如何解决无法在Append = True的镶木地板文件中存储的Dask数据帧上使用索引
我有一个用例,我想通过to_parquet(ddf,'TestParquet',append = True)将多个Dask数据帧存储到一个公共镶木存储中。
镶木地板文件的结构是通过向其写入的第一个数据帧设置的(没有append = True)。
每个数据框都有“分类索引”列。
类别从一开始就在所有数据框中都是已知的,并且没有一个数据框具有相同的类别。
数据帧在整个类别列表上进行了分区(因此每个分区在保存到拼花地板之前都具有空分区)。最终,一旦所有daframe保存到镶木地板上,所有类别/分区将包含数据。
问题:追加第二个数据框后,索引不能用于检索特定类别。
这是一个最小的可重现示例:
熊猫:0.24.2 快速拼花:0.4.1 快手:2.22.0
import pandas as pd
import dask.dataframe as dd
import fastparquet
data= {'Name': [ 'C','B','F','B'],'ID':[1,1,2,3,3],'Value':[2,4,3]}
df = pd.DataFrame(data)
df['Name']=df['Name'].astype(pd.api.types.CategoricalDtype(categories=['A','C','D','E','F'],ordered=True) )
ddf = dd.from_pandas(df,npartitions=2)
ddf=ddf.set_index('Name',sorted=False).repartition(divisions=['A',force=True)
ddf_parquet=dd.read_parquet('./TestParquet')
data2= {'Name': ['D','A','A'],4],'Value':[1,5,6]}
df2 = pd.DataFrame(data2)
df2['Name']=df2['Name'].astype(pd.api.types.CategoricalDtype(categories=['A',ordered=True) )
ddf2 = dd.from_pandas(df2,npartitions=2)
ddf2=ddf2.set_index('Name',force=True)
dd.to_parquet(ddf2,'./TestParquet',engine='fastparquet',append=True,ignore_divisions=True)
ddf_parquet2=dd.read_parquet('./TestParquet')
将第一个数据帧保存到镶木地板后,我可以使用索引而不会出现问题:
ddf_parquet.loc['B'].head()
ID Value
Name
F 3 2
F 2 4
但是,在附加第二个数据帧之后,尝试选择除第一个分区的索引值('B')以外的任何内容都会导致错误:
ddf_parquet2.loc['A'].head()
Traceback (most recent call last):
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4805,in get_slice_bound
return self._searchsorted_monotonic(label,side)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4756,in _searchsorted_monotonic
return self.searchsorted(label,side=side)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/base.py",line 1501,in searchsorted
return self._values.searchsorted(value,side=side,sorter=sorter)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/arrays/categorical.py",line 1370,in searchsorted
raise ValueError("Categorical not ordered\nyou can use "
ValueError: Categorical not ordered
you can use .as_ordered() to change the Categorical to an ordered one
During handling of the above exception,another exception occurred:
Traceback (most recent call last):
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/dask/dataframe/methods.py",line 42,in try_loc
return loc(df,iindexer,cindexer)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/dask/dataframe/methods.py",line 28,in loc
return df.loc[iindexer]
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexing.py",line 1500,in __getitem__
return self._getitem_axis(maybe_callable,axis=axis)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexing.py",line 1867,in _getitem_axis
return self._get_slice_axis(key,line 1533,in _get_slice_axis
slice_obj.step,kind=self.name)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4673,in slice_indexer
kind=kind)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4872,in slice_locs
start_slice = self.get_slice_bound(start,'left',kind)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4808,in get_slice_bound
raise err
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4802,in get_slice_bound
slc = self._get_loc_only_exact_matches(label)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/base.py",line 4772,in _get_loc_only_exact_matches
return self.get_loc(key)
File "/cba/local/mx/Linux_x86_64/python/Python-3.6.2/lib/python3.6/site-packages/pandas/core/indexes/category.py",line 438,in get_loc
raise KeyError(key)
KeyError: 'A'
我不知道什么,为什么在索引中找不到'A'键,以及为什么错误提示要对类别进行排序,因为在设置索引之前已对'名称'类别进行了排序。
一个观察结果:
-
,两个数据帧的划分均按预期设置为:
('A','B','C','D','E','F','F')
保存到Parquet并读回数据后,在保存第一个数据帧后将分区重新对齐到第一个数据帧的非空分区:
('B','F')
因此,我必须在第二个Daframe后面附加ignore_partitions = True,否则我将收到一条错误消息,指出划分是重叠的(这正是为什么我在保存最终分区/之前先对整个列表类别进行了分区/从第一个数据帧开始划分。
重置索引和重新索引实际上是不可行的,因为实际数据集非常庞大(保存到Parquet之前总共约200GB)。
设置每个数据框的划分以匹配其实际类别,最终会导致相同的错误。
任何帮助将不胜感激。
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