Python 3.X Changing the values ​​of the line elements in the cycle

How to change the value of the symbol of the line in the cycle? Code:

   def   getdetimepostfix  ():   Import  DATETIME;  dtvalue = DateTime.DATETEME.NOW ();  retvalue =  str  (dtvalue);   for  simm  in  retvalue:   if  sym ==  '' ':' :  sym = . '   Retu  RETVALUE;     
 I try to adjust the date, getting rid of "": "", but it is not possible to change individual characters. 
  
 or it is better to make a cycle of this type? Class = "" Data-Highlighted = ""> i =  0 ;   for  item  in  retalwale:   if  retvalue [i] ==  '': ':  retvalue [i] = ' '  i +  1 ;     
?