How can I assign keyboard shortcut for nautilus scripts?

How it can be done

When you right- click a file or folder for a nautilus script, the selected file is passed as an argument to the script. In most cases by something like:

import os
subject = os.getenv("NAUTILUS_SCRIPT_CURRENT_URI")

...using python3, in its simplest form.

If you replace this by:

import pyperclip

subprocess.call(["xdotool", "key", "Control_L+c"])
subject = pyperclip.paste()

...the currently selected file is used inside the script as an argument

What you need

To use this solution (16.04 and up), you need to install both xdotooland python3-pyperclip:

sudo apt-get install python3-pyperclip xdotool

The complete script, mentioned in comments

then becomes:

#!/usr/bin/env python3
import subprocess
import os
import sys
import pyperclip

# --- set the list of valid extensions below (lowercase)
# --- use quotes, *don't* include the dot!
ext = ["jpg", "jpeg", "png", "gif", "icns", "ico"]
# --- set the list of preferred filenames
# --- use quotes
specs = ["folder.png", "cover.png", "monkey.png"]
# ---

# retrieve the path of the targeted folder
subprocess.call(["xdotool", "key", "Control_L+c"])
dr = pyperclip.paste()

for root, dirs, files in os.walk(dr):
    for directory in dirs:
        folder = os.path.join(root, directory)
        fls = os.listdir(folder)
        try:
            first = [p for p in fls if p in specs]
            first = first[0] if first else min(
                p for p in fls if p.split(".")[-1].lower() in ext
                )
        except ValueError:
            pass
        else:
            subprocess.Popen([
                "gvfs-set-attribute", "-t", "string",
                os.path.abspath(folder), "metadata::custom-icon",
                "file://"+os.path.abspath(os.path.join(folder, first))
                ])

Adding this to a shortcut key will set the icons for all directories inside the selected one.

Adding it to a shortcut key (!)

Adding shortcut keys, running (scripts using-) xdotool commands to press another key combination can be tricky. To prevent both key combinations to interfere with each other, use:

/bin/bash -c "sleep 1 && python3 /path/to/script.py"

Explanation

When Ctrl+C is pressed while a file is selected, the path to the file is copied to the clipboard. We are simulating the key press with:

subprocess.call(["xdotool", "key", "Control_L+c"])

python's pyperclip module simply produces the path, stripped from file:// when using pyperclip.paste() (this will not literally paste, but make the path available inside the script).