Insert substring if found

Question

I'm given a string my_str in Python. What I want to do is: If my_str contains a substring str1, then insert a string str2 right after the substring str1 (and leave the rest of my_str intact.) Otherwise, do nothing. (Let's say that my_str contains no more than one substring being str1.)

The way I'm thinking is: Do a for-loop to find whether str1 exists within my_str

for i in range(0, len(my_str)-len(str1)):
  if my_str[i:i+len(str1)] == str1:
    my_str = my_str[0:i] + str2 + my_str[i:]

I'm curious whether there's any magic way that can do this shorter.

Don't name a variable str or file or dict or set or list, etc — inspectorG4dget
– inspectorG4dget, Commented Nov 28, 2012 at 20:28

sampson-chen · Accepted Answer · 2012-11-28 21:06:31Z

8

The easiest method is with str.replace():

>>> str1 = "blah"
>>> str2 = "new"
>>> "testblah".replace(str1, str1+str2)
'testblahnew'
>>> "testblahtest".replace(str1, str1+str2)
'testblahnewtest'
>>> "test".replace(str1, str1+str2)
'test'
>>> "blahtestblah".replace(str1, str1+str2)
'blahnewtestblahnew'

We simply replace the original value with the new string appended to itself, essentially slotting in the new value.

A quick tutorial on replace() for more examples.

edited Nov 28, 2012 at 21:06

sampson-chen

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answered Nov 28, 2012 at 20:30

Gareth Latty

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Comments

inspectorG4dget · Accepted Answer · 2012-11-28 20:54:58Z

1

def myReplace(myStr, str1, str2):
    try:
        i = myStr.index(str1)
        answer = myStr[:i+len(str1)] + str2 + myStr[i+len(str1):]
        return answer
    except ValueError:
        return myStr

Hope this helps

edited Nov 28, 2012 at 20:54

answered Nov 28, 2012 at 20:32

inspectorG4dget

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4 Comments

Anov Over a year ago

honest question- what is the advantage of doing it this way rather than using the built-in replace method?

inspectorG4dget Over a year ago

@Anov: the built-in replace is what I would normally go with. Since the OP seems to be a beginner, I gave an answer that's closer to what he was trying to implement. I would have the replace way of doing this in my answer to be complete, but as I finished writing this solution, I noticed that someone else had posted that already.

Ashwini Chaudhary Over a year ago

@inspectorG4dget In think you used the try-except to handle ValueError, if that's the case then use str.find(), it returns -1 if the substring is not found.

abarnert Over a year ago

The OP's version will replace every instance of str1 with str1+str2; yours will only replace the first. The OP said "Let's say that my_str contains no more than one substring being str1", but it's worth pointing out the difference. With replace, you can do it either way—pass 1 for count, or don't. You could also do the same with this solution by adding a loop and passing i+1 as the start to each index/find call.

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