pathjoin - demo joining Windows paths like C:foo on the right

In a process running on Windows, each drive has a current directory, and a path like X: or X:a, beginning with a drive letter not followed immediately by a \ or /, specifies a location relative to the current directory on that drive.

This raises the question of how to define the operation of joining such a path "under" an absolute path. For other paths, this is mostly straightforward:

• Taking an absolute path like X:\a and joining a fully relative path like b\c gives X:\a\b\c.
• Taking an absolute path like X:\a and joining an absolute path like Y:\b gives the absolute path Y:\b, because if you were located at X:\a, this would not affect the meaning of another absolute path.
• Taking an absolute path like X:\a and joining a path like \b\c that is relative to the current drive gives X:\b\c, because if you are located anywhere under the X: drive, then \b\c refers to X:\b\c.

But there are other cases. One of them is taking an absolute path like X:\a and joining another such path to it. This really divides into two sub-cases:

1. The other path may be on the same drive, such as taking X:\a and joining X:b. If we follow the rule of trying to form an absolute path to the location X:b as it would be resolved when at the location X:\a, then we should form X:\a\b. Some path joining implementations follow this rule, while others do not and instead simply return X:b.
2. The other path may be on a different drive, such as taking X:\a and joining Y:b. If we were to follow the rule of trying to form an absolute path to the location Y:b as it would be resolved when at the location X:\a, then the result would depend on the current directory on Y:, which would have to be obtained dynamically from the system (and which could fail, if there is currently no Y: drive). But this behavior would also be extremely weird, since Y: itself is such a path: taking X: and joining Y: should not give a path like Y:\aa\bb, even if that is the current directory on Y:. Path joining implementations do not follow this rule.

This results in two potentially unintuitive effects:

• Different path joining implementations treat problems like taking X:\a and joining X:b differently. Especially to users coming from a Unix background.
• Even implementations that produce an absolute path in that case will still produce a relative path in others such as taking X:\a and joining Y:b.

In particular, it may be intuitive that taking an absolute path and joining a relative path "under" it sometimes produces a relative path.

Examples

This repository contains a Rust program whose code is in src/main.rs, and a Python program whose code is at main.py, that print some information about the results on Windows of taking the absolute path C:\foo and joining C:bar or D:bar to it.

For the results below:

• All the examples below were run on Windows 10 (10.0.19045), though the Windows version probably does not matter, especially since the filesystem is not actually accessed to do the joining.
• The Rust code was built with Rust 1.80.1.
• The Python code was run in interpreters up from 3.8 up to 3.13 RC 1.
• Note how, in some versions of Python, the behavior differs between Path (which oop uses) and os.path.join (which classic uses).

C:\Users\ek\source\repos\pathjoin [main]> cargo run
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.01s
     Running `target\debug\pathjoin.exe`

left="C:\\", right="C:foo"
joined = "C:foo"
joined.is_absolute() = false
joined.is_relative() = true

left="C:\\", right="D:foo"
joined = "D:foo"
joined.is_absolute() = false
joined.is_relative() = true

C:\Users\ek\source\repos\pathjoin [main]> python3.8 main.py
Python 3.8.10

left='C:\\', right='C:foo'
oop = WindowsPath('C:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'C:\\foo'
isabs(classic) = True

left='C:\\', right='D:foo'
oop = WindowsPath('D:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'D:foo'
isabs(classic) = False

C:\Users\ek\source\repos\pathjoin [main]> python3.9 main.py
Python 3.9.13

left='C:\\', right='C:foo'
oop = WindowsPath('C:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'C:\\foo'
isabs(classic) = True

left='C:\\', right='D:foo'
oop = WindowsPath('D:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'D:foo'
isabs(classic) = False

C:\Users\ek\source\repos\pathjoin [main]> python3.10 main.py
Python 3.10.11

left='C:\\', right='C:foo'
oop = WindowsPath('C:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'C:\\foo'
isabs(classic) = True

left='C:\\', right='D:foo'
oop = WindowsPath('D:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'D:foo'
isabs(classic) = False

C:\Users\ek\source\repos\pathjoin [main]> python3.11 main.py
Python 3.11.9

left='C:\\', right='C:foo'
oop = WindowsPath('C:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'C:\\foo'
isabs(classic) = True

left='C:\\', right='D:foo'
oop = WindowsPath('D:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'D:foo'
isabs(classic) = False

C:\Users\ek\source\repos\pathjoin [main]> python3.12 main.py
Python 3.12.5

left='C:\\', right='C:foo'
oop = WindowsPath('C:/foo')
oop.is_absolute() = True
isabs(oop) = True
classic = 'C:\\foo'
isabs(classic) = True

left='C:\\', right='D:foo'
oop = WindowsPath('D:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'D:foo'
isabs(classic) = False

C:\Users\ek\source\repos\pathjoin [main]> python3.13 main.py
Python 3.13.0rc1

left='C:\\', right='C:foo'
oop = WindowsPath('C:/foo')
oop.is_absolute() = True
isabs(oop) = True
classic = 'C:\\foo'
isabs(classic) = True

left='C:\\', right='D:foo'
oop = WindowsPath('D:foo')
oop.is_absolute() = False
isabs(oop) = False
classic = 'D:foo'
isabs(classic) = False

License

Everything in this repository is licensed under 0BSD.