在具有动态名称的熊猫中创建新数据框也添加新列

Question

I have a dataframe df

 df = pd.DataFrame({'A':['-a',1,'a'], 
               'B':['a',np.nan,'c'],
               'ID':[1,2,2],
                't':[pd.tslib.Timestamp.now(),pd.tslib.Timestamp.now(),
                    np.nan]})

Added a new column

df['YearMonth'] = df['t'].map(lambda x: 100*x.year + x.month)

Now I want to write a function or macro which will do date comparasion, create a new dataframe also add a new column to dataframe.

I tried like this but seems I am going wrong:

def test(df,ym):
    df_new=df
    if(ym <= df['YearMonth']):
        df_new+"_"+ym=df_new
        return df_new+"_"+ym
    df_new+"_"+ym['new_col']=ym

Now when I call test function I want a new dataframe should get created named as df_new_201612 and this new dataframe should have one more column, named as new_col that has value of ym for all the rows.

test(df,201612)

The output of new dataframe is:

df_new_201612

A   B   ID  t                           YearMonth   new_col
-a  a   1   2016-12-05 12:37:56.374620  201612      201612 
1   NaN 2   2016-12-05 12:37:56.374644  201208      201612 
a   c   2   nat                         nan         201612 

Answer 1

Creating variables with dynamic names is typically a bad practice.

I think the best solution for your problem is to store your dataframes into a dictionary and dynamically generate the name of the key to access each dataframe.

import copy

dict_of_df = {}
for ym in [201511, 201612, 201710]:

    key_name = 'df_new_'+str(ym)    

    dict_of_df[key_name] = copy.deepcopy(df)

    to_change = df['YearMonth']< ym
    dict_of_df[key_name].loc[to_change, 'new_col'] = ym   

dict_of_df.keys()
Out[36]: ['df_new_201710', 'df_new_201612', 'df_new_201511']

dict_of_df
Out[37]: 
{'df_new_201511':     A    B  ID                       t  YearMonth  new_col
 0  -a    a   1 2016-12-05 07:53:35.943     201612   201612
 1   1  NaN   2 2016-12-05 07:53:35.943     201612   201612
 2   a    c   2 2016-12-05 07:53:35.943     201612   201612,
 'df_new_201612':     A    B  ID                       t  YearMonth  new_col
 0  -a    a   1 2016-12-05 07:53:35.943     201612   201612
 1   1  NaN   2 2016-12-05 07:53:35.943     201612   201612
 2   a    c   2 2016-12-05 07:53:35.943     201612   201612,
 'df_new_201710':     A    B  ID                       t  YearMonth  new_col
 0  -a    a   1 2016-12-05 07:53:35.943     201612   201710
 1   1  NaN   2 2016-12-05 07:53:35.943     201612   201710
 2   a    c   2 2016-12-05 07:53:35.943     201612   201710}

 # Extract a single dataframe
 df_2015 = dict_of_df['df_new_201511']

Answer 2

There is a more easy way to accomplish this using exec method. The following steps can be done to create a dataframe at runtime.

1.Create the source dataframe with some random values.

import numpy as np
import pandas as pd
    
df = pd.DataFrame({'A':['-a',1,'a'], 
                   'B':['a',np.nan,'c'],
                   'ID':[1,2,2]})

2.Assign a variable that holds the new dataframe name. You can even send this value as a parameter or loop it dynamically.

new_df_name = 'df_201612'

3.Create dataframe dynamically using exec method to copy data from source dataframe to the new dataframe dynamically and in the next line assign a value to new column.

exec(f'{new_df_name} = df.copy()')
exec(f'{new_df_name}["new_col"] = 123') 

4.Now the dataframe df_201612 will be available on the memory and you can execute print statement along with eval to verify this.

print(eval(new_df_name))