Run flake8 from project root by default.

Option `per-file-ignores` was introduced in flake8 version 3.7.0.
It allows to ignore specific errors in specific files using glob syntax.
For example `per-file-ignores = src/generated/*.py:F401` will
ignore `F401` error in all python files in `src/generated`.
Thus ale has to run flake8 from project root where .flake8 config
is placed otherwise glob won't match linted file.

ale_linters/python/flake8.vim

@@ -4,7 +4,7 @@
 call ale#Set('python_flake8_executable', 'flake8')
 call ale#Set('python_flake8_options', '')
 call ale#Set('python_flake8_use_global', get(g:, 'ale_use_global_executables', 0))
-call ale#Set('python_flake8_change_directory', 1)
+call ale#Set('python_flake8_change_directory', 'project')
 call ale#Set('python_flake8_auto_pipenv', 0)
 
 function! s:UsingModule(buffer) abort
@@ -38,10 +38,34 @@ function! ale_linters#python#flake8#RunWithVersionCheck(buffer) abort
     \)
 endfunction
 
+function! ale_linters#python#flake8#GetCdString(buffer) abort
+    let l:change_directory = ale#Var(a:buffer, 'python_flake8_change_directory')
+
+    " map legacy options to new ones
+    if l:change_directory is# 1
+        let l:change_directory = 'file'
+    elseif l:change_directory is# 0
+        let l:change_directory = 'off'
+    endif
+
+    if l:change_directory is# 'file'
+        return ale#path#BufferCdString(a:buffer)
+    elseif l:change_directory is# 'off'
+        return ''
+    endif
+
+    let l:project_root = ale#python#FindProjectRootIni(a:buffer)
+
+    if !empty(l:project_root)
+        return ale#path#CdString(l:project_root)
+    endif
+
+    return ale#path#BufferCdString(a:buffer)
+endfunction
+
 function! ale_linters#python#flake8#GetCommand(buffer, version) abort
-    let l:cd_string = ale#Var(a:buffer, 'python_flake8_change_directory')
-    \   ? ale#path#BufferCdString(a:buffer)
-    \   : ''
+    let l:cd_string = ale_linters#python#flake8#GetCdString(a:buffer)
+
     let l:executable = ale_linters#python#flake8#GetExecutable(a:buffer)
 
     let l:exec_args = l:executable =~? 'pipenv$'

doc/ale-python.txt

@@ -169,13 +169,14 @@ flake8                                                      *ale-python-flake8*
 
 g:ale_python_flake8_change_directory     *g:ale_python_flake8_change_directory*
                                          *b:ale_python_flake8_change_directory*
-  Type: |Number|
-  Default: `1`
+  Type: |String|
+  Default: `project`
 
-  If set to `1`, ALE will switch to the directory the Python file being
-  checked with `flake8` is in before checking it. This helps `flake8` find
-  configuration files more easily. This option can be turned off if you want
-  to control the directory Python is executed from yourself.
+  If set to `project`, ALE will switch to the project root before checking file.
+  If set to `file`, ALE will switch to directory the Python file being
+  checked with `flake8` is in before checking it.
+  You can turn it off with `off` option if you want to control the directory
+  Python is executed from yourself.
 
 
 g:ale_python_flake8_executable                 *g:ale_python_flake8_executable*

test/command_callback/test_flake8_command_callback.vader

@@ -34,13 +34,32 @@ Execute(The flake8 callbacks should return the correct default values):
   \]
 
 Execute(The option for disabling changing directories should work):
-  let g:ale_python_flake8_change_directory = 0
+  let g:ale_python_flake8_change_directory = 'off'
 
   AssertLinter 'flake8', [
   \ ale#Escape('flake8') . ' --version',
   \ ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
   \]
 
+Execute(The option for changing directory to project root should work):
+  silent execute 'file ' . fnameescape(g:dir . '/python_paths/namespace_package_tox/namespace/foo/bar.py')
+
+  AssertLinter 'flake8', [
+  \ ale#Escape('flake8') . ' --version',
+  \ ale#path#CdString(ale#python#FindProjectRootIni(bufnr('')))
+  \   . ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
+  \]
+
+Execute(The option for changing directory to file dir should work):
+  let g:ale_python_flake8_change_directory = 'file'
+  silent execute 'file ' . fnameescape(g:dir . '/python_paths/namespace_package_tox/namespace/foo/bar.py')
+
+  AssertLinter 'flake8', [
+  \ ale#Escape('flake8') . ' --version',
+  \ ale#path#BufferCdString(bufnr(''))
+  \   . ale#Escape('flake8') . ' --format=default --stdin-display-name %s -',
+  \]
+
 Execute(The flake8 command callback should let you set options):
   let g:ale_python_flake8_options = '--some-option'
 
@@ -163,5 +182,5 @@ Execute(Pipenv is detected when python_flake8_auto_pipenv is set):
   call ale#test#SetFilename('../python_fixtures/pipenv/whatever.py')
 
   AssertLinter 'pipenv',
-  \ ale#path#BufferCdString(bufnr(''))
+  \ ale#path#CdString(ale#python#FindProjectRootIni(bufnr('')))
   \   . ale#Escape('pipenv') . ' run flake8 --format=default --stdin-display-name %s -'