How to round certain values in list

I have a list in Python

a = [100.50, 121.50, 130.50, 140.50, 150.50, 160.50, 170.50, 180.50, 190.50, 200.50] 

I want to round first three values

 [100.00, 100.00, 100.00, 140.50, 150.50, 160.50, 170.50, 180.50, 190.50, 200.50]

I need to round down to 100.0 if they are < 130.0 and then similarly round down to 200 if they are < 230 and so on.

Answer

Based on comments it seems you want to round down to 100.0 if they are < 130.0 and then similarly round down to 200 if they are < 230 and so on.

So the following:

b = [100*(x//100) if x % 100 < 30 else x for x in a]

So for every element x of list a, if the remainder of dividing it by 100 is less than 30 we have to round it down other wise keep x.

For rounding down to 100, 200, … integer divide x by 100 and then multiply again by 100.

Example

a = [100.50, 121.50, 130.50, 220.50, 240.50, 310.50, 350.50]

Output

    [100.0, 100.0,   130.5,  200.0,  240.5,  300.0,  350.5]