How to replace multiple (non-unique) substrings in a string?

Question

I have this string -

test_str = 'hello_2Testsmthhello_1smthhello_1'

and I have a list of strings -

list1 = ['hello_1', 'hello_2']

Now I want to replace hello_1 and hello_2 with a period(.). This is what I've tried -

for i in list1:
 h = test_str.replace(str(i), ".")
#print (h)

This gives me the string - .Testsmthhello_1smthhello_1

However, the expected output is - .Testsmth.smth.

Can anyone help me here?

• Karl Knechtel: Where you have written h = test_str.replace(str(i), "."), do you expect this to change the value of test_str? Why or why not? If it doesn't change, then can you explain how you expect the loop to work?
• Karl Knechtel: In general, you should try to think about your code first, and step through the logic of it. Please read ericlippert.com/2014/03/05/how-to-debug-small-programs .
• Karl Knechtel: Okay, so the first time through the loop, you do h = test_str.replace(str(i), "."), so h ends up with the result of doing the first replacement, and test_str is the same as before. Right? So, what do you think will happen the second time through the loop, given that test_str didn't change? Again: try to think about your code first, and step through the logic of it.
• NST: @KarlKnechtel - Thanks for your advice.. I didn't think of it that way..

Answer 1

You can simply change the h in the loop to test_str :)

for i in list1:
 test_str = test_str.replace(str(i), ".")

In your original loop, you did successfully replace the substrings with . in test_str but you didn't save this change. test_str stayed the same and h got a brand new value in each loop. As you can see by printing the values during the loop.

for i in list1:
 print(i)
 h = test_str.replace(str(i), ".")
 print(h)
 print(test_str)

Hopefully I explained this clearly.

Answer 2

replace() returns a copy of the string with the replaced version so you need to assign it back to test_str:

test_str = 'hello_2Testsmthhello_1smthhello_1'
list1 = ['hello_1', 'hello_2']
for i in list1:
 test_str = test_str.replace(i, ".")
 
print(test_str) #.Testsmth.smth.

Answer 3

It is because test_str is replaced only once. Do not use h for your intent.

for i in list1:
 test_str = test_str.replace(str(i), ".")

If you do not want to change test_str, then make a copy as follows.

import copy
h = copy.copy(test_str)
for i in list1:
 h = h.replace(str(i), ".")

Answer 4

You can use regex for it

import re
test_str = 'hello_2Testsmthhello_1smthhello_1'
pattern = r'hello_\d'
print(re.sub(pattern,'.', test_str))