如何编写从给定日期返回 30 天前的日期的 python function

Question

Hi I am new to python and I am trying to write a function that returns the date 30 days ago from a given date. My function has two parameters as seen below:

import datetime

def get_date_x_days_ago (x_days_ago, date_from = None):
    d = datetime.timedelta(days=x_days_ago)
    if date_from is None:
        date_from = datetime.today()
    else:
        datetime.datetime.strptime(date_from, "YYYY-mm-dd")

    return date_from - d

print(get_date_x_days_ago(x_days_ago=30, date_from="2020-11-11"))

I cant seem to get the function to work as expected

Answer 1

You need to assign the parsed date from strptime to the date_from variable.

Also, see https://www.programiz.com/python-programming/datetime/strptime (under "Format Code List") for the corresponding date format codes, ie "%Y" denotes year with century.

import datetime

def get_date_x_days_ago (x_days_ago, date_from = None):
    d = datetime.timedelta(days=x_days_ago)
    if date_from is None:
        date_from = datetime.datetime.today()
    else:
        date_from = datetime.datetime.strptime(date_from, "%Y-%m-%d")
        # ^^^^ missing assignment

    return date_from - d

print(get_date_x_days_ago(x_days_ago=30, date_from="2020-11-11"))

Answer 2

Since you're finding a certain date from a date to x day before. So, first, convert your date_from to only date then subtract with x_days .

from datetime import timedelta, datetime, date
def get_date_x_days_ago(x_days_ago, date_from=None):
    return datetime.strptime(date_from, '%Y-%m-%d').date() - timedelta(x_days_ago) if date_from else date.today()-timedelta(x_days_ago)

print(get_date_x_days_ago(x_days_ago=30, date_from="2020-11-11"))

Output

get_date_x_days_ago(x_days_ago=30, date_from="2020-11-11")
2020-10-12

get_date_x_days_ago(x_days_ago=30)
2021-03-12