Complete Roguelike Tutorial, using python3+pysdl2, part 0

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This tutorial is a draft, an unfinished tutorial. Feel free to talk and edit but do it knowing that much is yet to be done.

This is part of a series of tutorials; the main page can be found here.


Setting up Python+PySDL2



If you haven't already done so, download and install Python 3.5. Any version of Python 3.x up to 3.5.x should be fine, but its not guaranteed to work.

PySDL2 is currently not working with Python 3.6. This tutorial was written and tested using Windows 7 x64, Python 3.5.2 x86, PySDL2 0.9.5 and SDL2 x86 2.0.5. It is advisable to go with 32 bit for compatibility's sake.


The easiest way to install PySDL2 is using pip:

$ python -m pip install pysdl2

If you would like another form of installation you can look for it at PySDL2's installing instrunctions.


Download the latest release of SDL2 and extract it somewhere. Be warned that both Python and SDL2 must either be both 32 bit, or both 64 bit. If you get dll loading errors, getting this wrong is the most likely cause. The SDL2 should be added to your PATH environment variable or placed at the project's folder. Another option is to tell PySDL2 where the library is located. You can do that py adding those lines at the start of your main python file (explained below):

import os
os.environ["PYSDL2_DLL_PATH"] = "C:\\lib\\SDL2-2.0.5-win32-x86"

Choice of code editor

If you're just starting out with Python, you'll find that many Python coders just use a simple editor and run their scripts from a console to see any debugging output. Most Python coders don't feel the need to use a fancy IDE! On Windows, Notepad++ is an excellent bet; most Linux programmers already have an editor of choice. Almost all editors allow you to configure shortcut keys (like F5 for instance) to quickly run the script you're editing, without having to switch to a console.

See this page if you want to set up a Notepad++ shortcut with a couple of nice features for debugging, or if you tried to roll your own and hit the infamous "module not found" error.

Personally I'm using Sublime Text 3 with the installed packages: Jedi - Python autocompletion; Python Flake8 Lint; Python Improved. Coloring, highlighting, linting, extending it pretty much however you want, etc. makes it like work like a fancy IDE - but lighter. It runs on my toaster! :)

Setting up your project

Your project folder

Now create your project's folder. Inside it, create two empty files and It'll make the tutorial easier to just use the same names for now, and you can always rename it later.


If you chose to keep the SDL2 library at the project folder, it should now look like this:

  +-SDL2.dll (.dll for Windows, .so for Linux).

We will omit the sdl library and txt from now on when we list the folder's content. If you have it, just remember that you will have those two additional files on top of what is shown.

You're ready to start editing stuff!

Defining constants

It's good practice to define constants, special numbers that might get reused. Constants are usually defined on a module level and written in all capital letters with underscores separating words, according to Python's style guide - its not required, but it should make your code more readable to other people, so we're sticking to this style. Let's create a file named at our project's folder and write on it:

"""Game constants."""
# size of a (square) tile's side in pixels.
# the width of the screen in pixels.
# the height of the screen in pixels
# maximum frames per second that should be drawn
# the window's background color (RGBA, from 0-255)
WINDOW_COLOR = (0, 0, 0, 255)

Now that we have our contants defined is time to create our scene manager!

Creating a scene Manager

Wait, manager? Ain't we making a game? SDL2 is a C library. PySDL2 is a python wrapper for that library. But remember we've said at the introduction that SDL2 is not a game engine? Neither is PySDL2, although it does provide higher level classes and methods to help us. We're going to create some classes to make our lives easier, more like a python game engine, less like a bunch of C methods. It will take some time until we can finally draw our character to the screen, but it will save us lots of re-work in the future. If you don't care about the implementation of the Manager and related classes, our boilerplate code, you can just download the 0.x release (part 0 of the tutorial = 0.x releases) of the project on GitHub and skip to the Part 1. The code should be reasonably well described, with lots of docstrings and comments (feel free file an issue on GitHub if something is not described well enough) so that you may be able to understand it all just by looking at (actually, reading) it. And you can always come back here if, on the later stages, you feel like you need to understand what's going on in that But even before we deal with the Manager, we're going to work on a Clock, the class that will control our frame rate among time. Something that the Manager itself will depend on.

