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CSV files

CSV (Comma Separated Values) is a very popular import and export data format used in spreadsheets and databases. Each line in a CSV file is a data record. Each record consists of one or more fields, separated by commas. While CSV is a very simple data format, there can be many differecies, such as different delimiters, new lines, or quoting characters.

Reading CSV files

Reading of CSV files into memory:

# example1.csv
1/2/2014,5,8,red
1/3/2014,5,2,green
1/4/2014,9,1,blue

import csv

with open('example1.csv') as csvfile:
    readCSV = csv.reader(csvfile, delimiter=',')
    for row in readCSV:
        print(row)
        print(row[0])
        print(row[0],row[1],row[2],)

Another example: reading a file with headers and data with specific encoding and delimiter. Print the header and create a data list (without the header line).

data = []
with open('data.csv', 'r', encoding='utf-8') as f:
    csvreader = csv.reader(f, delimiter=';')
    header = next(csvreader)  # skip header
    print(header)
    for row in csvreader:
        data.append([float(val) for val in row])

DictReader

The csv.DictReader class operates like a regular reader but maps the information read into a dictionary. The keys for the dictionary can be passed in with the fieldnames parameter or inferred from the first row of the CSV file.

# example2.csv
min,avg,max
1, 5.5, 10
2, 3.5, 5

import csv

with open('values.csv', 'r') as f:
    reader = csv.DictReader(f)
    for row in reader:
        print(row['min'], row['avg'], row['max'])

Writing to CSV files

The csv.writer() method returns a writer object which converts the user's data into delimited strings on the given file-like object.

import csv

nms = [[1, 2, 3, 4, 5, 6], [7, 8, 9, 10, 11, 12]]

with open('numbers2.csv', 'w') as f:
    writer = csv.writer(f)
    for row in nms:
        writer.writerow(row)

DictWriter

The csv.DictWriter class operates like a regular writer but maps Python dictionaries into CSV rows. The fieldnames parameter is a sequence of keys that identify the order in which values in the dictionary passed to the writerow() method are written to the CSV file.

import csv

with open('names.csv', 'w') as f:
    fnames = ['first_name', 'last_name']
    writer = csv.DictWriter(f, fieldnames=fnames)    

    writer.writeheader()  # writes the headers to the CSV file.
    writer.writerow({'first_name' : 'John', 'last_name': 'Smith'})
    writer.writerow({'first_name' : 'Robert', 'last_name': 'Brown'})
    writer.writerow({'first_name' : 'Julia', 'last_name': 'Griffin'})

Quoting CSV Files

With the CSV module, you can also perform a variety of quoting functions.

They are:

  • csv.QUOTE_ALL - Quote everything, regardless of type.
  • csv.QUOTE_MINIMAL - Quote fields with special characters
  • csv.QUOTE_NONNUMERIC - Quote all fields that are not integers or floats
  • csv.QUOTE_NONE - Do not quote anything on output