How to use read, readline, and readlines (python) Read a csv file

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Release date:2024/12/22　　　　　　　　　

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■Description
Reads text, csv, and other files. Note that csv.reader may be easier to use.

■Example: Reading a csv file
The following test.csv file will be used as the subject.

＜read：Read everything as a string＞

with open("test.csv", "r") as file: # r means read only
    data = file.read()

data # If I use print(data), the output looks different, so I'll output it as data

    → '4,5,7,1\n2,5,6,7\n3,9,10,11\n'

data[0] # Specify the 0th character

    → '4'

data[0:4] # Specify the 0th to 5th characters. The comma is counted as one character.

    → ',5,7'

＜readline：Read only one line as a string＞

with open("test.csv", "r") as file:
data = file.readline()

data

→ '4,5,7,1\n'

＜readlines：Read everything as a string and make a list＞

with open("test.csv", "r") as file:
    data = file.readlines()

data

    → ['4,5,7,1\n', '2,5,6,7\n', '3,9,10,11\n']

data[1] # Specify the first list

    → '2,5,6,7\n'

class=bor>＜Convert strings to numbers＞
You can convert them to numbers using numpy. We will continue to use the above result.

data[1].split(',') # split separates data with any character.

→ ['2', '5', '6', '7\n']

import numpy as np
np.asfarray(data[1].split(','))

→ array([2., 5., 6., 7.])

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