Flycheck定制Python的检查

问题的出现与分析

有一阵子没仔细琢磨Emacs的Python配置，最近几天在开发一个项目的时候遇到一个报错。

Suspicious state from syntax checker python-flake8: Flycheck checker python-flake8 returned non-zero exit code 1, but its output contained no errors: Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib64/python3.6/runpy.py", line 14, in <module>
    import importlib.machinery # importlib first so we can test #15386 via -m
  File "/usr/lib64/python3.6/importlib/__init__.py", line 57, in <module>
    import types
  File "/usr/lib64/python3.6/types.py", line 171, in <module>
    import functools as _functools
  File "/usr/lib64/python3.6/functools.py", line 21, in <module>
    from collections import namedtuple
  File "/usr/lib64/python3.6/collections/__init__.py", line 27, in <module>
    from keyword import iskeyword as _iskeyword
  File "/home/momoka/projects/qianka/hebe/hebe/services/subtask/keyword.py", line 3, in <module>
    import logging
  File "/usr/lib64/python3.6/logging/__init__.py", line 26, in <module>
    import sys, os, time, io, traceback, warnings, weakref, collections
  File "/usr/lib64/python3.6/traceback.py", line 5, in <module>
    import linecache
  File "/usr/lib64/python3.6/linecache.py", line 11, in <module>
    import tokenize
  File "/usr/lib64/python3.6/tokenize.py", line 33, in <module>
    import re
  File "/usr/lib64/python3.6/re.py", line 314, in <module>
    @functools.lru_cache(_MAXCACHE)
AttributeError: module 'functools' has no attribute 'lru_cache'

Try installing a more recent version of python-flake8, and please open a bug report if the issue persists in the latest release.  Thanks!

由于报错message的buffer frame太小，一开始没注意到是Flycheck报错（主要是对Emacs还不够精通），花精力去研究是否是jedi报错，找偏了方向。

当回过神来仔细阅读错误信息，发现报错内容是指 src_shell{functools}里缺少 src_shell{lru_cache}属性，这个属性是从3.2开始加入，并且本地项目开发都用的3.6，就比较奇怪。

再多花点时间仔细阅读错误栈后发现，是因为 src_shell{collections}包里会用到 src_shell{keyword}库¹。这个标准库从未见过，看说明只是用来判断某字符串内容是否为Python关键字的。

经过一番查询后，想调试出Flycheck到底是跑了哪个命令才出现的报错，能够重现那就能办法修复。这时需要按下 src_shell{C-c ! C-c}，选择checker并运行。

-*- mode: compilation; default-directory: "~/projects/qianka/hebe/hebe/services/subtask/" -*-
Compilation started at Wed Jul 25 09:52:59

python -c import\ sys\,runpy\;sys.path.pop\(0\)\;runpy.run_module\(\"flake8\"\) --format\=default - < /home/momoka/projects/qianka/hebe/hebe/services/subtask/subtask_cache.py
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib64/python3.6/runpy.py", line 14, in <module>
    import importlib.machinery # importlib first so we can test #15386 via -m
  File "/usr/lib64/python3.6/importlib/__init__.py", line 57, in <module>
    import types
  File "/usr/lib64/python3.6/types.py", line 171, in <module>
    import functools as _functools
  File "/usr/lib64/python3.6/functools.py", line 21, in <module>
    from collections import namedtuple
  File "/usr/lib64/python3.6/collections/__init__.py", line 27, in <module>
    from keyword import iskeyword as _iskeyword
  File "/home/momoka/projects/qianka/hebe/hebe/services/subtask/keyword.py", line 3, in <module>
    import logging
  File "/usr/lib64/python3.6/logging/__init__.py", line 26, in <module>
    import sys, os, time, io, traceback, warnings, weakref, collections
  File "/usr/lib64/python3.6/traceback.py", line 5, in <module>
    import linecache
  File "/usr/lib64/python3.6/linecache.py", line 11, in <module>
    import tokenize
  File "/usr/lib64/python3.6/tokenize.py", line 33, in <module>
    import re
  File "/usr/lib64/python3.6/re.py", line 314, in <module>
    @functools.lru_cache(_MAXCACHE)
AttributeError: module 'functools' has no attribute 'lru_cache'

Compilation exited abnormally with code 1 at Wed Jul 25 09:52:59

在项目根目录下运行 src_shell{python -c …}那句命令时并未报错，但请留意第一行的运行路径，似乎Flycheck运行时会将工作路径设置为源代码文件所在目录。

而恰巧该项目的同一个目录下便有一个源代码文件 src_shell{keyword.py}，恐怕是由于Python的引入机制²引起的问题，导致循环依赖的发生。

既然定位症结，那么将Flycheck运行checker的路径改到项目根目录即可。Flycheck在某个版本中加入了当前工作路径的支持³⁴，这里可以参考haskell-stack-ghc的配置。

解决方案

直接对 src_shell{flycheck.el} 源代码进行修改不合适，由于需要跟着package升级，一旦升级后即会失效，基于同样的理由，自己fork一个版本保持维护也相对麻烦。所以考虑直接定制一个新的checker⁵。从 src_shell{flycheck.el} 中将原本 src_shell{python-flake8} 抄出来一份，放在 src_shell{~/.emacs.d/my-flycheck.el} 中，并命名为 src_shell{my-python-flake8} 。

;; package -- Summary
;; my-flycheck.el
;;

;;; Commentary:

;;; Code:

(defun flycheck-python--find-default-directory (checker)
  "... `CHECKER`."
     (or
      (when (buffer-file-name)
        (flycheck--locate-dominating-file-matching
         (file-name-directory (buffer-file-name))
         "local\\'"))
      (when (buffer-file-name)
        (flycheck--locate-dominating-file-matching
         (file-name-directory (buffer-file-name))
         "requirements.txt\\'"))))

(flycheck-define-checker my-python-flake8
  "A Python syntax and style checker using Flake8.

Requires Flake8 3.0 or newer. See URL
`https://flake8.readthedocs.io/'."
  ;; Not calling flake8 directly makes it easier to switch between different
  ;; Python versions; see https://github.com/flycheck/flycheck/issues/1055.
  :command ("python"
            (eval (flycheck-python-module-args 'python-flake8 "flake8"))
            "--format=default"
            (config-file "--config" flycheck-flake8rc)
            (option "--max-complexity" flycheck-flake8-maximum-complexity nil
                    flycheck-option-int)
            (option "--max-line-length" flycheck-flake8-maximum-line-length nil
                    flycheck-option-int)
            "-")
  :standard-input t
  :error-filter (lambda (errors)
                  (let ((errors (flycheck-sanitize-errors errors)))
                    (seq-map #'flycheck-flake8-fix-error-level errors)))
  :error-patterns
  ((warning line-start
            "stdin:" line ":" (optional column ":") " "
            (id (one-or-more (any alpha)) (one-or-more digit)) " "
            (message (one-or-more not-newline))
            line-end))
  :enabled (lambda ()
             (or (not (flycheck-python-needs-module-p 'python-flake8))
                 (flycheck-python-find-module 'python-flake8 "flake8")))
  :verify (lambda (_) (flycheck-python-verify-module 'python-flake8 "flake8"))
  :modes python-mode
  :working-directory flycheck-python--find-default-directory)

(provide 'my-flycheck)
;;; my-flycheck.el ends here

请留意最后设置的 src_shell{:working-directory} 属性，调用了自定义的方法，该方法的含义是指依次向上寻找当前编辑源代码所在目录的父级，是否有含有 src_shell{local} （个人习惯用这个名字建立virtualenv）或者 src_shell{requirements.txt} 的，即表示为项目根目录，作为运行checker的目录。同时flychecker很智能的会将后续的参数自动修改为基于这个目录起始的相对路径。

Flycheck提供了一个变量设置 src_shell{flycheck-checker} 来指定当前使用哪个 checker 。接下来的问题是何时启用该 checker 呢，所有项目均启用显然也不合适。这时就想到了项目级或者目录级的变量设置⁶。

在需要启用定制checker的项目根目录下写入新文件 src_shell{.dir-locals.el} ，内容如下

((python-mode . ((flycheck-checker . my-python-flake8))))

该文件的含义是，表达式第一个参数表示需要在哪些模式下开启，这里仅需要 src_shel{python-mode} 即可，全部启用可以用 src_shell{nil} ，后面参数为设置变量内容。

当打开该目录下的文件时，Emacs会提示检测到目录级本地的变量设置unsafe，会提示是否接受，说明已经起效。

这里可以按以下步骤验证配置已经功能是否已经正常。

运行命令（ src_shell{M-x} ） src_shell{describe-variable} 或者按 src_shell_{C-h v} 再输入 src_shell{flycheck-checker} ，查看设置的checker。

再次打开源代码文件后按下 src_shel{C-c ! C-c} 查看输出。

-*- mode: compilation; default-directory: "~/projects/qianka/hebe/" -*-
Compilation started at Wed Jul 25 10:29:05

python -c import\ sys\,runpy\;sys.path.pop\(0\)\;runpy.run_module\(\"flake8\"\) --format\=default - < /home/momoka/projects/qianka/hebe/hebe/services/subtask/subtask_cache.py
stdin:43:9: F841 local variable '_' is assigned to but never used
stdin:135:1: E302 expected 2 blank lines, found 1
stdin:256:5: F841 local variable '_' is assigned to but never used
stdin:346:5: F841 local variable '_' is assigned to but never used

Compilation exited abnormally with code 1 at Wed Jul 25 10:29:05