[python] Changing file extension in Python

Suppose from index.py with CGI, I have post file foo.fasta to display file. I want to change foo.fasta's file extension to be foo.aln in display file. How can I do it?

An elegant way using pathlib.Path:

from pathlib import Path
p = Path('mysequence.fasta')
p.rename(p.with_suffix('.aln'))

As AnaPana mentioned pathlib is more new and easier in python 3.4 and there is new with_suffix method that can handle this problem easily:

from pathlib import Path
new_filename = Path(mysequence.fasta).with_suffix('.aln')

Starting from Python 3.4 there's pathlib built-in library. So the code could be something like:

from pathlib import Path

filename = "mysequence.fasta"
new_filename = Path(filename).stem + ".aln"

https://docs.python.org/3.4/library/pathlib.html#pathlib.PurePath.stem

I love pathlib :)

Sadly, I experienced a case of multiple dots on file name that splittext does not worked well... my work around:

file = r'C:\Docs\file.2020.1.1.xls'
ext = '.'+ os.path.realpath(file).split('.')[-1:][0]
filefinal = file.replace(ext,'.zip')
os.rename(file ,filefinal)

Using pathlib and preserving full path:

from pathlib import Path
p = Path('/User/my/path')
new_p = Path(p.parent.as_posix() + '/' + p.stem + '.aln')

Use this:

os.path.splitext("name.fasta")[0]+".aln"

And here is how the above works:

The splitext method separates the name from the extension creating a tuple:

os.path.splitext("name.fasta")

the created tuple now contains the strings "name" and "fasta". Then you need to access only the string "name" which is the first element of the tuple:

os.path.splitext("name.fasta")[0]

And then you want to add a new extension to that name:

os.path.splitext("name.fasta")[0]+".aln"

>> file = r'C:\Docs\file.2020.1.1.xls'
>> ext = '.'+ os.path.realpath(file).split('.')[-1:][0]
>> filefinal = file.replace(ext,'.zip')
>> os.rename(file ,filefinal) 

Bad logic for repeating extension, sample: 'C:\Docs\.xls_aaa.xls.xls'

import os
thisFile = "mysequence.fasta"
base = os.path.splitext(thisFile)[0]
os.rename(thisFile, base + ".aln")

Where thisFile = the absolute path of the file you are changing