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

Question

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'
latestValue='1'
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)?

Answer 1

Use this:

if mystr == '':
    mystr = latestValue
else:
    mystr = mystr[mystr.find(",")+1:] + "," + latestValue

This should be much faster than any solution which splits the list. It only finds the first occurrence of , and "removes" the beginning of the string. Also, if the list is empty, then mystr will be just latestValue (insignificant overhead added by this) -- thanks Paulo Scardine for pointing that out.

Answer 2

_, 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.