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(use-package fill-column-indicator
  :ensure t
  :config
  (setq fci-rule-color "LightSlateBlue")
  (setq fci-handle-truncate-lines t)
  (add-hook 'python-mode-hook (lambda ()
                                (setq-default fci-rule-column 79)
                                (fci-mode t)))
  (add-hook 'c-mode-hook (lambda ()
                           (setq-default fci-rule-column 80)
                           (fci-mode t))))

(use-package fill-column-indicator
  :ensure t
  :config
  (setq fci-rule-color "LightSlateBlue")
  (setq fci-handle-truncate-lines t)
  (defun my/fci-config (mode num)
    (add-hook mode (lambda ()
                     (progn
                       (setq-default fci-rule-column num)
                       (fci-mode t)))))
  (let (mode-config-hash)
    (setq mode-config-hash (make-hash-table :test 'equal))
    (puthash 'python-mode-hook 79 mode-config-hash)
    (puthash 'c-mode-hook 80 mode-config-hash)
    (puthash 'cperl-mode-hook 80 mode-config-hash)
    (maphash (lambda (k v) (my/fci-config k v)) mode-config-hash)))

(use-package fill-column-indicator
  :ensure t
  :config
  (setq fci-rule-color "LightSlateBlue")
  (setq fci-handle-truncate-lines t)
  (add-hook 'python-mode-hook (lambda ()
                                (setq-default fci-rule-column 79)
                                (fci-mode t)))
  (add-hook 'c-mode-hook (lambda ()
                           (setq-default fci-rule-column 80)
                           (fci-mode t))))

(use-package fill-column-indicator
  :ensure t
  :config
  (setq fci-rule-color "LightSlateBlue")
  (setq fci-handle-truncate-lines t)
  (defun my/fci-config (mode num)
    (add-hook mode (lambda ()
                     (progn
                       (setq-default fci-rule-column num)
                       (fci-mode t)))))
  (let (mode-config-hash)
    (setq mode-config-hash (make-hash-table :test 'equal))
    (puthash 'python-mode-hook 79 mode-config-hash)
    (puthash 'c-mode-hook 80 mode-config-hash)
    (puthash 'cperl-mode-hook 80 mode-config-hash)
    (maphash (lambda (k v) (my/fci-config k v)) mode-config-hash)))

But the above did not work. I try some debugging and realized that the line:

did not ran correctly. If I change it to a integer value:

But the code above did not work. I opened a Python or C file and the fci-mode was not enabled.

I try some debugging and realized that the line:

did not ran correctly. If I change it to an integer value: