Jake Jake - 5 months ago 21
Python Question

Merge a list of pandas dataframes

There has been many similar questions but none specifically to this.

I have a list of data frames and I need to merge them together using a unique column

(date)
. Field names are different so concat is out.

I can manually use
df[0].merge(df[1],on='Date').merge(df[3],on='Date)
etc. to merge each df one by one, but the issue is that the number of data frames in the list differs with user input.

Is there any way to merge that just combines all data frames in a list at one go? Or perhaps some for in loop at does that?

I am using Python 2.7.

Answer

You can use reduce function where dfList is your list of data frames:

import pandas as pd
reduce(lambda x, y: pd.merge(x, y, on = 'Date'), dfList)

As a demo:

df = pd.DataFrame({'Date': [1,2,3,4], 'Value': [2,3,3,4]})
dfList = [df, df, df]
dfList

# [   Date  Value
#  0     1      2
#  1     2      3
#  2     3      3
#  3     4      4,    Date  Value
#  0     1      2
#  1     2      3
#  2     3      3
#  3     4      4,    Date  Value
#  0     1      2
#  1     2      3
#  2     3      3
#  3     4      4]

reduce(lambda x, y: pd.merge(x, y, on = 'Date'), dfList)
#   Date  Value_x  Value_y  Value
# 0    1        2        2      2
# 1    2        3        3      3
# 2    3        3        3      3
# 3    4        4        4      4