如何使用Python或SQL计算X周的过去几天？

Question

I need to compute exactly 5 week days into the past. eg if today's date is 2015-05-07 I need my program to give me 2015-04-30.

So far I tried is this

today = datetime.date.today() - datetime.timedelta(5)

This gives 5 days exactly into the past. But I need 5th weekday into the past. The purpose I need this is because I have a database table which consists of some data and I need to write a Python script which fetches the data for last 5 days.

Answer 1

Five days in the past is simply:

select date_sub(CURDATE(), interval 5 day)

If by week day you mean Mon - Fri, then the logic is actually rather simple. 5 week days are one week, which is 7 days (at least in the United States). So the answer to your question is

select date_sub(CURDATE(), interval 7 day)

Note that other numbers of days would require more complex logic, using CASE .

Answer 2

Python datetime objects have a isoweekday() method to help with this. This method returns a numeric value corresponding to the day of the week. So for example, Monday is the integer 1. You want to skip Saturday and Sunday, these correspond to 6 and 7 respectfully. We can use this to backtrack:

from datetime import datetime, timedelta                                              

def workingday(d, no_of_days):                                                              
    backdate = d                                                                      
    count = 0                                                                         
    while count < no_of_days:                                                                  
        backdate = backdate - timedelta(1)                                              
        if backdate.isoweekday() in (6,7):                                            
            continue                                                                  
        count += 1                                                                    
    return backdate.strftime("%Y-%m-%d")


d = datetime.strptime("2015-05-07","%Y-%m-%d")                                        
print workingday(d, 5)

Output:

2015-04-30 

I have used your date format. But you can easily change this.

Instead of isoweekday() you can use weekday() but now Monday will be returned as 0 and Sunday will be returned as 6.