Add a fixer for Python for automatically adding blank lines before control statements

autoload/ale/fix/registry.vim

@@ -2,6 +2,11 @@
 " Description: A registry of functions for fixing things.
 
 let s:default_registry = {
+\   'add_blank_lines_for_python_control_statements': {
+\       'function': 'ale#handlers#python#AddLinesBeforeControlStatements',
+\       'suggested_filetypes': ['python'],
+\       'description': 'Add blank lines before control statements.',
+\   },
 \   'autopep8': {
 \       'function': 'ale#handlers#python#AutoPEP8',
 \       'suggested_filetypes': ['python'],

autoload/ale/handlers/python.vim

@@ -36,6 +36,26 @@ function! ale#handlers#python#HandlePEP8Format(buffer, lines) abort
     return l:output
 endfunction
 
+" Add blank lines before control statements.
+function! ale#handlers#python#AddLinesBeforeControlStatements(buffer, lines) abort
+    let l:new_lines = []
+    let l:last_indent_size = 0
+
+    for l:line in a:lines
+        let l:indent_size = len(matchstr(l:line, '^ *'))
+
+        if l:indent_size <= l:last_indent_size
+        \&& match(l:line, '\v^ *(return|if|for|while|break|continue)') >= 0
+            call add(l:new_lines, '')
+        endif
+
+        call add(l:new_lines, l:line)
+        let l:last_indent_size = l:indent_size
+    endfor
+
+    return l:new_lines
+endfunction
+
 function! ale#handlers#python#AutoPEP8(buffer, lines) abort
     return {
     \   'command': 'autopep8 -'

test/fixers/test_python_add_blank_lines_fixer.vader

@@ -0,0 +1,85 @@
+Before:
+  Save g:ale_fixers
+
+After:
+  Restore
+
+Given python(Some Python without blank lines):
+  def foo():
+      return 1
+
+
+  def bar():
+      return 1
+      return 4
+
+
+  def bar():
+      if x:
+          pass
+          for l in x:
+              pass
+          for l in x:
+              pass
+              break
+              continue
+      elif x:
+          pass
+          while x:
+              pass
+          while x:
+              pass
+      else:
+          pass
+      if x:
+          pass
+      elif x:
+          pass
+      else:
+          pass
+
+Execute(Blank lines should be added appropriately):
+  let g:ale_fixers = {'python': ['ale#handlers#python#AddLinesBeforeControlStatements']}
+  ALEFix
+
+Expect python(Newlines should be added):
+  def foo():
+      return 1
+
+
+  def bar():
+      return 1
+
+      return 4
+
+
+  def bar():
+      if x:
+          pass
+
+          for l in x:
+              pass
+
+          for l in x:
+              pass
+
+              break
+
+              continue
+      elif x:
+          pass
+
+          while x:
+              pass
+
+          while x:
+              pass
+      else:
+          pass
+
+      if x:
+          pass
+      elif x:
+          pass
+      else:
+          pass