嵌套列表中的字符串

Question

I have a list looking like this:

record1 = [["2020/02/19", 7.0], ["2020/02/20", 7.3], ["2020/02/21", 6.1]]

but I want to change the dates from yyyy/mm/dd to dd/mm/yyyy .

record1= [["19/02/2020", 7.0], ["20/02/2020", 7,3], ["21/02/2020", 6.1]

How can I do this?

I cannot just use ::-1 because then everything gets mixed up.

Answer 1

The proper way of doing it using datetime module:

record1 = [[datetime.strptime(d, '%Y/%m/%d').strftime('%d/%m/%Y'), v] for d, v in record1]

This converts it to datetime object, then formats it the way you intended

>>> record1
[['19/02/2020', 7.0], ['20/02/2020', 7.3], ['21/02/2020', 6.1]]

This would be a robust, explicit solution.

But... Some experiments showed that manipulating strings with split would be a faster solution:

from datetime import datetime
import time


def convert_with_datetime(record1):
    return [[datetime.strptime(d, '%Y/%m/%d').strftime('%d/%m/%Y'), v] for d, v in record1]

def convert_with_split(record1):
    return [['/'.join(d.split('/')[::-1]), v] for d, v in record1]

record1 = [["2020/02/19", 7.0], ["2020/02/20", 7.3], ["2020/02/21", 6.1]] * 10**6



start = time.time()
convert_with_datetime(record1)
end = time.time()
print('convert_with_datetime:', end - start)

start = time.time()
convert_with_split(record1)
end = time.time()
print('convert_with_split:', end - start)

The result is:

convert_with_datetime: 12.311486959457397
convert_with_split: 1.3060288429260254