How to replace some characters from the end of a string?

I want to replace characters at the end of a python string. I have this string:

s = "123123"

I want to replace the last 2 with x. Suppose there is a method called replace_last:

>>> replace_last(s, '2', '开发者_Python百科x')
'1231x3'

Is there any built-in or easy method to do this?

---

It's similar to python's str.replace():

>>> s.replace('2', 'x', 1)
'1x3123'

But it's from the end to beginning.

---

Using regular expression function re.sub to replace words at end of string

import re
s = "123123"
s = re.sub('23$', 'penguins', s)
print s

Prints:

1231penguins

or

import re
s = "123123"
s = re.sub('^12', 'penguins', s)
print s

Prints:

penguins3123

---

This is exactly what the rpartition function is used for:

S.rpartition(sep) -> (head, sep, tail)

Search for the separator sep in S, starting at the end of S, and return the part before it, the separator itself, and the part after it. If the separator is not found, return two empty strings and S.

I wrote this function showing how to use rpartition in your use case:

def replace_last(source_string, replace_what, replace_with):
    head, _sep, tail = source_string.rpartition(replace_what)
    return head + replace_with + tail

s = "123123"
r = replace_last(s, '2', 'x')
print r

Output:

1231x3

---

This is one of the few string functions that doesn't have a left and right version, but we can mimic the behaviour using some of the string functions that do.

>>> s = '123123'
>>> t = s.rsplit('2', 1)
>>> u = 'x'.join(t)
>>> u
'1231x3'

or

>>> 'x'.join('123123'.rsplit('2', 1))
'1231x3'

---

>>> s = "aaa bbb aaa bbb"
>>> s[::-1].replace('bbb','xxx',1)[::-1]
'aaa bbb aaa xxx'

For your second example

>>> s = "123123"
>>> s[::-1].replace('2','x',1)[::-1]
'1231x3'

---

When the wanted match is at the end of string, re.sub comes to the rescue.

>>> import re
>>> s = "aaa bbb aaa bbb"
>>> s
'aaa bbb aaa bbb'
>>> re.sub('bbb$', 'xxx', s)
'aaa bbb aaa xxx'
>>> 

---

Here is a solution based on a simplistic interpretation of your question. A better answer will require more information.

>>> s = "aaa bbb aaa bbb"
>>> separator = " "
>>> parts = s.split(separator)
>>> separator.join(parts[:-1] + ["xxx"])
'aaa bbb aaa xxx'

Update

(After seeing edited question) another very specific answer.

>>> s = "123123"
>>> separator = "2"
>>> parts = s.split(separator)
>>> separator.join(parts[:-1]) + "x" + parts[-1]
'1231x3'

Update 2

There is far better way to do this. Courtesy @mizipzor.

---

There is a way to handle replace to make it work backwards. The idea is to reverse the string, text to replace and new text using [::-1]. The logic stays the same.

For example:

>>> a = "123 456 123 456 123 456"

and we want to replace the last one, (or two, or n) occurrences of "123" with "abc"

>>> a[::-1].replace("123"[::-1], "abc"[::-1], 1)[::-1]
'123 456 123 456 abc 456'

This could be put as a function that behaves exactly like replace.

def replace_right(text, old, new, count=-1):
    """
    Return a copy of text with all occurrences of substring old
    replaced by new, starting from the right.

    If count is given and is not -1, only the first count
    occurrences are replaced.
    """
    return text[::-1].replace(old[::-1], new[::-1], count)[::-1]

Execution:

>>> replace_right(a, "123", "abc", 1)
'123 456 123 456 abc 456'

>>> replace_right(a, "123", "abc", 2)
'123 456 abc 456 abc 456'

---

There is a very easy way to do this, follow my steps

s = "stay am stay am  delete am"
#  want to remove the last am
s = s[:s.rfind('am')]
print(s)

Here at first, we find the index number of am's last occurrence, then take 0th index to that particular index.

---

I got a tricky answer, but it is not efficient enough

fname = '12345.png.pngasdfg.png'
suffix = '.png'
fname_rev = fname[::-1]
suffix_rev = suffix[::-1]
fullname_rev = fname_rev.replace(suffix_rev, '', 1)
fullname = fullname_rev[::-1]
fullname '12345.png.pngasdfg'

Built-in function replace() takes three arguments str.replace(old, new, max_time) So you can delete the last mached string from the origional string

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For the second example, I would recommend rsplit, as it is very simple to use and and directly achieves the goal. Here is a little function with it:

def replace_ending(sentence, old, new):
    if sentence.endswith(old):
        i = sentence.rsplit(old,1)
        new_sentence =new.join(i)
        return new_sentence
    return sentence

print(replace_ending("aaa bbb aaa bbb", "bbb", "xxx")) 

Output:

aaa bbb aaa xxx