Tick the Clock

Pygame, a python library based on SDL version 1.x, had a Clock. There is another library, based on Pygame, that is built around SDL2, named pygame-sdl2, that has a Clock, but it's made using cython, not python (this could actually be considered good, considering performance, but at this tutorial we're aiming at pure Python, because one language is enough for a tutorial). We're not reinventing the wheel, but we're adapting that cython Clock to a python one. We're not going to dive deeper into this process, just know that this is a pure Python port of that. You should download the ported version we're going to use from here. Make sure its placed under util/ in your project's folder.

  • Note*: pygame-sdl2's code is released under zlib license. That means you can do almost everything you want with it, but it remains a copyrighted work. That being said, you can use it, even commercially, but we're not going to place the code here. Just get it through git and use it.

Now we're going to work on

Creating a scene Manager

Firt of all we're going to need a few imports:

# ctypes will be required for a single use at startup, don't let it scare you!
import ctypes
import os
# tell sdl2 where your library is
os.environ["PYSDL2_DLL_PATH"] = "C:\\lib\\SDL2-2.0.5-win32-x86"
# import sdl2
import sdl2
# and sdl2.ext, where the pythonic part of the pysdl2 resides
import sdl2.ext
# import the constants we've defined
# impor our pythonic Clock
from util.time import Clock

Next we're going to instantiate sdl2.ext.Resources to help us handling our resources:

Resources = sdl2.ext.Resources(
    os.path.join(os.path.dirname(__file__), "resources"))

Then we're going to create the Manager class. As the first lines of its initialization we're going to unpack some arguments related to the constants we've defined. The description should make it clear enough:

class Manager(object):
    """Manage scenes and the main game loop.
    At each loop the events are passed down to the active scene and it's
    update method is called.
    def __init__(
        self, width=None, height=None, cols=None, rows=None, tile_size=None,
        limit_fps=None, window_color=None
            width (int): the width of the screen in pixels. Defaults to
            height (int): the height of the screen in pixels. Defaults to
            tile_size (int): size of a (square) tile's side in pixels.
                Defaults to constants.TILE_SIZE
            limit_fps (int): maximum frames per second that should be drawn.
                Defaults to constants.LIMIT_FPS
            window_color (4-tuple): the window's background color, as a tuple
                of 4 integers representing Red, Greehn, Blue and Alpha values
                (0-255). Defaults to constants.WINDOW_COLOR
            m = Manager()  # start with default parameters
            m.set_scene(SceneBase)  # set a scene. This is a blank base scene
            m.execute()  # call the main loop
        # Set the default arguments
        self.width = width or SCREEN_WIDTH
        self.height = height or SCREEN_HEIGHT
        self.tile_size = tile_size or TILE_SIZE
        self.limit_fps = limit_fps or LIMIT_FPS
        self.window_color = window_color or WINDOW_COLOR
        # Number of tile_size-sized drawable columns and rows on screen
        self.cols = self.width // self.tile_size
        self.rows = self.height // self.tile_size

The way we've built our Manager so far allow us to consider the constants as default values but still accept values passed in during its initialization. An example of that will be shown when we first draw our character. We're also going to set a blank scene (None) at start, requiring that, after the Manager instantiation, a proper scene is passed to it before starting the main loop (unless you want't to stare at blank scrren).

        # Initialize with no scene
        self.scene = None

And finally we're going to write some SDL stuff, mostly via PySDL2.ext utilities, so that we don't have to it on each scene we create:

