Prerequisites: Reading and Writing data in CSV, Creating CSV filesĀ
CSV is a Comma-Separated Values file, which allows plain-text data to be saved in a tabular format. These files are stored in our system with a .csv extension. CSV files differ from other spreadsheet file types (like Microsoft Excel) because we can only have a single sheet in a file, and they cannot save cells, columns, or rows. Also, we cannot save formulas in this format.
To parse CSV files in Python, we make use of the csv library. The CSV library contains objects that are used to read, write and process data from and to CSV files. Letās see how we can add numbers into our CSV files using csv library.
Steps to read numbers in a CSV file:
- Create a python file (example: gfg.py).
- Import the csv library.
- Create a nested-list āmarksā which stores the student roll numbers and their marks in maths and python in a tabular format.
- Open a new csv file (or an existing csv file) in the āwā mode of the writer object and other necessary parameters (here delimiter & quoting).
- Write into it the list āmarksā with the help of writerows method.
- In order to read the rows, make use of reader object and store each row(which is also a list) in a new list āoutputā.
- Print the list output for verifying the code.
Reading numbers in a CSV file without quotes:
In order to write in our CSV file āmy_csvā, Ā we make use of the writerows() method of the writer object. But to read numbers as they are, we will make use of an optional parameter of the writer object, which is āquotingā. The āquotingā parameter tells the writer which character is to be quoted.
If quoting is set to csv.QUOTE_NONNUMERIC, then .writerow() will quote all fields which contain text data and convert all numeric fields to the float data type.
Code:
Python3
import csvĀ
# creating a nested list of roll numbers,# subjects and marks scored by each roll numbermarks = [Ā Ā Ā Ā ["RollNo", "Maths", "Python"],Ā Ā Ā Ā [1000, 80, 85],Ā Ā Ā Ā [2000, 85, 89],Ā Ā Ā Ā [3000, 82, 90],Ā Ā Ā Ā [4000, 83, 98],Ā Ā Ā Ā [5000, 82, 90]]Ā
# using the open method with 'w' mode# for creating a new csv file 'my_csv' with .csv extensionwith open('my_csv.csv', 'w', newline = '') as file:Ā Ā Ā Ā writer = csv.writer(file, quoting = csv.QUOTE_NONNUMERIC,Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā delimiter = ' ')Ā Ā Ā Ā writer.writerows(marks)Ā
# opening the 'my_csv' file to read its contentswith open('my_csv.csv', newline = '') as file:Ā Ā Ā Ā Ā Ā Ā reader = csv.reader(file, quoting = csv.QUOTE_NONNUMERIC,Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā delimiter = ' ')Ā Ā Ā Ā Ā Ā Ā Ā Ā # storing all the rows in an output listĀ Ā Ā Ā output = []Ā Ā Ā Ā for row in reader:Ā Ā Ā Ā Ā Ā Ā Ā output.append(row[:])Ā
for rows in output:Ā Ā Ā Ā print(rows) |
Output:
output of gfg.py
And this is how it looks in the CSV file āmy_csv.csvā which gets created once we run the above code:
my_csv.csv
Reading numbers in a CSV file with quotes:
If quoting is set to csv.QUOTE_ALL then .writerow() will quote all fields and the numbers will now be stored in quotes. To read the numbers from each row, we make use of the reader object from CSV library and store all the rows within a list āoutputā, which we would also print afterward.
Code:
Python3
import csvĀ
# creating a nested list of roll numbers,# subjects and marks scored by each roll numbermarks = [Ā Ā Ā Ā ["RollNo", "Maths", "Python"],Ā Ā Ā Ā [1000, 80, 85],Ā Ā Ā Ā [2000, 85, 89],Ā Ā Ā Ā [3000, 82, 90],Ā Ā Ā Ā [4000, 83, 98],Ā Ā Ā Ā [5000, 82, 90]]Ā
# using the open method with 'w' mode# for creating a new csv file 'my_csv' with .csv extensionwith open('my_csv.csv', 'w', newline = '') as file:Ā Ā Ā Ā writer = csv.writer(file, quoting = csv.QUOTE_ALL,Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā delimiter = ' ')Ā Ā Ā Ā writer.writerows(marks)Ā
# opening the 'my_csv' file to read its contentswith open('my_csv.csv', newline = '') as file:Ā Ā Ā Ā reader = csv.reader(file,Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā quoting = csv.QUOTE_ALL,Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā Ā delimiter = ' ')Ā Ā Ā Ā Ā Ā Ā Ā Ā # storing all the rows in an output listĀ Ā Ā Ā output = []Ā Ā Ā Ā for row in reader:Ā Ā Ā Ā Ā Ā Ā Ā output.append(row[:])Ā
for rows in output:Ā Ā Ā Ā print(rows) |
Ā Output:
output of gfg.py
And this how the above input gets stored within āmy_csv.csvā file:
my_csv.csv
