pandas: melt 100+ variables into 100+ new dataframes

问题

Pretty new to stackoverflow, please bear with me if the format looks odd..

I have a big set of data with 100+ columns of data structured like:

countrya countryb year variable1 variable2 ...... varaible100

I want to have the 100 variables separated into 100 new dataframes and save them into csvs.

Below is the code I have for creating 1 new csv.

dfm1=pd.melt(df, id_vars=['countrya','countryb','year'], value_vars=['variable1'],
         value_name='variable1')
dfm1.drop('variable',axis=1)
dfm1.to_csv('newdf1.csv')

How can I automate the process? Thank you!

回答1:

Here is one way. First, create the data frame.

import pandas as pd

df = pd.DataFrame({
    'country_a': [1, 2, 3],
    'country_b': [4, 5, 6],
    'year': [2018, 2019, 2020],
    'var_a': ['a', 'b', 'c'],
    'var_b': ['x', 'y', 'z']
})

print(df)
   country_a  country_b  year var_a var_b
0          1          4  2018     a     x
1          2          5  2019     b     y
2          3          6  2020     c     z

Second, iterate over the fields with your column names.

base_fields = df.columns[:3].to_list()    # columns in every file
var_fields = df.columns[3:]               # var_a, var_b, ...

for var_field in var_fields:
    file_name = f'{var_field}.csv'
    with open(file_name, 'wt') as handle:
        fields = base_fields + [var_field]
        df.loc[:, fields].to_csv(handle)
        
        print(f'wrote {fields} to {file_name}')


wrote ['country_a', 'country_b', 'year', 'var_a'] to var_a.csv
wrote ['country_a', 'country_b', 'year', 'var_b'] to var_b.csv
                                          ^              ^
                                          last field and file name change

回答2:

You can use a for loop against all variables, and call your function inside it(assuming your sample code is correct)

def split(df, variable_name):
  dfm1=pd.melt(df, id_vars=['countrya','countryb',variable_name], value_vars=[variable_name], value_name=variable_name)
  dfm1.drop('variable',axis=1) # I don't know what's this line used for
  dfm1.to_csv('newdf_{}.csv'.format(variable_name))

for variable_name in ['variable1', 'variable2']:
  split(df, variable_name)

来源：https://stackoverflow.com/questions/63386562/pandas-melt-100-variables-into-100-new-dataframes

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python

pandas

automation

melt