completion-ignored-filenames?

Question

Is there a way to do ignore whole file names the same way as 'completion-ignored-extensions' ignores filename extensions?

For example, I would like to blacklist __pycache__/, so that if the default directory contains __init__.py and __pycache__/ (and these two are the only files whose names start with underscore), I can do C-x C-f _ TAB and get right to __init__.py as the only choice, without Emacs offering me __pycache__/ after me hitting TAB once more to reveal the second underscore. Is this possible?

Answer 1

Not too sure what the context of your question is. If you're using function read-file-name then you can pass it a PREDICATE argument that will exclude any files or dirs you want to exclude as completion candidates. C-h f read-file-name tells us:

Sixth arg PREDICATE, if non-nil, should be a function of one argument; then a file name is considered an acceptable completion alternative only if PREDICATE returns non-nil with the file name as its argument.

So a PREDICATE value of foo would exclude file __pycache__:

(defun foo (filename)
  "Return non-nil if FILENAME is not `__pycache__'."
  (setq filename  (directory-file-name filename))
  (not (string= filename "__pycache__")))

(read-file-name "File: " nil nil t nil 'foo)

The reason for using directory-file-name is so that this works for a FILENAME value that is a directory name, as well as a value that is an ordinary file name.

Answer 2

From the documentation of completion-ignored-extensions

It ignores directory names if they match any string in this list which
ends in a slash.

so you can simply add __pycache__/ to the variable

(add-to-list 'completion-ignored-extensions
             "__pycache__/")