Python adding prefix to list elements in python

Question

I have this list of strings

lst = ["/foo/dir/c-.*.txt","/foo/dir2/d-.*.svc","/foo/dir3/es-.*.info"]

and I have a prefix string :

/root

Is there any pythonic way to add the prefix string to each element in the list, so the end result will look like this:

lst = ["/root/foo/dir/c-.*.txt","/root/foo/dir2/d-.*.svc","/root/foo/dir3/es-.*.info"]

If it can be done without iterating and creating new list ...

`lst = [prefix+item for item in lst]` - this is to reassign the result back to orig. list — Daniel Hao, Jul 18 '21 at 15:03
You might want os.path.join() to use OS agnostic paths rather than / — Chris_Rands, Jul 18 '21 at 15:04
This is quite simple really, you could write out a for loop- where did you efforts go wrong? You'll have to iterate, directly or implicitly of course — Chris_Rands, Jul 18 '21 at 15:06

Abhi · Answer 1 · 2021-07-18T15:05:21.483

used:

List Comprehensions

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

F=Strings

F-strings provide a way to embed expressions inside string literals, using a minimal syntax. It should be noted that an f-string is really an expression evaluated at run time, not a constant value. In Python source code, an f-string is a literal string, prefixed with 'f', which contains expressions inside braces. The expressions are replaced with their values.

lst = ["/foo/dir/c-.*.txt","/foo/dir2/d-.*.svc","/foo/dir3/es-.*.info"]
prefix = '/root'
lst =[ f'{prefix}{path}' for path in lst] 

print(lst)

score 1 · Answer 2 · answered Jul 18 '21 at 14:59

1

Use list comprehensions and string concatenation:

lst = ["/foo/dir/c-.*.txt","/foo/dir2/d-.*.svc","/foo/dir3/es-.*.info"]
   
print(['/root' + p for p in lst])

# ['/root/foo/dir/c-.*.txt', '/root/foo/dir2/d-.*.svc', '/root/foo/dir3/es-.*.info']

answered Jul 18 '21 at 14:59

funnydman

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eroot163pi · Accepted Answer · 2021-07-18T15:13:16.087

1

I am not sure of pythonic, but this will be also on possible way

list(map(lambda x: '/root' + x, lst))

Here there is comparison between list comp and map List comprehension vs map

Also thanks to @chris-rands learnt one more way without lambda

list(map('/root'.__add__, lst))

edited Jul 18 '21 at 15:13

answered Jul 18 '21 at 15:00

eroot163pi

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cute! very pythonic! – user63898 Jul 18 '21 at 15:05
what is the diff between your solution to list comprehensions – user63898 Jul 18 '21 at 15:05
`map()` is a built-in function that allows you to turn all the items in an iterable without using an explicit for loop. But it wouldn't be considered pythonic by many! – PCM Jul 18 '21 at 15:07
3

actually using map with lambda is normally considered less pythonic, although it's taste really for functional programmers ;) – Chris_Rands Jul 18 '21 at 15:08
@user63898 [check this for map vs list comp](https://stackoverflow.com/questions/1247486/list-comprehension-vs-map) – Abhi Jul 18 '21 at 15:08
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Yeah, it is indeed considered unpythonic https://stackoverflow.com/questions/1247486/list-comprehension-vs-map – eroot163pi Jul 18 '21 at 15:09
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Of course you don't need lambda here though, e.g. `list(map('/root'.__add__, lst))` (or better import add from operator) – Chris_Rands Jul 18 '21 at 15:11
Thanks @Chris_Rands I took the liberty of adding it above – eroot163pi Jul 18 '21 at 15:13

score 0 · Answer 4 · answered Jul 18 '21 at 15:03

0

Just simply write:

lst = ["/foo/dir/c-.*.txt","/foo/dir2/d-.*.svc","/foo/dir3/es-.*.info"]
prefix="/root"

res = [prefix + x for x in lst]
print(res)

answered Jul 18 '21 at 15:03

devReddit

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score 0 · Answer 5 · answered Jul 18 '21 at 15:06

0

A simple list comprehension -

lst = ["/foo/dir/c-.*.txt","/foo/dir2/d-.*.svc","/foo/dir3/es-.*.info"]
prefix = '/root'

print([prefix + string for string in lst]) # You can give it a variable if you want

answered Jul 18 '21 at 15:06

PCM

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Python adding prefix to list elements in python

5 Answers5