        # Initialize the video system - this implicitly initializes some
        # necessary parts within the SDL2 DLL used by the video module.
        # You SHOULD call this before using any video related methods or
        # classes.
        # Create a new window (like your browser window or editor window,
        # etc.) and give it a meaningful title and size. We definitely need
        # this, if we want to present something to the user.
        self.window = sdl2.ext.Window(
            "Tiles", size=(self.width, self.height),
        # Create a renderer that supports hardware-accelerated sprites.
        self.renderer = sdl2.ext.Renderer(self.window)
        # Create a sprite factory that allows us to create visible 2D elements
        # easily.
        self.factory = sdl2.ext.SpriteFactory(
            sdl2.ext.TEXTURE, renderer=self.renderer)
        # Creates a simple rendering system for the Window. The
        # SpriteRenderSystem can draw Sprite objects on the window.
        self.spriterenderer = self.factory.create_sprite_render_system(
        # By default, every Window is hidden, not shown on the screen right
        # after creation. Thus we need to tell it to be shown now.
        # Enforce window raising just to be sure.
        # Initialize the keyboard state controller.
        # PySDL2/SDL2 shouldn't need this but the basic procedure for getting
        # key mods and locks is not working for me atm.
        # So I've implemented my own controller.
        self.kb_state = KeyboardStateController()
        # Initialize a mouse starting position. From here on the manager will
        # be able to work on distances from previous positions.
        # Initialize a clock utility to help us control the framerate
        self.clock = Clock()
        # Make the Manager alive. This is used on the main loop.
        self.alive = True
    def _get_mouse_state(self):
        """Get the mouse state.
        This is only required during initialization. Later on the mouse
        position will be passed through events.
        # This is an example of what PySDL2, below the hood, does for us.
        # Here we create a ctypes int (i.e. a C type int)
        x = ctypes.c_int(0)
        y = ctypes.c_int(0)
        # And pass it by reference to the SDL C function (i.e. pointers)
        sdl2.mouse.SDL_GetMouseState(ctypes.byref(x), ctypes.byref(y))
        # The variables were modified by SDL, but are still of C type
        # So we need to get their values as python integers
        self._mouse_x = x.value
        self._mouse_y = y.value
        # Now we hope we're never going to deal with this kind of stuff again
        return self._mouse_x, self._mouse_y

The long comments and docstring should provide some information about what we just did. We initialize SDL2 (sdl2.ext.init()); create a (borderless, in this case) window (sdl2.ext.Window); create a renderer that supports hardware acceleration (sdl2.ext.Renderer; it uses textures instead of surfaces, works with/on the GPU and provides a nice performance gain, should you require it for drawing tons of sprites); we also create a sprite factory (sdl2.ext.SpriteFactory) that will help make sprite creation easier for us later on; we ask the window to be shown ( and raised (sdl2.SDL_RaiseWindow(window)) in case some input got our focus; we instantiate a KeyboardStateController what will be described below (because the default PySDL2 way of handling keyboard mods and locks, although easier in theory, simply doesn't work for my computer/keyboard); then we get our initial mouse state (starting position); finally we instantiate our Clock and set the Manager state to alive!

Now we have to create our main loop that will keep the game running, process and dispatch events (input and output events). We keep it small and call other helper functions to do specialized work:

    def run(self):
        """Main loop handling events and updates."""
        while self.alive:
        return sdl2.ext.quit()

The on_event method takes a bit more of work to evaluate the events received and dispatch them accordingly (mouse events, keyboard events and its specific types such as press/release/etc.):

    def on_event(self):
        """Handle the events and pass them to the active scene."""
        scene = self.scene
        if scene is None:
        for event in sdl2.ext.get_events():
            # Exit events
            if event.type == sdl2.SDL_QUIT:
                self.alive = False
            # Redraw in case the focus was lost and now regained
            if event.type == sdl2.SDL_WINDOWEVENT_FOCUS_GAINED:
            # on_mouse_motion, on_mouse_drag
            if event.type == sdl2.SDL_MOUSEMOTION:
                x = event.motion.x
                y = event.motion.y
                buttons = event.motion.state
                self._mouse_x = x
                self._mouse_y = y
                dx = x - self._mouse_x
                dy = y - self._mouse_y
                if buttons & sdl2.SDL_BUTTON_LMASK:
                    scene.on_mouse_drag(event, x, y, dx, dy, "LEFT")
                elif buttons & sdl2.SDL_BUTTON_MMASK:
                    scene.on_mouse_drag(event, x, y, dx, dy, "MIDDLE")
                elif buttons & sdl2.SDL_BUTTON_RMASK:
                    scene.on_mouse_drag(event, x, y, dx, dy, "RIGHT")
                    scene.on_mouse_motion(event, x, y, dx, dy)
            # on_mouse_press
            elif event.type == sdl2.SDL_MOUSEBUTTONDOWN:
                x = event.button.x
                y = event.button.y
                button_n = event.button.button
                if button_n == sdl2.SDL_BUTTON_LEFT:
                    button = "LEFT"
                elif button_n == sdl2.SDL_BUTTON_RIGHT:
                    button = "RIGHT"
                elif button_n == sdl2.SDL_BUTTON_MIDDLE:
                    button = "MIDDLE"
                double = bool(event.button.clicks - 1)
                scene.on_mouse_press(event, x, y, button, double)
            # on_mouse_scroll (wheel)
            elif event.type == sdl2.SDL_MOUSEWHEEL:
                offset_x = event.wheel.x
                offset_y = event.wheel.y
                scene.on_mouse_scroll(event, offset_x, offset_y)
            # for keyboard input, set the key symbol and keyboard modifiers
            mod = self.kb_state.process(event)
            sym = event.key.keysym.sym
            # on_key_release
            if event.type == sdl2.SDL_KEYUP:
                scene.on_key_release(event, sym, mod)
            # on_key_press
            elif event.type == sdl2.SDL_KEYDOWN:
                scene.on_key_press(event, sym, mod)

