pandas query() 方法中的错误？
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Bug in pandas query() method?
提问by AKKO
I was experimenting several use cases for the pandas query() method, and tried one argument that threw an exception, but yet caused an unwanted modification to the data in my DataFrame.
我正在试验 pandas query() 方法的几个用例，并尝试了一个引发异常的参数，但仍导致对我的 DataFrame 中的数据进行了不必要的修改。
In [549]: syn_fmax_sort
Out[549]:
build_number name fmax
0 390 adpcm 143.45
1 390 aes 309.60
2 390 dfadd 241.02
3 390 dfdiv 10.80
....
211 413 dfmul 215.98
212 413 dfsin 11.94
213 413 gsm 194.70
214 413 jpeg 197.75
215 413 mips 202.39
216 413 mpeg2 291.29
217 413 sha 243.19
[218 rows x 3 columns]
So I wanted to use query()to just take out a subset of this dataframe that contains all the build_numberof 392, so I tried:
所以我想用来query()取出这个数据帧的一个子集，其中包含所有build_number392，所以我尝试了：
In [550]: syn_fmax_sort.query('build_number = 392')
That threw a ValueError: cannot label index with a null keyexception, but not only that, it returned back the full dataframe to me,and caused all the build_numberto be set to 392:
这引发了一个ValueError: cannot label index with a null key异常，但不仅如此，它还将完整的数据帧返回给我，并导致所有build_number设置为 392：
In [551]: syn_fmax_sort
Out[551]:
build_number name fmax
0 392 adpcm 143.45
1 392 aes 309.60
2 392 dfadd 241.02
3 392 dfdiv 10.80
....
211 392 dfmul 215.98
212 392 dfsin 11.94
213 392 gsm 194.70
214 392 jpeg 197.75
215 392 mips 202.39
216 392 mpeg2 291.29
217 392 sha 243.19
[218 rows x 3 columns]
However, I have since figured out how to get value 392 only, if I used syn_fmax_sort.query('391 < build_number < 393'), it works/
但是，我已经想出了如何仅获得 392 的值，如果我使用了syn_fmax_sort.query('391 < build_number < 393')，它就可以工作/
So my question is: Is the behavior that I observed above when I queried the dataframe wrongly due to a bug in the query()method?
所以我的问题是：当我错误地查询数据帧时，我在上面观察到的行为是由于query()方法中的错误吗？
回答by EdChum
It looks like you had a typo, you probably wanted to use ==rather than =, a simple example shows the same problem:
看起来你有一个错字，你可能想使用==而不是=，一个简单的例子显示了同样的问题：
In [286]:
df = pd.DataFrame({'a':np.arange(5)})
df
Out[286]:
a
0 0
1 1
2 2
3 3
4 4
In [287]:
df.query('a = 3')
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-287-41cfa0572737> in <module>()
----> 1 df.query('a = 3')
C:\WinPython-64bit-3.4.2.4\python-3.4.2.amd64\lib\site-packages\pandas\core\frame.py in query(self, expr, **kwargs)
1923 # when res is multi-dimensional loc raises, but this is sometimes a
1924 # valid query
-> 1925 return self[res]
1926
1927 def eval(self, expr, **kwargs):
C:\WinPython-64bit-3.4.2.4\python-3.4.2.amd64\lib\site-packages\pandas\core\frame.py in __getitem__(self, key)
1778 return self._getitem_multilevel(key)
1779 else:
-> 1780 return self._getitem_column(key)
1781
1782 def _getitem_column(self, key):
C:\WinPython-64bit-3.4.2.4\python-3.4.2.amd64\lib\site-packages\pandas\core\frame.py in _getitem_column(self, key)
1785 # get column
1786 if self.columns.is_unique:
-> 1787 return self._get_item_cache(key)
1788
1789 # duplicate columns & possible reduce dimensionaility
C:\WinPython-64bit-3.4.2.4\python-3.4.2.amd64\lib\site-packages\pandas\core\generic.py in _get_item_cache(self, item)
1066 res = cache.get(item)
1067 if res is None:
-> 1068 values = self._data.get(item)
1069 res = self._box_item_values(item, values)
1070 cache[item] = res
C:\WinPython-64bit-3.4.2.4\python-3.4.2.amd64\lib\site-packages\pandas\core\internals.py in get(self, item, fastpath)
2856 loc = indexer.item()
2857 else:
-> 2858 raise ValueError("cannot label index with a null key")
2859
2860 return self.iget(loc, fastpath=fastpath)
ValueError: cannot label index with a null key
It looks like internally it's trying to build an index using your query and it then checks the length and as it's 0 it raises a ValueErrorit probably should be KeyError, I don't know how it's evaluated your query but perhaps it's unsupported at the moment the ability to assign values to columns.
看起来它在内部尝试使用您的查询构建索引，然后检查长度，因为它是 0，所以ValueError它可能应该是KeyError，我不知道它是如何评估您的查询的，但目前可能不受支持为列赋值。
