Search python fixers in virtual environment (#632)

* Search python fixers in virtual environment

* Add tests for python fixers

autoload/ale/fixers/autopep8.vim

@@ -2,7 +2,12 @@
 " Description: Fixing files with autopep8.
 
 function! ale#fixers#autopep8#Fix(buffer) abort
+    let l:executable = ale#handlers#python#GetExecutable(a:buffer, 'autopep8')
+    if empty(l:executable)
+        return 0
+    endif
+
     return {
-    \   'command': 'autopep8 -'
+    \   'command': ale#Escape(l:executable) . ' -'
     \}
 endfunction

autoload/ale/fixers/isort.vim

@@ -2,12 +2,17 @@
 " Description: Fixing Python imports with isort.
 
 function! ale#fixers#isort#Fix(buffer) abort
+    let l:executable = ale#handlers#python#GetExecutable(a:buffer, 'isort')
+    if empty(l:executable)
+        return 0
+    endif
+
     let l:config = ale#path#FindNearestFile(a:buffer, '.isort.cfg')
     let l:config_options = !empty(l:config)
     \   ? ' --settings-path ' . ale#Escape(l:config)
     \   : ''
 
     return {
-    \   'command': 'isort' . l:config_options . ' -',
+    \   'command': ale#Escape(l:executable) . l:config_options . ' -',
     \}
 endfunction

autoload/ale/fixers/yapf.vim

@@ -2,12 +2,17 @@
 " Description: Fixing Python files with yapf.
 
 function! ale#fixers#yapf#Fix(buffer) abort
+    let l:executable = ale#handlers#python#GetExecutable(a:buffer, 'yapf')
+    if empty(l:executable)
+        return 0
+    endif
+
     let l:config = ale#path#FindNearestFile(a:buffer, '.style.yapf')
     let l:config_options = !empty(l:config)
     \   ? ' --style ' . ale#Escape(l:config)
     \   : ''
 
     return {
-    \   'command': 'yapf --no-local-style' . l:config_options,
+    \   'command': ale#Escape(l:executable) . ' --no-local-style' . l:config_options,
     \}
 endfunction

autoload/ale/handlers/python.vim

@@ -45,3 +45,24 @@ function! ale#handlers#python#HandlePEP8Format(buffer, lines) abort
 
     return l:output
 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
+function! ale#handlers#python#GetExecutable(buffer, cmd_name) abort
+    let l:virtualenv = ale#python#FindVirtualenv(a:buffer)
+
+    if !empty(l:virtualenv)
+        let l:ve_executable = l:virtualenv . '/bin/' . a:cmd_name
+
+        if executable(l:ve_executable)
+            return l:ve_executable
+        endif
+    endif
+
+    if executable(a:cmd_name)
+        return a:cmd_name
+    endif
+
+    return ''
+endfunction