elif isinstance(data, dict):
--> 392 mgr = init_dict(data, index, columns, dtype=dtype)
393 elif isinstance(data, ma.MaskedArray):
394 import numpy.ma.mrecords as mrecords
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/internals/construction.pyc in init_dict(data, index, columns, dtype)
210 arrays = [data[k] for k in keys]
211
--> 212 return arrays_to_mgr(arrays, data_names, index, columns, dtype=dtype)
213
214
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/internals/construction.pyc in arrays_to_mgr(arrays, arr_names, index, columns, dtype)
49 # figure out the index, if necessary
50 if index is None:
---> 51 index = extract_index(arrays)
52 else:
53 index = ensure_index(index)
/nail/home/shawn/pg/research_ipython/virtualenv_run/local/lib/python2.7/site-packages/pandas/core/internals/construction.pyc in extract_index(data)
306
307 if not indexes and not raw_lengths:
--> 308 raise ValueError('If using all scalar values, you must pass'
309 ' an index')
310
ValueError: If using all scalar values, you must pass an index
我可以省略result_type='expand'和meta kwarg来获得一个我在方法中返回的DataFrame完整的DataFrame，但是我确实希望它可以内联扩展。我在Python 2.7.6上使用Dask 1.1.4和Pandas 0.24.1。
编辑：我发现以后可以像这样扩展行：
new_frame = df.apply(custom_fn, axis=1)
dd.concat([
data for _, data in new_frame.iteritems()
], interleave_partitions=True).head()
这有点混乱，但至少目前看来似乎可行。