code

while True:  # endless loop
    print("Please enter index of column: 0 or 1 or 2")
    print("followed by index of row: 0 or 1 or 2")
    print("like for example: 0 1")
    command = input("and press ENTER: > ")
    command = command.strip()  # remove leading and trailing spaces
    column_string = command[0]  # the first char
    row_string = command[-1]  # the last (!) char
    if column_string in ["0", "1", "2"] and row_string in ["0", "1", "2"]:
        break
    print("Wrong input. Please try again \n")  # make an extra new line at the end!
print("column:", column_string, "row:", row_string)

diff

This code is a “fresh start” and did not evolved from previous code, so a diff is not meaningful here

code discussion

• line 2 - 3: Ideally the line / column numbering would be human-friendly: first line has number 1, not 0. And columns could be named “A B C” instead of numbers. I will come back to this later in step005, for now the line / column numbering remains programmer-friendly instead user-friendly. Remember, in python, the indexing starts always with 0, now with 1.
• line 6: This code allows the player to type any (or None at all) blanks or other chars between the column and the row index. Also leading and trailing spaces are removed by the .strip() command. Lowercase / Uppercase input should be no problem as the script accepts numbers only, but python does provide a .lower() and .upper() functions as well.
• line 10: The single 'break' command breaks out of the current loop. The python code continues below the while loop, at line 12 (because line 12 has the same indentation as line 1)
• line 11: A print() command ends per default with a new line. By including \n at the end of the string, we get a new line AND an extra new line.

output

Please enter index of column: 0 or 1 or 2
followed by index of row: 0 or 1 or 2
like for example: 0 1
and press ENTER: > x2
Wrong input. Please try again

Please enter index of column: 0 or 1 or 2
followed by index of row: 0 or 1 or 2
like for example: 0 1
and press ENTER: > 12
column: 1 row: 2

Please enter index of column: 0 or 1 or 2
followed by index of row: 0 or 1 or 2
like for example: 0 1
and press ENTER: > 0,0
column: 0 row: 0

Please enter index of column: 0 or 1 or 2
followed by index of row: 0 or 1 or 2
like for example: 0 1
and press ENTER: > 2   0
column: 2 row: 0

code

cells = [["x", "x", " "],
         [" ", "x", "o"],
         ["o", " ", "o"],
         ]


def is_free(row, column):   # function with two mandatory arguments
    """checks a single coordinate in the cells array
    returns True if the cell is free (contains a Space)
    otherwise returns False"""
    try:
        content = cells[row][column]
    except IndexError:
        return "this cell does not exist"
    except:
        return "not even a legal index"
    # slow but readable
    if content == " ":
        return True
    return False
    # faster but harder to read:
    # return True if content == " " else False

# ---- testing ------
print("0,0:", is_free(0, 0))
print("2,1:", is_free(2, 1))
print("5,6:", is_free(5, 6))
print("x,0:", is_free("x", 0))

diff

This code is a 'fresh start' and did not evolved from a previous version, so a diff would not be meaningful here

code discussion

• line 11: When working with arrays, one of the most common errors is adressing an element that simply does not exist, usually because the array is too small. Here the critical line 11 is embedded inside a try: block. Python is then trying to execute the line. When an error occurs, the code below the corresponding except block is excuted. Otherwise, the program continues after the try/except block. As the most common error with arrays is an IndexError, this error get it's own specific error handler (line 13). If it is any other error, the the more general except: block in line 15 is executed.
• line 13: This error handler is only responsible if an IndexError occurs
• line 15: except: catches every error that is not already handled by previous exceptions.
• line 24: testing a function: There is a lot to learn about writing tests for a function. The method above is sometimes called “Gutenberg”-testing because it involves print(). Ideally, you write test even before writing the function, meaning you think about what output the function should return on different kinds of input. Then, you write the function it self, and then you test it whenever the code changes. More on that topic later, or take a look now into Doctest and Unittest.

output

0,0: False
2,1: True
5,6: this cell does not exist
x,0: not even a legal index

code

"""tic tac toe for 2 players, without win checking"""

# ----- defnine some top-level variables ------
cells = [[" ", " ", " "],
         [" ", " ", " "],
         [" ", " ", " "],
         ]
# ore more elegant:
# cells = [[" " for x in range(3)] for y in range(3)]
symbols = ("x", "o")    # tuples are read-only
greeting = "This is turn {}. Player {}, where do you put your '{}'?"
text = "Please enter numbers (0 or 1 or 2) for column and row\n"
text += "   like for example: '1 2' or '02' or '2x1'\n"
text += "   and press ENTER >>> "


