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Python3.x Python csv Module

CSV (Comma-Separated Values) files are a common file format used to store tabular data.

CSV files consist of plain text, where each line represents a row of data in a table, and each column is separated by commas (or other delimiters).

CSV files are commonly used for data exchange because they are simple and easy to process.

Python provides a built-in csv module for reading and writing CSV files. This module simplifies the process of handling CSV files, allowing developers to easily manipulate tabular data.

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1. Reading CSV Files

To read a CSV file, you can use the csv.reader object. Here is a simple example:

Example

import csv

# Open CSV file

with open('data.csv', mode='r', encoding='utf-8')as file:

# Create csv.reader object

 csv_reader =csv.reader(file)

# Read data line by line

for row in csv_reader:

print(row)

Code Explanation:

• open('data.csv', mode='r', encoding='utf-8'): Opens the file named data.csv in read-only mode and specifies the encoding as UTF-8.
• csv.reader(file): Creates a csv.reader object to read the file contents.
• for row in csv_reader: Reads the file contents line by line, with each row of data parsed into a list.

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2. Writing CSV Files

To write to a CSV file, you can use the csv.writer object. Here is an example:

Example

import csv

# Data to be written

 data =[

['Name','Age','City'],

['Alice','30','New York'],

['Bob','25','Los Angeles']

]

# Open CSV file

with open('output.csv', mode='w', encoding='utf-8', newline='')as file:

# Create csv.writer object

 csv_writer =csv.writer(file)

# Write data

for row in data:

 csv_writer.writerow(row)

Code Explanation:

• open('output.csv', mode='w', encoding='utf-8', newline=''): Opens the file named output.csv in write mode and specifies the encoding as UTF-8. newline='' is used to prevent blank lines on Windows systems.
• csv.writer(file): Creates a csv.writer object to write to the file contents.
• csv_writer.writerow(row): Writes each row of data to the file.

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3. Reading and Writing CSV Files Using Dictionaries

The csv module also provides DictReader and DictWriter classes, which can parse each row of a CSV file into a dictionary, or write dictionaries to a CSV file.

Reading CSV Files with DictReader:

Example

import csv

with open('data.csv', mode='r', encoding='utf-8')as file:

 csv_dict_reader =csv.DictReader(file)

for row in csv_dict_reader:

print(row)

Writing CSV Files with DictWriter:

Example

import csv

data =[

{'Name': 'Alice','Age': '30','City': 'New York'},

{'Name': 'Bob','Age': '25','City': 'Los Angeles'}

]

with open('output.csv', mode='w', encoding='utf-8', newline='')as file:

 fieldnames =['Name','Age','City']

 csv_dict_writer =csv.DictWriter(file, fieldnames=fieldnames)

# Write header

 csv_dict_writer.writeheader()

# Write data

for row in data:

 csv_dict_writer.writerow(row)

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Common Attributes and Methods

Core Methods of the csv Module

Method	Description	Example
csv.reader()	Reads CSV data from a file object	reader = csv.reader(file)
csv.writer()	Writes data to a CSV file	writer = csv.writer(file)
csv.DictReader()	Reads CSV rows as dictionaries (with header)	dict_reader = csv.DictReader(file)
csv.DictWriter()	Writes dictionaries to a CSV file (requires fieldnames)	dict_writer = csv.DictWriter(file, fieldnames)
csv.register_dialect()	Registers a custom CSV dialect (e.g., delimiter)	csv.register_dialect('mydialect', delimiter='|')
csv.unregister_dialect()	Removes a registered dialect	csv.unregister_dialect('mydialect')
csv.list_dialects()	Lists all registered dialects	print(csv.list_dialects())

Common Methods for csv.reader and csv.writer Objects

Method	Description	Applicable Object
__next__()	Iterates to the next row (or use for loop)	reader
writerow(row)	Writes a single row of data	writer
writerows(rows)	Writes multiple rows of data (list of lists)	writer

Features of csv.DictReader and csv.DictWriter Objects

Feature/Method	Description	Example
fieldnames	List of field names (DictReader automatically gets it from the first row)	dict_reader.fieldnames
writeheader()	Writes the header row (specific to DictWriter)	dict_writer.writeheader()

Common Parameter Descriptions

Parameter	Description	Example Value	Applicable Method
delimiter	Field delimiter	',' (default), '\t'	reader/writer
quotechar	Quote character (encloses special fields)	'"' (default)	reader/writer
quoting	Quoting rule	csv.QUOTE_ALL (quote all)	reader/writer
skipinitialspace	Ignore spaces after delimiter	True/False	reader
lineterminator	Line terminator	'\r\n' (default)	writer
dialect	Predefined dialect name	'excel' (default)	All methods

Examples

1. Reading a CSV File

Example

import csv

with open('data.csv','r')as file:

 reader =csv.reader(file, delimiter=',')

for row in reader:

print(row)# Each row is a list

2. Writing a CSV File

Example

data =[['Name','Age'],['Alice',25],['Bob',30]]

with open('output.csv','w', newline='')as file:

 writer =csv.writer(file)

 writer.writerows(data)# Write multiple rows

3. Using DictReader and DictWriter (with headers)

Example

# Read

with open('data.csv','r')as file:

 dict_reader =csv.DictReader(file)

for row in dict_reader:

print(row['Name'], row['Age'])# Access via field name

# Write

 fieldnames =['Name','Age']

with open('output.csv','w', newline='')as file:

 dict_writer =csv.DictWriter(file, fieldnames=fieldnames)

 dict_writer.writeheader()# Write header

 dict_writer.writerow({'Name': 'Alice','Age': 25})

4. Custom Dialect (e.g., processing TSV files)

Example

csv.register_dialect('tsv', delimiter='\t', quoting=csv.QUOTE_NONE)

with open('data.tsv','r')as file:

 reader =csv.reader(file, dialect='tsv')

for row in reader:

print(row)