python - Pandas groupby 到 to_csv

Question

想要将 Pandas groupby 数据帧输出到 CSV。尝试了各种 StackOverflow 解决方案，但没有奏效。

Python 3.6.1， Pandas 0.20.1

groupby 结果如下:

id  month   year    count
week                
0   9066    82  32142   895
1   7679    84  30112   749
2   8368    126 42187   872
3   11038   102 34165   976
4   8815    117 34122   767
5   10979   163 50225   1252
6   8726    142 38159   996
7   5568    63  26143   582

想要一个看起来像的 csv

week  count
0   895
1   749
2   872
3   976
4   767
5   1252
6   996
7   582

当前代码:

week_grouped = df.groupby('week')
week_grouped.sum() #At this point you have the groupby result
week_grouped.to_csv('week_grouped.csv') #Can't do this - .to_csv is not a df function. 

阅读 SO 解决方案:

output groupby to csv file pandas

week_grouped.drop_duplicates().to_csv('week_grouped.csv')

结果: AttributeError:无法访问“DataFrameGroupBy”对象的可调用属性“drop_duplicates”，尝试使用“apply”方法

Python pandas - writing groupby output to file

week_grouped.reset_index().to_csv('week_grouped.csv')

结果: AttributeError:“无法访问‘DataFrameGroupBy’对象的可调用属性‘reset_index’，尝试使用‘apply’方法”

Answer 1

尝试这样做:

week_grouped = df.groupby('week')
week_grouped.sum().reset_index().to_csv('week_grouped.csv')

这会将整个数据帧写入文件。如果您只需要这两列，

week_grouped = df.groupby('week')
week_grouped.sum().reset_index()[['week', 'count']].to_csv('week_grouped.csv')

这里逐行解释原代码:

# This creates a "groupby" object (not a dataframe object) 
# and you store it in the week_grouped variable.
week_grouped = df.groupby('week')

# This instructs pandas to sum up all the numeric type columns in each 
# group. This returns a dataframe where each row is the sum of the 
# group's numeric columns. You're not storing this dataframe in your 
# example.
week_grouped.sum() 

# Here you're calling the to_csv method on a groupby object... but
# that object type doesn't have that method. Dataframes have that method. 
# So we should store the previous line's result (a dataframe) into a variable 
# and then call its to_csv method.
week_grouped.to_csv('week_grouped.csv')

# Like this:
summed_weeks = week_grouped.sum()
summed_weeks.to_csv('...')

# Or with less typing simply
week_grouped.sum().to_csv('...')