# ---- functions ----
def is_free(row, column):
    """checks a single coordinate in the the cells array
    returns True if the cell is free (contains a Space)
    otherwise returns False"""
    try:
        content = cells[row][column]
    except IndexError:
        print("this cell does not exist")
        return False
    return True if content == " " else False


def display():
    """displays the 3x3 array 'cells' with heading row and column"""
    print(r"r\c 0:  1:  2:")  # header line. r\c is not an escape sequence, leading r -> raw string
    for index, row in enumerate(cells):
        print("{}: ".format(index), end="")  # no new line at end of print
        for element in row:
            print("[{}] ".format(element), end="")  # no new line at end of print
        print()  # print only a new line
    print()  # empty line after board


for turns in range(9):  # play 9 legal moves, then the board is full
    display()
    player = turns % 2  # modulo: the remainder of a division by 2.
    print(greeting.format(turns, player, symbols[player]))
    while True:  # ask until legal move
        command = input(text)
        command = command.strip()
        if len(command) < 2:
            print("Enter 2 coordinates. Try again!")
            continue
        raw_column, raw_row = command[0], command[-1]  # 2 variables, 2 values
        try:
            row, column = int(raw_row), int(raw_column)  # 2 variables, 2 values and only one line inside try: block
        except ValueError:
            print("Enter numbers only. Try again")
            continue  # go back to the start of the while loop
        if is_free(row, column):
            print("input accepted\n")  # extra new line
            cells[row][column] = symbols[player]
            break  # breaks out of this while loop
    # print("*** next turn! *****")
print("all fields are full. Game Over")

diff

code discussion

• line 1: docstring
• line 3: declaring variables at top-level. It is not necessary to declare variables before declaring functions. It is only necessary that when the function is called, all necessary top-level variables for this function must be declared. However it is good practice to declare top-level variables, (especially constants) first.
• line 4: Creating a 2d-array and assigning it to the variable cells. The 2d-array is created as a nested list (a list with list's inside. Inside each inner list are three empty strings.
• line 4-7: Because the brackets, the nested list spans over several physical lines. For python, line 4-7 is one single line.
• line 9: see step002 for how to create a nested list using list comrehensions and the range() function in a single line of code. This line is out-commented now but will later replace lines 4-7.
• line 10: the variable symbols is a tuple (because using the round brackets ()) and not a list (with square brackets []). The difference of using a tuple instead of a list: tuples are read-only or immutable, wile list's are mutable. Meaning it is later possible to replace an item in a list (like the topleft element in cells) but it is not possible to replace just the o element in symbols. In short:

  cells[0][0] = "y"

  is possible. But

  symbols[0] = "E"

