python: flake8: fix length processing in f-strings

"F821 undefined name 'bar'" with `f'foo bar'` refers to the beginning of
the f-string, and therefore the entry's `length` property should not be
set.

This changes the flake8 postprocessing to use the generic length
postprocessor, which results in no `length` attribute for this entry
(since it checks if the text is there).

A proper fix would be to search for the next `{bar}`, and adjust the
lnum/col also.

autoload/neomake/makers/ft/python.vim

@@ -130,13 +130,14 @@ function! neomake#makers#ft#python#Flake8EntryProcess(entry) abort
         let type = ''
     endif
 
-    let l:token = matchstr(a:entry.text, "'.*'")
-    if strlen(l:token)
-        " remove quotes
-        let l:token = substitute(l:token, "'", '', 'g')
-        if a:entry.type ==# 'F' && (a:entry.nr == 401 ||  a:entry.nr == 811)
-            " The unused column is incorrect for import errors and redefinition
-            " errors.
+    let token_pattern = '\v''\zs[^'']+\ze'
+    if a:entry.type ==# 'F' && (a:entry.nr == 401 || a:entry.nr == 811)
+        " Special handling for F401 (``module`` imported but unused) and
+        " F811 (redefinition of unused ``name`` from line ``N``).
+        " The unused column is incorrect for import errors and redefinition
+        " errors.
+        let token = matchstr(a:entry.text, token_pattern)
+        if !empty(token)
             let l:view = winsaveview()
             call cursor(a:entry.lnum, a:entry.col)
             " The number of lines to give up searching afterwards
@@ -172,9 +173,11 @@ function! neomake#makers#ft#python#Flake8EntryProcess(entry) abort
             endif
 
             call winrestview(l:view)
-        endif
 
-        let a:entry.length = strlen(l:token) " subtract the quotes
+            let a:entry.length = strlen(l:token)
+        endif
+    else
+        call neomake#postprocess#generic_length_with_pattern(a:entry, token_pattern)
     endif
 
     let a:entry.text = a:entry.type . a:entry.nr . ' ' . a:entry.text

autoload/neomake/postprocess.vim

@@ -1,12 +1,13 @@
 " Generic postprocessor to add `length` to `a:entry`.
-" The pattern can be overridden on `self`, and should adhere to this:
+" The pattern can be overridden on `self` and should adhere to this:
 "  - the matched word should be returned as the whole match (you can use \zs
 "    and \ze).
 "  - enclosing patterns should be returned as \1 and \2, where \1 is used as
 "    offset when the first entry did not match.
 " See tests/postprocess.vader for tests/examples.
+" See neomake#postprocess#generic_length_with_pattern for a non-dict variant.
 function! neomake#postprocess#generic_length(entry) abort dict
-    if a:entry.bufnr == bufnr('%') && a:entry.lnum > 0 && a:entry.col
+    if a:entry.lnum > 0 && a:entry.col
         let pattern = get(self, 'pattern', '\v(["''`])\zs[^\1]{-}\ze(\1)')
         let start = 0
         let best = 0
@@ -18,7 +19,8 @@ function! neomake#postprocess#generic_length(entry) abort dict
             let l = len(m[0])
             if l > best
                 " Ensure that the text is there.
-                if getline(a:entry.lnum)[a:entry.col-1 : a:entry.col-2+l] == m[0]
+                let line = getbufline(a:entry.bufnr, a:entry.lnum)[0]
+                if line[a:entry.col-1 : a:entry.col-2+l] == m[0]
                     let best = l
                 endif
             endif
@@ -38,6 +40,12 @@ function! neomake#postprocess#generic_length(entry) abort dict
     endif
 endfunction
 
+" Wrapper to call neomake#process#generic_length (a dict function).
+function! neomake#postprocess#generic_length_with_pattern(entry, pattern) abort
+    let this = {'pattern': a:pattern}
+    return call('neomake#postprocess#generic_length', [a:entry], this)
+endfunction
+
 " Deprecated: renamed to neomake#postprocess#generic_length.
 function! neomake#postprocess#GenericLengthPostprocess(entry) abort dict
     return neomake#postprocess#generic_length(a:entry)

tests/ft_python.vader

@@ -113,19 +113,20 @@ Execute (python: flake8: errorformat/postprocess: F811):
   bwipe!
 
 Execute (neomake#makers#ft#python#Flake8EntryProcess):
-  let entry = {'type': 'F', 'nr': 841, 'text': "local variable 'foo' is assigned to but never used"}
+  let bufnr = bufnr('%')
+  let entry = {'type': 'F', 'nr': 841, 'text': "local variable 'foo' is assigned to but never used", 'lnum': 1, 'col': 1, 'bufnr': bufnr}
   call neomake#makers#ft#python#Flake8EntryProcess(entry)
   AssertEqual entry.type, 'W'
 
-  let entry = {'type': 'F', 'nr': 999, 'text': "something"}
+  let entry = {'type': 'F', 'nr': 999, 'text': "something", 'lnum': 1, 'col': 1, 'bufnr': bufnr}
   call neomake#makers#ft#python#Flake8EntryProcess(entry)
   AssertEqual entry.type, 'E'
 
-  let entry = {'type': 'F', 'nr': 404, 'text': "not found"}
+  let entry = {'type': 'F', 'nr': 404, 'text': "not found", 'lnum': 1, 'col': 1, 'bufnr': bufnr}
   call neomake#makers#ft#python#Flake8EntryProcess(entry)
   AssertEqual entry.type, 'W'
 
-  let entry = {'type': 'F', 'nr': 407, 'text': "no future"}
+  let entry = {'type': 'F', 'nr': 407, 'text': "no future", 'lnum': 1, 'col': 1, 'bufnr': bufnr}
   call neomake#makers#ft#python#Flake8EntryProcess(entry)
   AssertEqual entry.type, 'E'