我想做什么？
pd.read_csv(... nrows=###)可以读取文件的前nrow。我希望在使用pd.read_hdf(...)时也这样做。在
有什么问题？
我被documentation搞糊涂了。start和{}看起来像我需要的，但是当我尝试它时，返回一个ValueError。我尝试的第二件事是使用nrows=10，认为它可能是允许的**kwargs。当我这样做时，不会抛出错误，但也会返回完整的数据集，而不是仅仅返回10行。在
问题：如何正确地从HDF文件中读取较小的行子集？(编辑：不必先把整件事读入内存！)在
以下是我的互动环节：>>> import pandas as pd
>>> df = pd.read_hdf('storage.h5')
Traceback (most recent call last):
File "", line 1, in
df = pd.read_hdf('storage.h5')
File "C:\Python35\lib\site-packages\pandas\io\pytables.py", line 367, in read_hdf
raise ValueError('key must be provided when HDF5 file '
ValueError: key must be provided when HDF5 file contains multiple datasets.
>>> import h5py
>>> f = h5py.File('storage.h5', mode='r')
>>> list(f.keys())[0]
'table'
>>> f.close()
>>> df = pd.read_hdf('storage.h5', key='table', start=0, stop=10)
Traceback (most recent call last):
File "", line 1, in
df = pd.read_hdf('storage.h5', key='table', start=0, stop=10)
File "C:\Python35\lib\site-packages\pandas\io\pytables.py", line 370, in read_hdf
return store.select(key, auto_close=auto_close, **kwargs)
File "C:\Python35\lib\site-packages\pandas\io\pytables.py", line 740, in select
return it.get_result()
File "C:\Python35\lib\site-packages\pandas\io\pytables.py", line 1447, in get_result
results = self.func(self.start, self.stop, where)
File "C:\Python35\lib\site-packages\pandas\io\pytables.py", line 733, in func
columns=columns, **kwargs)
File "C:\Python35\lib\site-packages\pandas\io\pytables.py", line 2890, in read
return self.obj_type(BlockManager(blocks, axes))
File "C:\Python35\lib\site-packages\pandas\core\internals.py", line 2795, in __init__
self._verify_integrity()
File "C:\Python35\lib\site-packages\pandas\core\internals.py", line 3006, in _verify_integrity
construction_error(tot_items, block.shape[1:], self.axes)
File "C:\Python35\lib\site-packages\pandas\core\internals.py", line 4280, in construction_error
passed, implied))
ValueError: Shape of passed values is (614, 593430), indices imply (614, 10)
>>> df = pd.read_hdf('storage.h5', key='table', nrows=10)
>>> df.shape
(593430, 614)
编辑：
我刚尝试使用where：
^{pr2}$
收到一个TypeError，该错误指示一个固定的格式存储(默认的format值df.to_hdf(...))：TypeError: cannot pass a where specification when reading from a
Fixed format store. this store must be selected in its entirety
这是否意味着如果格式是固定格式，我就不能选择行的子集？