python - 为什么我会收到此错误 “ValueError: Wrong number of items passed 2, placement implies 4”，即使代码没问题？

Question

我正在使用这段代码，它给了我这个错误，ValueError: Wrong number of items passed 2, placement implies 4. 它应该在 csv 文件中打印均值、中位数和众数通过使用其他 csv 文件中的数据。当我使用此代码时，它会将平均值和中位数打印到指定的 csv 文件，但不会打印模式。这是我正在使用的代码(如下)。

import pandas as pd
df = pd.read_csv('one_hello.csv', names=['teacher', 'student',\
         's.grade', 't.average','t.median','t.mode'])

df['Star Rating'] = df['Star Rating'].astype(int) #convert to integer.

df['t.mean'] = df.groupby('teacher')['s.grade'].transform('mean')
df['t.median'] = df.groupby('teacher')['s.grade'].transform('median')
df['t.mode'] = df.groupby('teacher')['s.grade'].transform(lambda group: group.mode())
df.to_csv('two_hello.csv') #write to file

文件“test_one”如下所示:

teacher         student         student grade
Jon             marin           99
Jon             Rob             81
Jon             marly           90
Jon             Ticy            90
Bon             lue             76
Bon             martin          76
Bon             marie           56
Ton             Seri            43
Ton             Loku            99

我希望文件“test_two”看起来像这样:

teacher         student         s.grade    t.avg    t.median    t.mode 
Jon             marin           99          90         90           90
Jon             Rob             81          90          90          90  
Jon             marly           90          90          90          90
Jon             Ticy            90          90          90          90
Bon             lue             76          69.3        76          76
Bon             martin          76          69.3        76          76
Bon             marie           56          69.3        76          76
Ton             Seri            43          69.3        71          43
Ton             Loku            99          69.3        71          99

但是，它只在另一个文件上打印“平均值”和“中值”，并给我这样的错误。

`Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/site-packages/pandas/core/groupby.py", line 3049, in transform
    s = klass(res, indexer)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/series.py", line 250, in __init__
    data = SingleBlockManager(data, index, fastpath=True)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/internals.py", line 4117, in __init__
    fastpath=True)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/internals.py", line 2719, in make_block
    return klass(values, ndim=ndim, fastpath=fastpath, placement=placement)
  File "/usr/local/lib/python2.7/site-packages/pandas/core/internals.py", line 115, in __init__
    len(self.mgr_locs)))
ValueError: Wrong number of items passed 2, placement implies 4`

Answer 1

假设 one_hello.csv 是 test_one 并且 Star Rating 是 s.grade 这应该可行。我只更改了 read_csv() 中的列名映射以匹配源文件:

import io
import pandas as pd


source = io.StringIO("""
Jon, marin, 99
Jon, Rob, 81
Jon, marly, 90
Jon, Ticy, 90
Bon, lue, 76
Bon, martin, 76
Bon, marie, 56
Ton, Seri, 43
Ton, Loku, 99
""")

df = pd.read_csv(source, names=['teacher', 'student', 's.grade'], dtype={'s.grade': int})
df['t.mean'] = df.groupby('teacher')['s.grade'].transform('mean')
df['t.median'] = df.groupby('teacher')['s.grade'].transform('median')
df['t.mode'] = df.groupby('teacher')['s.grade'].transform(lambda group: group.mode())
df.to_csv('out.csv')
print(df)

输出:

  teacher  student  s.grade     t.mean  t.median  t.mode
0     Jon    marin       99  90.000000        90      90
1     Jon      Rob       81  90.000000        90      90
2     Jon    marly       90  90.000000        90      90
3     Jon     Ticy       90  90.000000        90      90
4     Bon      lue       76  69.333333        76      76
5     Bon   martin       76  69.333333        76      76
6     Bon    marie       56  69.333333        76      76
7     Ton     Seri       43  71.000000        71      43
8     Ton     Loku       99  71.000000        71      99