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Snake!

You've played Snake before, right? Let's learn how to make Snake for ourselves!

Setup

First, make sure you've followed the instructions on how to install Python. Second, go to [https://github.com/ruggeri/snake-game/downloads][2] to download the zip file containing the project materials. Extract the zip. [2]: https://github.com/ruggeri/snake3

Next, in Terminal.app, run the configure script to install the Tornado web server:

~/projects/snake3$ ./configure
Creating /Users/edwardruggeri/projects/snake3/lib/site.py
Searching for tornado
Reading http://pypi.python.org/simple/tornado/
Reading http://www.tornadoweb.org/
Best match: tornado 2.2.1
...

Now go ahead and launch the snake program with the run script:

~/projects/snake3$ ./run
Server started.

Open snake.html in Chrome; you should see just the message "HELLO!" in the middle of the screen. That means everything is working!

Finally, open snake.py in your editor; this is where we'll write your Python code! Here's what it looks like now:

from canvas import *
from snake_lib import *

#### YOUR CODE GOES HERE:

def move(direction):
    hello_list = ['H', 'E', 'L', 'L', 'O', '!']
    
    x = 5
    y = 5
    for letter in hello_list:
        put_char((x,y), letter)
        x = x + 1

First steps

The file we'll work the most with is snake.py. Take a look at the bit after def move(direction):. This part prints "HELLO!". Let's look at this line:

hello_list = ['H', 'E', 'L', 'L', 'O', '!']

To the right of the = is a list of items. We can make lots of different lists in Python:

[0, 1, 2, 3]
['John', 'Paul', 'George', 'Ringo']

Lists start with a [ and end with a ]. Then we list each item, separated by commas.

The hello_list part to the left of the = is a variable name. By assigning a value to a variable, we can use that name later on. So

beatles = ['John', 'Paul', 'George', 'Ringo']

lets us use the name beatles to mean ['John', ...] later on.

Now you try! Open up IDLE or the Python interpreter and make a list of your friends. Assign the list to the variable friends.

Printing and for loops

Let's say hello to your friends! Here's how we could say hello to a single friend:

>>> print("Hey Joe!")
Hey Joe!

print is called a function. Functions do work for us; the part in parentheses is what a function works on. print takes "Hey Joe!" and prints it out.

Go ahead and print some messages in IDLE or the python interpreter!

Okay, let's see how to say hello to each of my cats.

>>> cats = ["Earl", "Breakfast", "Gizmo", "Kiki"]
>>> for cat in cats:
...     print("Hello " + cat + "!")
...
Hello Earl!
Hello Breakfast!
Hello Gizmo!
Hello Kiki!

What all is this? The first line defines the variable cats and sets it equal to my list of cats. Next comes for cat in cats:; this tells Python to loop through each item in cats, each time assigning the value to a new variable cat. The last part, print("Hello " + cat + "!") is called the body. We run the commands in the body for each cat in the list. Notice that cat is different every time; it's set to each of my cats in the order of the list.

Okay! Now you try! Use a for loop in IDLE to greet some of your friends!

Back to hello_list

Let's take another look at the code in snake.py.

def move(direction):
    hello_list = ['H', 'E', 'L', 'L', 'O', '!']
    
    x = 5
    y = 5
    for letter in hello_list:
        put_char((x,y), letter)
        x = x + 1

The key here is the function put_char. This prints a letter, or character at a specific location. The location is a point in the grid. Here's an example of a grid:

 |0|1|2|3|4|5
0| | | | | | 
-------------
1| | | | | | 
-------------
2| | | | | |Q
-------------
3| | | | | | 
-------------
4| | |P| | | 
-------------
5| | | |R| |

This is a five-by-five grid; I've marked three points: P, Q, and R. P is two columns over and four rows down; we say that it is at position (2, 4). Q is 5 columns over and 2 rows down; we say its position is (5, 2). What position is R at?

We can tell our program where to draw letters by giving put_char a position and a letter; let's look closely at

 put_char((x,y), letter)

The first bit, (x,y) is the position; x and y are variables which will represent how many columns to go over and how many rows to go down. letter is also a variable; it's the letter to print.

Try swapping (y,x) for (x,y); what do you think will happen?

The last part of our for loop's body is x = x + 1. Why don't you try adding the line y = y + 1; what do you think will happen?

Go ahead and change the list so your own name is printed!

Drawing the border

Let's draw a border for our Snake game along each of the edges. Here, I'll draw one side:

def move(direction):
    for y in range(HEIGHT):
        put_char((0, y), "W")

I'm putting a "W" for "Wall". Here's a couple new things. HEIGHT is a variable with the number of rows; there's another variable you can use named WIDTH for the number of columns. You can find where HEIGHT and WIDTH are defined in canvas.py.

What is this range guy? He's another function, and you can see range is being given the HEIGHT. range(HEIGHT) returns a list of numbers up to HEIGHT:

>>> HEIGHT
30
>>> range(HEIGHT)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]

When we write for y in range(HEIGHT):, we're saying that we should run the body 30 times, for y = 0, y = 1, y = 2, for each of the heights.

Go ahead and add loops to print the other three borders!

Defining functions and blinking text

So far we've ignored the line def move(direction):. This is defines our own function, move; so far we've been editing the body. move takes a single option, direction, which we haven't used yet.

Our program consists of a big loop that looks like this:

  1. Read some keys from the keyboard
  2. Decide which direction to move the snake in
  3. Clear the grid
  4. Call our move function, giving us the direction to move
  5. Inside move, we use put_char to add characters to the grid
  6. Print the grid out
  7. Pause a few moments
  8. Repeat from step 1

Let's see this in action. Let's print some random "X"s to the screen. Here's how to do it:

def move(direction):
  x = randint(0, WIDTH)
  y = randint(0, HEIGHT)
  put_char((x,y), "X")

randint is another function; randit(0, WIDTH) chooses a random number between 0 and WIDTH. So we choose two random numbers, and put an "X" at that point.

Go ahead and try out the change! See how the "X" jumps around? That's because each time move is run, we draw a new random "X".

Now you try! Use a for loop to draw 100 random "X"s!

State

THIS PART IS A WORK IN PROGRESS

We use if and else for the first time here. The first part of the if runs only if BLINK is True, otherwise, the else part is run.

How does this work? BLINK starts out as True, so we first enter the if part. This part sets BLINK to false for the next time around, and prints "HELLO". The second time around, BLINK is False, so we run the else: part. This doesn't print anything, but it does set BLINK back to True. We then repeat the process.

You can ignore the line global BLINK. We have to write this because BLINK is first defined outside of the function move. Python complains if we leave it out.

Time for a challenge problem! Instead of blinking the whole word "Hello!", try printing a different letter each time move is called. Instead of setting BLINK to True or False, set it to the position of the letter to print in the list. Each time print the next letter.

You'll need to be able to get an item from a list. Here's how to do it:

>>> letters = ['a', 'b', 'c']
>>> letters[0]
'a'
>>> letters[1]
'b'
>>> letters[2]
'c'