What is the most effective way to concatenate two strings and delete everything before the first ',' in Python?


In Python, I have a string which is a comma separated list of values. e.g. '5,2,7,8,3,4'

I need to add a new value onto the end and remove the first value,

e.g. '5,22,7,814,3,4' -> '22,7,814,3,4,1'

Currently, I do this as follows:

mystr = '5,22,7,814,3,4'
mylist = mystr.split(',')
mystr = ''
for i in range(len(mylist)-1):
   if i==0:
      mystr += mylist[i+1]
   if i>0:
      mystr += ','+mylist[i+1]

mystr += ','+latestValue

This runs millions of times in my code and I've identified it as a bottleneck, so I'm keen to optimize it to make it run faster.

What is the most efficient to do this (in terms of runtime)?

_, sep, rest = mystr.partition(",")
mystr = rest + sep + latestValue

It also works without any changes if mystr is empty or a single item (without comma after it) due to str.partition returns empty sep if there is no sep in mystr.

You could use mystr.rstrip(",") before calling partition() if there might be a trailing comma in the mystr.