Concatenate a list of pandas dataframes together

Question

I have a list of Pandas dataframes that I would like to combine into one Pandas dataframe. I am using Python 2.7.10 and Pandas 0.16.2

I created the list of dataframes from:

import pandas as pd
dfs = []
sqlall = "select * from mytable"

for chunk in pd.read_sql_query(sqlall , cnxn, chunksize=10000):
    dfs.append(chunk)

This returns a list of dataframes

type(dfs[0])
Out[6]: pandas.core.frame.DataFrame

type(dfs)
Out[7]: list

len(dfs)
Out[8]: 408

Here is some sample data

# sample dataframes
d1 = pd.DataFrame({'one' : [1., 2., 3., 4.], 'two' : [4., 3., 2., 1.]})
d2 = pd.DataFrame({'one' : [5., 6., 7., 8.], 'two' : [9., 10., 11., 12.]})
d3 = pd.DataFrame({'one' : [15., 16., 17., 18.], 'two' : [19., 10., 11., 12.]})

# list of dataframes
mydfs = [d1, d2, d3]

I would like to combine d1, d2, and d3 into one pandas dataframe. Alternatively, a method of reading a large-ish table directly into a dataframe when using the chunksize option would be very helpful.

Asked By: Whitebeard

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Source

Answer 1

Given that all the dataframes have the same columns, you can simply concat them:

import pandas as pd
df = pd.concat(list_of_dataframes)

Answered By: DeepSpace

Answer 2

If the dataframes DO NOT all have the same columns try the following:

df = pd.DataFrame.from_dict(map(dict,df_list))

Answered By: meyerson

Answer 3

You also can do it with functional programming:

from functools import reduce
reduce(lambda df1, df2: df1.merge(df2, "outer"), mydfs)

Answered By: Jay Wong

Answer 4

concat also works nicely with a list comprehension pulled using the “loc” command against an existing dataframe

df = pd.read_csv('./data.csv') # ie; Dataframe pulled from csv file with a "userID" column

review_ids = ['1','2','3'] # ie; ID values to grab from DataFrame

# Gets rows in df where IDs match in the userID column and combines them 

dfa = pd.concat([df.loc[df['userID'] == x] for x in review_ids])

Answered By: Lelouch

Answer 5

Just to add few more details:

Example:

list1 = [df1, df2, df3]

import pandas as pd

Row-wise concatenation & ignoring indexes
```
pd.concat(list1, axis=0, ignore_index=True)
```
Note: If column names are not same then NaN would be inserted at different column values
Column-wise concatenation & want to keep column names
```
pd.concat(list1, axis=1, ignore_index=False)
```
If ignore_index=True, column names would be filled with numbers starting from 0 to (n-1), where n is the count of unique column names

Answered By: rmswrp

Answer 6

panders concat works also as well in addition with functools

from functors import reduce as reduce
import pandas as pd;
deaf = pd.read_csv("http://www.aol.com/users/data.csv")
for q in range(0, Len(deaf)):
  new = map(lambda x: reduce(pd.concat(x))

Answered By: jorgiojohnas

Concatenate a list of pandas dataframes together

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