How can I replace a substring in a Python pathlib.Path?

Is there an easy way to replace a substring within a pathlib.Path object in Python? The pathlib module is nicer in many ways than storing a path as a str and using os.path, glob.glob etc, which are built in to pathlib. But I often use files that follow a pattern, and often replace substrings in a path to access other files:

data/demo_img.png
data/demo_img_processed.png
data/demo_spreadsheet.csv

Previously I could do:

img_file_path = "data/demo_img.png"
proc_img_file_path = img_file_path.replace("_img.png", "_img_proc.png")
data_file_path = img_file_path.replace("_img.png", "_spreadsheet.csv")

pathlib can replace the file extension with the with_suffix() method, but only accepts extensions as valid suffixes. The workarounds are:

import pathlib
import os


img_file_path = pathlib.Path("data/demo_img.png")
proc_img_file_path = pathlib.Path(str(img_file_path).replace("_img.png", "_img_proc.png"))
# os.fspath() is available in Python 3.6+ and is apparently safer than str()
data_file_path = pathlib.Path(os.fspath(img_file_path).replace("_img.png", "_img_proc.png"))

Converting to a string to do the replacement and reconverting to a Path object seems laborious. Assume that I never have a copy of the string form of img_file_path, and have to convert the type as needed.

Solution

You are correct. To replace old with new in Path p, you need:

p = Path(str(p).replace(old, new))

EDIT

We turn Path p into str so we get this str method:

Help on method_descriptor:

replace(self, old, new, count=-1, /)

Return a copy with all occurrences of substring old replaced by new.

Otherwise we'd get this Path method:

Help on function replace in module pathlib:

replace(self, target)

Rename this path to the given path, clobbering the existing destination if it exists, and return a new Path instance pointing to the given path.