Use local versions of yapf on Windows, and get the callback tests to pass

autoload/ale/fixers/yapf.vim

@@ -11,7 +11,7 @@ function! ale#fixers#yapf#Fix(buffer) abort
     \   ['yapf'],
     \)
 
-    if !executable(l:executable)
+    if !ale#python#IsExecutable(l:executable)
         return 0
     endif
 

autoload/ale/python.vim

@@ -1,6 +1,7 @@
 " Author: w0rp <devw0rp@gmail.com>
 " Description: Functions for integrating with Python linters.
 
+let s:sep = has('win32') ? '\' : '/'
 " bin is used for Unix virtualenv directories, and Scripts is for Windows.
 let s:bin_dir = has('unix') ? 'bin' : 'Scripts'
 let g:ale_virtualenv_dir_names = get(g:, 'ale_virtualenv_dir_names', [
@@ -11,7 +12,6 @@ let g:ale_virtualenv_dir_names = get(g:, 'ale_virtualenv_dir_names', [
 \   'virtualenv',
 \])
 
-
 function! ale#python#FindProjectRootIni(buffer) abort
     for l:path in ale#path#Upwards(expand('#' . a:buffer . ':p:h'))
         if filereadable(l:path . '/MANIFEST.in')
@@ -58,9 +58,14 @@ function! ale#python#FindVirtualenv(buffer) abort
         endif
 
         for l:dirname in ale#Var(a:buffer, 'virtualenv_dir_names')
-            let l:venv_dir = ale#path#Simplify(l:path . '/' . l:dirname)
+            let l:venv_dir = ale#path#Simplify(
+            \   join([l:path, l:dirname], s:sep)
+            \)
+            let l:script_filename = ale#path#Simplify(
+            \   join([l:venv_dir, s:bin_dir, 'activate'], s:sep)
+            \)
 
-            if filereadable(ale#path#Simplify(l:venv_dir . '/' . s:bin_dir . '/activate'))
+            if filereadable(l:script_filename)
                 return l:venv_dir
             endif
         endfor
@@ -69,6 +74,12 @@ function! ale#python#FindVirtualenv(buffer) abort
     return ''
 endfunction
 
+" Run an executable check for Python scripts.
+" On Windows, 1 will be returned if the file is merely readable.
+function! ale#python#IsExecutable(path) abort
+    return has('win32') ? filereadable(a:path) : executable(a:path)
+endfunction
+
 " Given a buffer number and a command name, find the path to the executable.
 " First search on a virtualenv for Python, if nothing is found, try the global
 " command. Returns an empty string if cannot find the executable
@@ -81,9 +92,11 @@ function! ale#python#FindExecutable(buffer, base_var_name, path_list) abort
 
     if !empty(l:virtualenv)
         for l:path in a:path_list
-            let l:ve_executable = ale#path#Simplify(l:virtualenv . '/' . s:bin_dir . '/' . l:path)
+            let l:ve_executable = ale#path#Simplify(
+            \   join([l:virtualenv, s:bin_dir, l:path], s:sep)
+            \)
 
-            if executable(l:ve_executable)
+            if ale#python#IsExecutable(l:ve_executable)
                 return l:ve_executable
             endif
         endfor

test/fixers/test_yapf_fixer_callback.vader

@@ -9,9 +9,13 @@ Before:
   silent cd command_callback
   let g:dir = getcwd()
 
+  let b:bin_dir = has('win32') ? 'Scripts' : 'bin'
+
 After:
   Restore
 
+  unlet! b:bin_dir
+
   call ale#test#RestoreDirectory()
 
 Execute(The yapf callback should return the correct default values):
@@ -22,7 +26,7 @@ Execute(The yapf callback should return the correct default values):
   call ale#test#SetFilename('python_paths/with_virtualenv/subdir/foo/bar.py')
 
   AssertEqual
-  \ {'command': ale#Escape(g:dir . '/python_paths/with_virtualenv/env/bin/yapf')},
+  \ {'command': ale#Escape(ale#path#Winify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/yapf'))},
   \ ale#fixers#yapf#Fix(bufnr(''))
   \
 Execute(The yapf should include the .style.yapf file if present):
@@ -31,8 +35,8 @@ Execute(The yapf should include the .style.yapf file if present):
   AssertEqual
   \ {
   \   'command':
-  \     ale#Escape(g:dir . '/python_paths/with_virtualenv/env/bin/yapf')
+  \     ale#Escape(ale#path#Winify(g:dir . '/python_paths/with_virtualenv/env/' . b:bin_dir . '/yapf'))
   \     . ' --no-local-style'
-  \     . ' --style ' . ale#Escape(g:dir . '/python_paths/with_virtualenv/dir_with_yapf_config/.style.yapf'),
+  \     . ' --style ' . ale#Escape(ale#path#Winify(g:dir . '/python_paths/with_virtualenv/dir_with_yapf_config/.style.yapf')),
   \ },
   \ ale#fixers#yapf#Fix(bufnr(''))