So what we've done here is: check the type of the event and deliver it to whatever method we're going to create on the game scenes to handle that type of event. The output (graphic) part is simpler:

    def on_update(self):
        """Update the active scene."""
        scene = self.scene
        if self.alive:
            # clear the window with its color
            if scene:
                # call the active scene's on_update
            # present what we have to the screen
    def present(self):
        """Flip the GPU buffer."""

And finally we need to set up each scene that we pass to the Maanger:

    def set_scene(self, scene=None, **kwargs):
        """Set the scene.
            scene (SceneBase): the scene to be initialized
            kwargs: the arguments that should be passed to the scene
        self.scene = scene(manager=self, **kwargs)

keyboard state controller

The KeyboardStateController class will keep track of two things: if alt, ctrl and/or shift are held; if caps, num and/or scroll lock are on/off. As I've said above the default SDL2/PySDL2 way of handling this simpy don't work for me, thus a new way of tracking needs to be created:

class KeyboardStateController:
    """A class that keeps track of keyboard modifiers and locks."""
    def __init__(self):
        # evernthing defaults to False
        self._shift = False
        self._ctrl = False
        self._alt = False
        self.caps = False
        self.num = False
        self.scroll = False
    def alt(self):
        """Evaluate if the alt key only is held."""
        return self.combine(ctrl=True)
    def ctrl(self):
        """Evaluate if the ctrl key only is held."""
        return self.combine(ctrl=True)
    def shift(self):
        """Evaluate if the ctrl key only is held."""
        return self.combine(shift=True)
    def combine(self, alt=False, ctrl=False, shift=False):
        """Evaluate key combinations."""
        return all(
            (self._alt == alt,
             self._ctrl == ctrl,
             self._shift == shift)
    def process(self, event):
        """Process the current event and update the keyboard state."""
        down = True if event.type == sdl2.SDL_KEYDOWN else False
        self._process_mods(event.key.keysym.sym, down)
        if not down:
        return self
    def _process_locks(self, key):
        """Process the locks."""
        for lock, sym in (
            ("caps", sdl2.SDLK_CAPSLOCK),
            ("num", sdl2.SDLK_NUMLOCKCLEAR),
            ("scroll", sdl2.SDLK_SCROLLLOCK)
            if key == sym:
                _prev_lock = getattr(self, lock)
                setattr(self, lock, not _prev_lock)
    def _process_mods(self, key, down):
        """Process the modifiers."""
        for mod, syms in (
            ("_ctrl", (sdl2.SDLK_LCTRL, sdl2.SDLK_RCTRL)),
            ("_shift", (sdl2.SDLK_LSHIFT, sdl2.SDLK_RSHIFT)),
            ("_alt", (sdl2.SDLK_LALT, sdl2.SDLK_RALT))
            if key in syms:
                setattr(self, mod, down)

No need to go into details here: it process an event and set its state accordingly. We can check for simple mods (e.g. kb_state.shift) or locks (e.g. kb_state.caps) or multiple mods (kb_state.combine(shift=True, ctrl=True)).

