Support pylama for python (#2266)

* Add pylama for python
* Consolidate python traceback handling

autoload/ale/python.vim

@@ -27,6 +27,7 @@ function! ale#python#FindProjectRootIni(buffer) abort
         \|| filereadable(l:path . '/pycodestyle.cfg')
         \|| filereadable(l:path . '/flake8.cfg')
         \|| filereadable(l:path . '/.flake8rc')
+        \|| filereadable(l:path . '/pylama.ini')
         \|| filereadable(l:path . '/Pipfile')
         \|| filereadable(l:path . '/Pipfile.lock')
             return l:path
@@ -110,6 +111,44 @@ function! ale#python#FindExecutable(buffer, base_var_name, path_list) abort
     return ale#Var(a:buffer, a:base_var_name . '_executable')
 endfunction
 
+" Handle traceback.print_exception() output starting in the first a:limit lines.
+function! ale#python#HandleTraceback(lines, limit) abort
+    let l:nlines = len(a:lines)
+    let l:limit = a:limit > l:nlines ? l:nlines : a:limit
+    let l:start = 0
+
+    while l:start < l:limit
+        if a:lines[l:start] is# 'Traceback (most recent call last):'
+            break
+        endif
+
+        let l:start += 1
+    endwhile
+
+    if l:start >= l:limit
+        return []
+    endif
+
+    let l:end = l:start + 1
+
+    " Traceback entries are always prefixed with 2 spaces.
+    " SyntaxError marker (if present) is prefixed with at least 4 spaces.
+    " Final exc line starts with exception class name (never a space).
+    while l:end < l:nlines && a:lines[l:end][0] is# ' '
+        let l:end += 1
+    endwhile
+
+    let l:exc_line = l:end < l:nlines
+    \   ? a:lines[l:end]
+    \   : 'An exception was thrown.'
+
+    return [{
+    \   'lnum': 1,
+    \   'text': l:exc_line . ' (See :ALEDetail)',
+    \   'detail': join(a:lines[(l:start):(l:end)], "\n"),
+    \}]
+endfunction
+
 " Detects whether a pipenv environment is present.
 function! ale#python#PipenvPresent(buffer) abort
     return findfile('Pipfile.lock', expand('#' . a:buffer . ':p:h') . ';') isnot# ''