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

Use pathlib.Path.joinpath:

(Path(__file__).parent).joinpath('.env')

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__).

Will work on Windows with '\' and Unix (including Mac OS X) with '/'.

for posixpath here's the straightforward code

In [22]: os.path.join??
Type:       function
String Form:<function join at 0x107c28ed8>
File:       /usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/posixpath.py
Definition: os.path.join(a, *p)
Source:
def join(a, *p):
    """Join two or more pathname components, inserting '/' as needed.
    If any component is an absolute path, all previous path components
    will be discarded."""
    path = a
    for b in p:
        if b.startswith('/'):
            path = b
        elif path == '' or path.endswith('/'):
            path +=  b
        else:
            path += '/' + b
    return path

don't have windows but the same should be there with '\'