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import sys
import os

if __name__ == "__main__": # When the script is run
	if len( sys.argv ) == 1: # Check if it was run with an argument
		dir = input("Folder?\t") # If no, ask for user input
	else:
		dir = sys.argv[1] # Take first argument of the command

if os.path.isdir( dir ) == False: # Does the folder exists?
	sys.exit("Folder doesn't exist.") # then kill the script

# A valid folder is passed as argument
print("The folder exist!") # Your action