Pandas: import multiple csv files into dataframe using a loop and hierarchical indexing

Question

I would like to read multiple CSV files (with a different number of columns) from a target directory into a single Python Pandas DataFrame to efficiently search and extract data.

Example file:

Events 
1,0.32,0.20,0.67
2,0.94,0.19,0.14,0.21,0.94
3,0.32,0.20,0.64,0.32
4,0.87,0.13,0.61,0.54,0.25,0.43 
5,0.62,0.21,0.77,0.44,0.16

Here is what I have so far:

# get a list of all csv files in target directory
my_dir = "C:\\Data\\"
filelist = []
os.chdir( my_dir )
for files in glob.glob( "*.csv" ) :
    filelist.append(files)

# read each csv file into single dataframe and add a filename reference column 
# (i.e. file1, file2, file 3) for each file read
df = pd.DataFrame()
columns = range(1,100)
for c, f in enumerate(filelist) :
    key = "file%i" % c
    frame = pd.read_csv( (my_dir + f), skiprows = 1, index_col=0, names=columns )
    frame['key'] = key
    df = df.append(frame,ignore_index=True)

(the indexing isn't working properly)

Essentially, the script below is exactly what I want (tried and tested) but needs to be looped through 10 or more csv files:

df1 = pd.DataFrame()
df2 = pd.DataFrame()
columns = range(1,100)
df1 = pd.read_csv("C:\\Data\\Currambene_001y09h00m_events.csv", 
                  skiprows = 1, index_col=0, names=columns)
df2 = pd.read_csv("C:\\Data\\Currambene_001y12h00m_events.csv", 
                  skiprows = 1, index_col=0, names=columns)
keys = [('file1'), ('file2')]
df = pd.concat([df1, df2], keys=keys, names=['fileno'])

I have found many related links, however I am still not able to get this to work:

• Reading Multiple CSV Files into Python Pandas Dataframe
• Merge of multiple data frames of different number of columns into one big data frame
• Import multiple csv files into pandas and concatenate into one DataFrame

Answer 1

You need to decide in what axis you want to append your files. Pandas will always try to do the right thing by:

1. Assuming that each column from each file is different, and appending digits to columns with similar names across files if necessary, so that they don't get mixed;
2. Items that belong to the same row index across files are placed side by side, under their respective columns.

The trick to appending efficiently is to tip the files sideways, so you get the desired behaviour to match what pandas.concat will be doing. This is my recipe:

from pandas import *
files = !ls *.csv # IPython magic
d = concat([read_csv(f, index_col=0, header=None, axis=1) for f in files], keys=files)

Notice that read_csv is transposed with axis=1, so it will be concatenated on the column axis, preserving its names. If you need, you can transpose the resulting DataFrame back with d.T.

EDIT:

For different number of columns in each source file, you'll need to supply a header. I understand you don't have a header in your source files, so let's create one with a simple function:

def reader(f):
    d = read_csv(f, index_col=0, header=None, axis=1)
    d.columns = range(d.shape[1])
    return d

df = concat([reader(f) for f in files], keys=files)