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Some Reserved Keywords

    You have already seen various reserved keywords, for example, and, or, not, is, in, for, while, if, elif, else, break, continue, del and import. Reserved keywords and built-in functions are core (function­alities) of Python programming. Many modules, such as math, operator, random, turtle, tkinter, etc. are already installed when you have installed the Python software, but these modules are not automatically available for operations or usage.

    This separation of different modules from core Python application is done for optimization of your scripts (codes written in yourFileName.py file) and avoiding unnecessary inclusion of other modules. To know more about any module, you have to first import that module, and thereafter, you can type dir( name_of_module )   or print( dir( name_of_module ) )   either in Python CLI >>> or execute scripts (saved in yourFileName.py file) from Command Prompt/Terminal of your operating system. Also, you can get help documentation of any method/function of that module by typing help( name_of_module.name_of_method ).

Example 1

import keyword

print( dir(keyword) )   # To know how many, and which methods are available in a module. Output is shown below.

print()   # It creates a new line between above and below statements.
print( keyword.kwlist, '\n', type(keyword.kwlist), '\n')   # It shows all reserved keywords in a list data-type. Output is shown below.

print( keyword.softkwlist, '\n', type(keyword.softkwlist), '\n' )   # It shows keywords in context in a list data-type. Output is shown below.

print( dir(keyword.kwlist) )   # It does not show list of keywords, but all methods of the output data-type, that is, list.

    When you have installed the Python application, as instructions provided in previous page, you have checked on ☑ Documentation option. Thus, you can find the python3109.chm file (for Python 3.10.9) or python3913.chm file (for Python 3.9.13) or html folder (for Python 3.11.5) in the doc folder (inside the folder where you have installed the Python software). You should also visit official https://doc.python.org web-page for documentation of desired Python versions.

    I have installed at least three versions of Python software in my PC, that is, Python 3.9.13, Python 3.10.11, and Python 3.11.5. You may have to install different versions of Python in your remote servers/PC or also in your local computer, when you will learn Cloud Computing, Machine Learning, or deploy your sofisticated WEB-APPLICATION on an INSTANCE (a remote OS installed by you on different Cloud Service Providers).

    Since reserved keywords are core of Python software, you can see in above result that only kwlist and softkwlist are available methods. Also, iskeyword and issoftkeyword methods are available to check whether any string literal is a reserved/soft keyword or not. If you remove the pair of single quotes in following example, you will get SyntaxError: invalid syntax even on else keyword. match case keywords are introduced in Python 3.10, and it is same as switch statement in other programming languages.

Example 2

import keyword   # You should import a module only once in a file.
print( keyword.iskeyword('else') )        # Outputs True
print( keyword.iskeyword('match') )    # Outputs False

print( keyword.issoftkeyword('match') )   # Outputs True (introduced in Python 3.10, outputs False in previous versions.)

Example 4

import keyword
print( help( keyword.kwlist[34] ) )

match case

    Similarly, you can know about any of above reserved keywords. However, some new keywords, that is, match and case are introduced in Python 3.10. In the match case block, a variable name (assigned with number, string, boolean, or None) is placed after the match keyword, and it is checked with each case (option). Unlike if elif else block of codes, match case block checks the literal only once. Syntax of match case block is as follows:

Example 5

name = "user"
match name:
    case "user":
        print(f"Welcome {name}, you are in the class.")
    case "public":
        print("Sorry, you have to login first.")
    case "unknown":
        print("You have not registered to our class.")

A simple CLI App

    In the following example, two, four, six, eight, ten blank spaces are left in sequence for respective level of indentations. Do not use Tab key in your keyboard, when writing codes in Text Wranggler (in Mac OS) or Notepad (in MS Windows OS), or other simple text-editors. It may cause IndentationError or SyntaxError. A empty list is created for adding notes in your simple CLI app, and it is assigned to variable name, my_notes. Thereafter, an infinite while loop will ask every time, your action to be taken. You can also use string literal, as described in above example 5, instead of integer option to be matched.

    You should first type-cast the output of input() function to integer data-type, otherwise TypeError may be shown. You have to first check whether input data is numeric data-type or not (with isnumeric() string method). The user may press Enter or Return key without entering any number. This will assign the output an empty string to variable name user_action. Then, the int() built-in function will truncate the numeric data-type to integer, if the user inputs any floating (decimal) number. The continue statement under the (first) else will skip the while loop, and ask the user to enter a numeric value.

    Thereafter, the match block starts, and you should peruse each case option separate from other cases. In this way, you will find the following example, a very simple CLI app, which will be modified in next chapters. More built-in functions and some custom functions will be added, with the help of def keyword. The first two cases are very simple, and they add and show items in the my_notes list. The third and fourth cases also include NESTED for loop, each of which shows a number before each item. The user will input the number for desired item, and it will be edited or deleted from the my_notes list.

Example 6

my_notes = []
while True:
  user_action = input("Press 1 to add, 2 to show, 3 to edit, 4 to delete a note: ")
  if user_action.isnumeric():
    take_action = int(user_action)
  else:
    continue
  match take_action:
    case 1:
      add_note = input("Enter your note here (when complete, press Enter or Return key): \n")
      my_notes.append( add_note )
    case 2:
      print( "\n".join(my_notes) )
    case 3:
      if my_notes == []:
        print("Your notebook is empty. Please add something.")
      else:
        for index, item in enumerate(my_notes):
          print( index + 1, item )
        edit_num = int(input("Enter the number of item to be edited: "))
        my_notes[edit_num - 1] = input("Write something to edit: ")
    case 4:
      if my_notes == []:
        print("Your notebook is empty. Please add something.")
      else:
        for index, item in enumerate(my_notes):
          print( index + 1, item )
        del_num = int(input("Enter the number of item to be deleted: "))
        del my_notes[del_num - 1]
        print( "\n".join(my_notes) )   # Or, you can display the deleted item here!
    case _:
      break

    The _ (underscore) symbol is placed after the case keyword to select other (default) options, which are not mentioned in previous cases, and the user has opted them by mistake or deliberately. However, in the above example, since the string literals are already parsed by the first if block, the user can only input the numeric value for all options. When the user enters 5 or any number other than 1, 2, 3, 4 then the above program will be completed (or terminated).

    The above example is not the finished version of the simple CLI app, and you can see that when the user enters any number other than items available in my_notes list for editing or deleting it, ERROR will be shown. You should try to insert if else block to check whether the input provided by the user is numeric or not. Also, you should try to check whether the number provided by the user is not more than last index number of that list. You can do it with the help of len() built-in function.

    In next pages, you will learn to create custom functions, and save your notebook (that is, texts written by the user for each item of the my_notes list) in a file with .txt file. Hope, you have enjoyed the above example, and don't find it difficult. If you've any question regarding the same, you can mail me at help@ribhu.org.