Put a symbol before and after each character in a string

Question

I would like to add brackets to each character in a string. So

"HelloWorld"

should become:

"[H][e][l][l][o][W][o][r][l][d]"

I tried a straightforward loop:

word = "HelloWorld"
newWord = ""
for letter in word:
    newWord += "[%s]" % letter

But can I do it more efficiently? I'm concerned about the repeated string concatenations.

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_{See also: How to insert a character after every 2 characters in a string — note that techniques based on .join will not add anything before the first (or after the last) character, unless explicitly separately added.}

Answer 1

>>> s = "HelloWorld"
>>> ''.join('[{}]'.format(x) for x in s)
'[H][e][l][l][o][W][o][r][l][d]'

If string is huge then using str.join with a list comprehension will be faster and memory efficient than using a generator expression(https://stackoverflow.com/a/9061024/846892):

>>> ''.join(['[{}]'.format(x) for x in s])
'[H][e][l][l][o][W][o][r][l][d]'

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From Python performance tips:

Avoid this:

s = ""
for substring in list:
    s += substring

Use s = "".join(list) instead. The former is a very common and catastrophic mistake when building large strings.

Answer 2

The most pythonic way would probably be with a generator comprehension:

>>> s = "HelloWorld"
>>> "".join("[%s]" % c for c in s)
'[H][e][l][l][o][W][o][r][l][d]'

Ashwini Chaudhary's answer is very similar, but uses the modern (Python3) string format function. The old string interpolation with % still works fine and is a bit simpler.

A bit more creatively, inserting ][ between each character, and surrounding it all with []. I guess this might be a bit faster, since it doesn't do as many string interpolations, but speed shouldn't be an issue here.

>>> "[" + "][".join(s) + "]"
'[H][e][l][l][o][W][o][r][l][d]'