Windows has a concept of current directory for each drive. Because of that, "c:sourcedir" means "sourcedir" inside the current C: directory, and you'll need to specify an absolute directory.

Any of these should work and give the same result, but I don't have a Windows VM fired up at the moment to double check:

"c:/sourcedir"
os.path.join("/", "c:", "sourcedir")
os.path.join("c:/", "sourcedir")

The idea of os.path.join() is to make your program cross-platform (linux/windows/etc).

Even one slash ruins it.

So it only makes sense when being used with some kind of a reference point like os.environ['HOME'] or os.path.dirname(__file__).

os.path is string-based while pathlib is object oriented

os.path operates on strings while pathlib is object-oriented (the documentation at this link speaks about Object-oriented filesystem paths). So if "/home/test/test.txt" is the absolute path (in POSIX format) to access to the file test.txt in the directory /home/test/, the instructions:

from os import path

path_1 = path.join("/home", "test", "test.txt")

(executed in a Linux system) return in path_1, the string "/home/test/test.txt" (link to os.path.join() documentation.)

On the other hand, the following commands:

from pathlib import Path

path_2 = Path("/home") / "test" / "test.txt"

return an object called path_2 which represents the file.

A function and a method available with the 2 approaches

By the previous initialization, essentially:

• The string path_1 can be used as argument of other functions of the module os.path. For example, to check if a file exists:

  from os import path
  
  if path.exists(path_1):
      # the file exists
      do stuff
  

• The object path_2 is an instance of the class pathlib.Path, so with this object it is possible to call all methods of the class Path.

  Furthermore, if path_2 is created on a POSIX compliant system (such as Linux), its type is PosixPath (a subclass of Path), else if path_2 is created on a Windows system, its type is WindowsPath (a subclass of the class Path).

  An example of the methods available on the object path_2 is the exists() method. The snippet of code below shows how it is used:

  from pathlib import Path
  
  if path_2.exists():
      # the file exists
      do stuff
  

Link

See this article to learn about other differences between the two ways of managing paths with Python: Paths in Python: Comparing os.path and pathlib modules

You could use the f-string in python (see documentation).

In your case:

request_path  = f"{BASE_DIR}/{filepath}"

Be aware that you need an extra slash "/" between your file location variables (BASE_DIR and filepath) to complete the full file location properly.

Hope this helps!