How do I read and write CSV files with Python?

Question

I have a file example.csv with the contents

1,"A towel,",1.0
42," it says, ",2.0
1337,is about the most ,-1
0,massively useful thing ,123
-2,an interstellar hitchhiker can have.,3

How do I read this example.csv with Python?

Similarly, if I have

data = [(1, "A towel,", 1.0),
        (42, " it says, ", 2.0),
        (1337, "is about the most ", -1),
        (0, "massively useful thing ", 123),
        (-2, "an interstellar hitchhiker can have.", 3)]

How do I write data to a CSV file with Python?

Answer 1

import csv
with open(fileLocation+'example.csv',newline='') as File: #the csv file is stored in a File object

    reader=csv.reader(File)       #csv.reader is used to read a file
    for row in reader:
        print(row)

Answer 2

Here are some minimal complete examples how to read CSV files and how to write CSV files with Python.

Python 2+3: Reading a CSV file

Pure Python

# -*- coding: utf-8 -*-

import csv
import sys

# Define data
data = [(1, "A towel,", 1.0),
        (42, " it says, ", 2.0),
        (1337, "is about the most ", -1),
        (0, "massively useful thing ", 123),
        (-2, "an interstellar hitchhiker can have.", 3)]

# Write CSV file
kwargs = {'newline': ''}
mode = 'w'
if sys.version_info < (3, 0):
    kwargs.pop('newline', None)
    mode = 'wb'

with open('test.csv', mode, **kwargs) as fp:
    writer = csv.writer(fp, delimiter=',')
    # writer.writerow(["your", "header", "foo"])  # write header
    writer.writerows(data)

# Read CSV file
kwargs = {'newline': ''}
mode = 'r'
if sys.version_info < (3, 0):
    kwargs.pop('newline', None)
    mode = 'rb'
with open('test.csv', mode, **kwargs) as fp:
    reader = csv.reader(fp, delimiter=',', quotechar='"')
    # next(reader, None)  # skip the headers
    data_read = [row for row in reader]

print(data_read)

After that, the contents of data_read are

[['1', 'A towel,', '1.0'],
 ['42', ' it says, ', '2.0'],
 ['1337', 'is about the most ', '-1'],
 ['0', 'massively useful thing ', '123'],
 ['-2', 'an interstellar hitchhiker can have.', '3']]

Unicode and Python 2.X

If you want to write Unicode, you have to install unicodecsv. Do not open the file with codecs.open but simply with open. Write it with

import unicodecsv as csv
# Write CSV file
with open('test.csv', 'w', newline='') as fp:
    writer = csv.writer(fp, encoding='utf-8')
    # writer.writerow(["your", "header", "foo"])  # write header
    writer.writerows(data)

Related

• How do I write data into csv format as string (not file)?
• How can I use io.StringIO() with the csv module?: This is interesting if you want to serve a CSV on-the-fly with Flask, without actually storing the CSV on the server.

mpu

Have a look at my utility package mpu for a super simple and easy to remember one:

import mpu.io
data = mpu.io.read('example.csv', delimiter=',', quotechar='"', skiprows=None)
mpu.io.write('example.csv', data)

Pandas

import pandas as pd

# Read the CSV into a pandas data frame (df)
#   With a df you can do many things
#   most important: visualize data with Seaborn
df = pd.read_csv('myfile.csv', sep=',')
print(df)

# Or export it in many ways, e.g. a list of tuples
tuples = [tuple(x) for x in df.values]

# or export it as a list of dicts
dicts = df.to_dict().values()

See read_csv docs for more information. Please note that pandas automatically infers if there is a header line, but you can set it manually, too.

If you haven't heard of Seaborn, I recommend having a look at it.

Other

Reading CSV files is supported by a bunch of other libraries, for example:

• dask.dataframe.read_csv
• spark.read.csv

Created CSV file

1,"A towel,",1.0
42," it says, ",2.0
1337,is about the most ,-1
0,massively useful thing ,123
-2,an interstellar hitchhiker can have.,3

Common file endings

.csv

Working with the data

After reading the CSV file to a list of tuples / dicts or a Pandas dataframe, it is simply working with this kind of data. Nothing CSV specific.

Alternatives

• JSON: Nice for writing human-readable data; VERY commonly used (read & write)
• CSV: Super simple format (read & write)
• YAML: Nice to read, similar to JSON (read & write)
• pickle: A Python serialization format (read & write)
• MessagePack (Python package): More compact representation (read & write)
• HDF5 (Python package): Nice for matrices (read & write)
• XML: exists too *sigh* (read & write)

For your application, the following might be important:

• Support by other programming languages
• Reading / writing performance
• Compactness (file size)

See also: Comparison of data serialization formats

In case you are rather looking for a way to make configuration files, you might want to read my short article Configuration files in Python

Answer 3

Writing a CSV file

First you need to import csv

For eg:

import csv

with open('eggs.csv', 'wb') as csvfile:
    spamwriter = csv.writer(csvfile, delimiter=' ',
                        quotechar='|', quoting=csv.QUOTE_MINIMAL)
    spamwriter.writerow(['Spam'] * 5 + ['Baked Beans'])
    spamwriter.writerow(['Spam', 'Lovely Spam', 'Wonderful Spam'])