我想根据列的选择从现有的数据帧创建视图或数据帧。
例如，我想从一个dataframe df1中创建一个dataframe df2，该dataframe df1包含除其中两个外的所有列。我试着这样做，但没有成功:
import numpy as np
import pandas as pd
# Create a dataframe with columns A,B,C and D
df = pd.DataFrame(np.random.randn(100, 4), columns=list('ABCD'))
# Try to create a second dataframe df2 from df with all columns except 'B' and D
my_cols = set(df.columns)
my_cols.remove('B').remove('D')
# This returns an error ("unhashable type: set")
df2 = df[my_cols]
我做错了什么?也许更普遍的是，熊猫必须有什么机制来支持从数据框架中选择和排除任意列集?
另一个选项，不需要在循环中删除或过滤:
import numpy as np
import pandas as pd
# Create a dataframe with columns A,B,C and D
df = pd.DataFrame(np.random.randn(100, 4), columns=list('ABCD'))
# include the columns you want
df[df.columns[df.columns.isin(['A', 'B'])]]
# or more simply include columns:
df[['A', 'B']]
# exclude columns you don't want
df[df.columns[~df.columns.isin(['C','D'])]]
# or even simpler since 0.24
# with the caveat that it reorders columns alphabetically
df[df.columns.difference(['C', 'D'])]
下面是如何创建一个不包含列列表的DataFrame副本:
df = pd.DataFrame(np.random.randn(100, 4), columns=list('ABCD'))
df2 = df.drop(['B', 'D'], axis=1)
但是要小心!你在你的问题中提到了视图，这表明如果你改变了df，你会希望df2也改变。(就像数据库中的视图一样。)
这个方法不能实现:
>>> df.loc[0, 'A'] = 999 # Change the first value in df
>>> df.head(1)
A B C D
0 999 -0.742688 -1.980673 -0.920133
>>> df2.head(1) # df2 is unchanged. It's not a view, it's a copy!
A C
0 0.251262 -1.980673
还要注意，@piggybox的方法也是如此。(尽管这个方法很漂亮，很圆滑，而且很Pythonic。我不会这么做的!!)
有关视图与副本的更多信息，请参阅这个SO答案和这个答案所指向的Pandas文档的这一部分。
另一个选项，不需要在循环中删除或过滤:
import numpy as np
import pandas as pd
# Create a dataframe with columns A,B,C and D
df = pd.DataFrame(np.random.randn(100, 4), columns=list('ABCD'))
# include the columns you want
df[df.columns[df.columns.isin(['A', 'B'])]]
# or more simply include columns:
df[['A', 'B']]
# exclude columns you don't want
df[df.columns[~df.columns.isin(['C','D'])]]
# or even simpler since 0.24
# with the caveat that it reorders columns alphabetically
df[df.columns.difference(['C', 'D'])]
