80 — Tools: reading command-line arguments

So far every program asked for its input while running. A real tool reads its instructions up front: nme r dice 6 should roll a six-sided die without asking anything. Those words after the file name are command-line arguments — Python puts them in the list sys.argv.

#Steps

1. import sys gives you sys.argv: the program path plus every word typed after it. Save greet.nme:

   # greet.nme — greet whoever is named on the command line.
   # Run: nme r greet Mina
   
   import sys
   
   name = sys.argv[1]
   show f"hello {name}"
   

   copyrun it →

   nme r greet Mina
   

   copy

   hello Mina
   

   copy

   sys.argv[0] is the program's path and sys.argv[1] is the first real argument — the same list a normal python greet.py Mina command makes.

2. A missing argument must not crash confusingly. len(sys.argv) counts the words; check it before reading one. Save dice.nme:

   # dice.nme — roll a die with the number of sides you ask for.
   # Run: nme r dice 6
   
   use random latest
   
   import sys
   
   if len(sys.argv) < 2:
       show "usage: nme r dice <sides>"
   else:
       sides = int(sys.argv[1])
       show random_number(1, sides)
   

   copyrun it →

   nme r dice 6
   

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   3
   

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   int(sys.argv[1]) converts the word "6" into the number 6, as in guide 02; without the check, nme r dice would crash on an empty list. A "usage:" line that shows the right command is the friendly way to answer a missing argument.

3. Arguments make a command-line tool: one program, several commands. A todo list stores its items in todo.json (guide 14) and takes add, done, and list commands. Save todo.nme:

   # todo.nme — a todo tool that takes commands on the command line.
   # Run: nme r todo add "buy milk"
   #      nme r todo list
   #      nme r todo done 1
   
   use file latest
   
   import sys
   
   todo_file = "todo.json"
   
   def load_todos():
       try:
           return json_load(todo_file)
       except Exception:
           return []
   
   def save_todos(todos):
       json_save(todo_file, todos)
   
   def show_todos(todos):
       for i, item in enumerate(todos):
           mark = "x" if item["done"] else " "
           show f"{i + 1}. [{mark}] {item['text']}"
   
   command = sys.argv[1] if len(sys.argv) > 1 else "list"
   todos = load_todos()
   
   if command == "add":
       text = sys.argv[2] if len(sys.argv) > 2 else "no text"
       todos.append({"text": text, "done": False})
       save_todos(todos)
       show f"added: {text}"
   elif command == "done":
       number = int(sys.argv[2]) - 1
       todos[number]["done"] = True
       save_todos(todos)
       show "marked done"
   else:
       show_todos(todos)
   

   copyrun it →

   load_todos returns an empty list when the file does not exist yet (guide 68 explains try/except), and enumerate numbers the list starting at 1. Each item is a dict with a text and a done flag — the shape that json_save writes and json_load reads back.

4. Run the tool as a chain of commands:

   nme r todo add "buy milk"
   nme r todo add "learn argv"
   nme r todo done 1
   nme r todo
   

   copy

   added: buy milk
   added: learn argv
   marked done
   1. [x] buy milk
   2. [ ] learn argv
   

   copy

   The quotes around "buy milk" keep the two words together in one argument — without them, buy and milk would be sys.argv[2] and sys.argv[3]. The file todo.json is the tool's memory between runs.

#Try it yourself

Add a clear command that empties the list, or a rm command that removes one number (todos.pop(number)). Change the dice tool to accept a list of sides and roll each (for side in sys.argv[1:]). Then make a tool of your own: a unit converter (nme r convert 100 km to mi) is a good first one.

#What you learned

• sys.argv holds the program path and every word typed after the file.
• len(sys.argv) guards against missing arguments; a usage: line tells the user the right command.
• int(sys.argv[1]) turns an argument word into a number.
• A command word (add, done) plus data makes one program into many tools, like the mini bank's looped menu in guide 61 but given up front.
• Quotes group several words into a single argument.