  is not possible, because symbol is defined as a tuple. In practice, you make your python program a tiny bit less slow / memory-hungry when using tuples instead of lists. The main advantage is that you make your intentions clear while coding.
• line 11: Notice that the string greeting has empty placeholders in it {} but no trailing .format(). The .format() will be attached later, in line 44
• line 12-14: instead of creating a multi-line in triple-quotes, the variable text is here first defined and then modified by the + operator. Notice the \\n linebreak inside the string!
• line 18: scope of variables: The is_free() function needs row and column as arguments, but not the cells array! is_free() can still access the cells variable because cells was defined at top-level.
• line 24: The is_free() function only checks for IndexError (cell outside the array) and not for general errors with an except nor for ValueError, like getting the row and column argument as strings instead of integers. In this code example, the code calling the is_free() function is responsible for calling it only with the correct integer arguments. Notice that this fact (both arguments are of type integer) is not documented in the docstring of the function nor anywhere else. Generally, trusting the fact that some other part of code will never call a function with the wrong arguments is a sign of over-confidence. Especially when both code parts are written by yourself! See type-hinting
• line 30: the function display() display needs no arguments at all but is able to display the cells array, because cells was declared at top-level.
• line 33: enumerate() returns two values, and those are assigned to the two variables index and row.
• line 35: row itself is a list (of strings) and can be iterated using a for loop.
• line 41: range(9) creates an iterable (think of it as a list) with those 9 elements in it: [0,1,2,3,4,5,6,7,8]]. The game board has 9 fields only.
• line 42: function call: because this line is inside the for loop, the function display() will be called 9 times.
• line 43: Modulo-Operator, (%) calculates the remainder of a division. See the next headline below. The effect is that the value of player changes between 0 and 1
• line 46: Whatever the user enters by input() will be stored in the variable command. While the userinput will hopefully be 2 numbers, input() stores everything as string
• line 47: the built-in string method .strip() can be attached to any string variable and is very useful when handling player input. .strip() removes leading and trailing spaces and other invisible characters.
• line 48: another built-in python method: len() returns the number of elements in an object. If the object is a string, it returns the number of characters inside this string. This line makes sure the user entered at least 2 values (remember, leading and trailing spaces were already removed by .strip() in line 47.
• line 50: flow control: The continue statement works only inside a loop and goes back to the beginning of the loop. In our case, this is the while loop in line 42. Also see break
• line 51: flow control: Notice that the if statement in line 48 does not need an else branch. If the program flow reaches line 51, the program has made sure already that the command string consist of two values, otherwise the continue statement in line 50 would have lead the program flow back to the beginning of the while loop in line 45. command is now split into two variables: the first char of the user input (with index 0) is assigned to the variable raw_column, the last char of the user input (index -1) is assigned to the variable raw_row. Please note that the last char is not necessary the second char! Thanks to the .strip(), we know that the first and last char is not a whitespace.
• line 53: The userinput is two values, but what kind of values? For sure, they are until now of type string. By using int() twice, this line tries to convert the userinput from type string into type integer. This type-converting is also called casting. It may go wrong, for example because the user entered One, Two instead of 1,2. Good thing we are inside a try block:
• line 54: When the casting in line 50 goes wrong, python throws an ValueError, and this Error is handled here.
• line 57: If no ValueError was raised then the code continues here. Note that there is no general except statement to catch all other (Non-Value) Errors. Because I am very sure that no possible other error can occur. This is a classical case of hubris and should be avoided: Never underestimate the creativity of users! Especially when asking them for user input! Anyway, what does this line: it is the same as if is_free(row, column) == True. It's a function call to the is_free() function. The function is_free() returns a boolean value (True or False). This return value is then used in the if statement. Notice that it is a good idea to create meaningful names for functions. A function named free_cell() for example does not make it so clear what kind of return values are to be expected and what they mean. When you name your functions is_anything it is more clear that you expect a boolean return value when anything becomes True.
• line 60: The break command breaks out of the actual (for or while) loop. Unlike other programming languages, python has no command to break out to a specific loop if you are inside nested loops.
• line 61: This line is out-commented but should help you to read the code better: The indentation shows that you are still inside the while loop.

output

r\c 0:  1:  2:
0: [ ] [ ] [ ] 
1: [ ] [ ] [ ] 
2: [ ] [ ] [ ] 

This is turn 0. Player 0, where do you put your 'x'?
Please enter numbers (0 or 1 or 2) for column and row
   like for example: '1 2' or '02' or '2x1'
   and press ENTER >>> 2 2
input accepted

r\c 0:  1:  2:
0: [ ] [ ] [ ] 
1: [ ] [ ] [ ] 
2: [ ] [ ] [x] 

This is turn 1. Player 1, where do you put your 'o'?
Please enter numbers (0 or 1 or 2) for column and row
   like for example: '1 2' or '02' or '2x1'
   and press ENTER >>> 01
input accepted

r\c 0:  1:  2:
0: [ ] [ ] [ ] 
1: [o] [ ] [ ] 
2: [ ] [ ] [x] 

This is turn 2. Player 0, where do you put your 'x'?
Please enter numbers (0 or 1 or 2) for column and row
   like for example: '1 2' or '02' or '2x1'
   and press ENTER >>> 4
Enter 2 coordinates. Try again!
Please enter numbers (0 or 1 or 2) for column and row
   like for example: '1 2' or '02' or '2x1'
   and press ENTER >>> 44
this cell does not exist
Please enter numbers (0 or 1 or 2) for column and row
   like for example: '1 2' or '02' or '2x1'
   and press ENTER >>> 0x2
input accepted

r\c 0:  1:  2:
0: [ ] [ ] [ ] 
1: [o] [ ] [ ] 
2: [x] [ ] [x] 

This is turn 3. Player 1, where do you put your 'o'?