Writing CSV files in Python

29 Dec, 2019
CSV (Comma Separated Values) is a simple file format used to store tabular data, such as a spreadsheet or database. CSV file stores tabular data (numbers and text) in plain text. Each line of the file is a data record. Each record consists of one or more fields, separated by commas. The use of the comma as a field separator is the source of the name for this file format.
Python provides an in-built module called csv to work with CSV files. There are various classes provided by this module for writing to CSV:
Using csv.writer class
Using csv.DictWriter class
csv.writer class is used to insert data to the CSV file. This class returns a writer object which is responsible for converting the user’s data into a delimited string. A csvfile object should be opened with newline='' otherwise newline characters inside the quoted fields will not be interpreted correctly.
Syntax: csv.writer(csvfile, dialect=’excel’, **fmtparams)
Parameters:csvfile: A file object with write() method.dialect (optional): Name of the dialect to be used.fmtparams (optional): Formatting parameters that will overwrite those specified in the dialect.
csv.writer class provides two methods for writing to CSV. They are writerow() and writerows().
writerow(): This method writes a single row at a time. Field row can be written using this method.Syntax:writerow(fields)

Syntax:
writerow(fields)

writerows(): This method is used to write multiple rows at a time. This can be used to write rows list.Syntax:Writing CSV files in Python
writerows(rows)

Syntax:
Writing CSV files in Python
writerows(rows)

Example:
# Python program to demonstrate# writing to CSV    import csv     # field names fields = ['Name', 'Branch', 'Year', 'CGPA']     # data rows of csv file rows = [ ['Nikhil', 'COE', '2', '9.0'],          ['Sanchit', 'COE', '2', '9.1'],          ['Aditya', 'IT', '2', '9.3'],          ['Sagar', 'SE', '1', '9.5'],          ['Prateek', 'MCE', '3', '7.8'],          ['Sahil', 'EP', '2', '9.1']]     # name of csv file filename = "university_records.csv"    # writing to csv file with open(filename, 'w') as csvfile:     # creating a csv writer object     csvwriter = csv.writer(csvfile)             # writing the fields     csvwriter.writerow(fields)             # writing the data rows     csvwriter.writerows(rows)
Output:
This class returns a writer object which maps dictionaries onto output rows.
Syntax: csv.DictWriter(csvfile, fieldnames, restval=”, extrasaction=’raise’, dialect=’excel’, *args, **kwds)
Parameters:csvfile: A file object with write() method.fieldnames: A sequence of keys that identify the order in which values in the dictionary should be passed.restval (optional): Specifies the value to be written if the dictionary is missing a key in fieldnames.extrasaction (optional): If a key not found in fieldnames, the optional extrasaction parameter indicates what action to take. If it is set to raise a ValueError will be raised.dialect (optional): Name of the dialect to be used.
csv.DictWriter provides two methods for writing to CSV. They are:
writeheader(): writeheader() method simply writes the first row of your csv file using the pre-specified fieldnames.Syntax:writeheader()

Syntax:
writeheader()

writerows(): writerows method simply writes all the rows but in each row, it writes only the values(not keys).Syntax:writerows(mydict)

Syntax:
writerows(mydict)

Example:
# importing the csv module import csv     # my data rows as dictionary objects mydict =[{'branch': 'COE', 'cgpa': '9.0', 'name': 'Nikhil', 'year': '2'},          {'branch': 'COE', 'cgpa': '9.1', 'name': 'Sanchit', 'year': '2'},          {'branch': 'IT', 'cgpa': '9.3', 'name': 'Aditya', 'year': '2'},          {'branch': 'SE', 'cgpa': '9.5', 'name': 'Sagar', 'year': '1'},          {'branch': 'MCE', 'cgpa': '7.8', 'name': 'Prateek', 'year': '3'},          {'branch': 'EP', 'cgpa': '9.1', 'name': 'Sahil', 'year': '2'}]     # field names fields = ['name', 'branch', 'year', 'cgpa']     # name of csv file filename = "university_records.csv"    # writing to csv file with open(filename, 'w') as csvfile:     # creating a csv dict writer object     writer = csv.DictWriter(csvfile, fieldnames = fields)             # writing headers (field names)     writer.writeheader()             # writing data rows     writer.writerows(mydict) 
Output:
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