如何在 for 循环运行时更改 python/django 中的 for 循环变量

Question

I want to change variables that define a for loop while the loop is running. It will make more sense when you see the code, so here it is:

days = form.instance.days

for i in range(0, days + 1):
    days_added = i
    current_end = class_start_date + datetime.timedelta(days=days_added)
    current_end_day = calendar.day_name[datetime.datetime.strptime(str(current_end), '%Y-%m-%d').weekday()]
    if current_end_day == 'Saturday' or current_end_day == 'Sunday':
        days = days + 1

You see, when I run the code days = days + 1 , I want the days for for i in range(0, days + 1): to be updated, so that the number of total loops of the forloop will be increased by 1 whenever days = days + 1 occurs. days = form.instance.days is increased by 1, but days in for i in range(0, days + 1): is not updated. I hope you guys could help. Thanks.

Answer 1

When you write

for i in <expr>:

the expression only gets evaluated once, so you cannot achieve what you want with a for loop. You can use a while instead:

i = 0
while i < days + 1:
    days_added = i
    current_end = class_start_date + datetime.timedelta(days=days_added)
    current_end_day = calendar.day_name[datetime.datetime.strptime(str(current_end), '%Y-%m-%d').weekday()]
    if current_end_day == 'Saturday' or current_end_day == 'Sunday':
        days = days + 1
    i += 1

Now the i < days + 1 condition gets evaluated each time, and it will use the updated value of days .

Answer 2

A while loop may be better suited for your use case. You need to establish a condition for it to stop looping while avoiding falling on an infinite loop.