Add a has() wrapper we can override for tests, and add a function for generating paths up to the root directory to search through

autoload/ale.vim

@@ -105,6 +105,14 @@ function! ale#ResetLintFileMarkers() abort
     let s:should_lint_file_for_buffer = {}
 endfunction
 
+let g:ale_has_override = get(g:, 'ale_has_override', {})
+
+" Call has(), but check a global Dictionary so we can force flags on or off
+" for testing purposes.
+function! ale#Has(feature) abort
+    return get(g:ale_has_override, a:feature, has(a:feature))
+endfunction
+
 " Given a buffer number and a variable name, look for that variable in the
 " buffer scope, then in global scope. If the name does not exist in the global
 " scope, an exception will be thrown.

autoload/ale/path.vim

@@ -92,3 +92,27 @@ function! ale#path#IsBufferPath(buffer, filename) abort
 
     return resolve(l:buffer_filename) ==# l:resolved_filename
 endfunction
+
+" Given a path, return every component of the path, moving upwards.
+function! ale#path#Upwards(path) abort
+    let l:pattern = ale#Has('win32') ? '\v/+|\\+' : '\v/+'
+    let l:sep = ale#Has('win32') ? '\' : '/'
+    let l:parts = split(simplify(a:path), l:pattern)
+    let l:path_list = []
+
+    while !empty(l:parts)
+        call add(l:path_list, join(l:parts, l:sep))
+        let l:parts = l:parts[:-2]
+    endwhile
+
+    if ale#Has('win32') && a:path =~# '^[a-zA-z]:\'
+        " Add \ to C: for C:\, etc.
+        let l:path_list[-1] .= '\'
+    elseif a:path[0] ==# '/'
+        " If the path starts with /, even on Windows, add / and / to all paths.
+        call add(l:path_list, '')
+        call map(l:path_list, '''/'' . v:val')
+    endif
+
+    return l:path_list
+endfunction

autoload/ale/python.vim

@@ -0,0 +1,2 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Functions for integrating with Python linters.

test/test_path_upwards.vader

@@ -0,0 +1,46 @@
+After:
+  let g:ale_has_override = {}
+
+Execute(ale#path#Upwards should return the correct path components for Unix):
+  " Absolute paths should include / on the end.
+  AssertEqual
+  \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
+  \ ale#path#Upwards('/foo/bar/baz')
+  AssertEqual
+  \ ['/foo/bar/baz', '/foo/bar', '/foo', '/'],
+  \ ale#path#Upwards('/foo/bar/baz///')
+  " Relative paths do not.
+  AssertEqual
+  \ ['foo/bar/baz', 'foo/bar', 'foo'],
+  \ ale#path#Upwards('foo/bar/baz')
+  AssertEqual
+  \ ['foo2/bar', 'foo2'],
+  \ ale#path#Upwards('foo//..////foo2////bar')
+  " Expect an empty List for empty strings.
+  AssertEqual [], ale#path#Upwards('')
+
+Execute(ale#path#Upwards should return the correct path components for Windows):
+  let g:ale_has_override = {'win32': 1}
+
+  AssertEqual
+  \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
+  \ ale#path#Upwards('C:\foo\bar\baz')
+  AssertEqual
+  \ ['C:\foo\bar\baz', 'C:\foo\bar', 'C:\foo', 'C:\'],
+  \ ale#path#Upwards('C:\foo\bar\baz\\\')
+  AssertEqual
+  \ ['/foo\bar\baz', '/foo\bar', '/foo', '/'],
+  \ ale#path#Upwards('/foo/bar/baz')
+  AssertEqual
+  \ ['foo\bar\baz', 'foo\bar', 'foo'],
+  \ ale#path#Upwards('foo/bar/baz')
+  AssertEqual
+  \ ['foo\bar\baz', 'foo\bar', 'foo'],
+  \ ale#path#Upwards('foo\bar\baz')
+  " simplify() is used internally, and should sort out \ paths when actually
+  " running Windows, which we can't test here.
+  AssertEqual
+  \ ['foo2\bar', 'foo2'],
+  \ ale#path#Upwards('foo//..///foo2////bar')
+  " Expect an empty List for empty strings.
+  AssertEqual [], ale#path#Upwards('')