Creating a base scene

Every line that we write here is a line that we won't have to write at each scene later on. Don't give up! We're gaining in the long term. We should now create a base scene that would be inherited by the custom scenes we create. This scene should know who is its Manager and be able to access some of its attributes and methods easily. And it should act show how input and output are usually handled/passed by the Manager. Some of the properties here might not be used by you at all. But know that, if you need it, most of the essential ones are here. And as usual we're hoping the dosctrings and comments help explain what we forget:

class SceneBase(object):
    """Basic scene of the game.
    New Scenes should be subclasses of SceneBase.
    def __new__(cls, manager, **kwargs):
        """Create a new instance of a scene.
        A reference to the manager is stored before returning the instance.
        This is made preventively because many properties are related to the
            manager (Manager): the running instance of the Manager
        scene = super().__new__(cls)
        scene.manager = manager
        return scene
    def __init__(self, **kwargs):
    # properties
    def height(self):
        """Main window height.
        return self.manager.height
    def width(self):
        """Main window width.
        return self.manager.width
    def factory(self):
        """Reference to sdl2.ext.SpriteFactory instance.
        return self.manager.factory
    def kb_state(self):
        """Reference to KeyboardStateController instance.
        return self.manager.kb_state
    def renderer(self):
        """Reference to sdl2.ext.Renderer instance.
        return self.manager.renderer
    def sdlrenderer(self):
        """Reference to sdl2.SDL_Renderer instance.
        return self.manager.renderer.sdlrenderer
    def spriterenderer(self):
        """Reference to sdl2.ext.TextureSpriteRenderSystem instance.
        return self.manager.spriterenderer
    # other methods
    def quit(self):
        """Stop the manager main loop."""
        self.manager.alive = False
    # event methods
    def on_key_press(self, event, sym, mod):
        """Called on keyboard input, when a key is **held down**.
            event ( The base event, as passed by SDL2.
                Unless specifically needed, sym and mod should be used
            sym (int): Integer representing code of the key pressed. For
                printable keys ``chr(key)`` should return the corresponding
            mod (KeyboardStateController): the keyboard state for modifiers
                and locks. See :class:KeyboardStateController
    def on_key_release(self, event, sym, mod):
        """Called on keyboard input, when a key is **released**.
        By default if the Escape key is pressed the manager quits.
        If that behaviour is desired you can call ``super().on_key_release(
        event, sym, mod)`` on a child class.
            event ( The base event, as passed by SDL2.
                The other arguments should be used for a higher level
                interaction, unless specifically needed.
            sym (int): Integer representing code of the key pressed. For
                printable keys ``chr(key)`` should return the corresponding
            mod (KeyboardStateController): the keyboard state for modifiers
                and locks. See :class:KeyboardStateController
        if sym == sdl2.SDLK_ESCAPE:
    def on_mouse_drag(self, event, x, y, dx, dy, button):
        """Called when mouse buttons are pressed and the mouse is dragged.
            event ( The base event, as passed by SDL2.
                The other arguments should be used for a higher level
                interaction, unless specifically needed.
            x (int): horizontal coordinate, relative to window.
            y (int): vertical coordinate, relative to window.
            dx (int): relative motion in the horizontal direction
            dy (int): relative motion in the vertical direction
            button (str, "RIGHT"|"MIDDLE"|"LEFT"): string representing the
                button pressed.
    def on_mouse_motion(self, event, x, y, dx, dy):
        """Called when the mouse is moved.
            event ( The base event, as passed by SDL2.
                The other arguments should be used for a higher level
                interaction, unless specifically needed.
            x (int): horizontal coordinate, relative to window.
            y (int): vertical coordinate, relative to window.
            dx (int): relative motion in the horizontal direction
            dy (int): relative motion in the vertical direction
    def on_mouse_press(self, event, x, y, button, double):
        """Called when mouse buttons are pressed.
            event ( The base event, as passed by SDL2.
                The other arguments should be used for a higher level
                interaction, unless specifically needed.
            x (int): horizontal coordinate, relative to window.
            y (int): vertical coordinate, relative to window.
            button (str, "RIGHT"|"MIDDLE"|"LEFT"): string representing the
                button pressed.
            double (bool, True|False): boolean indicating if the click was a
                double click.
    def on_mouse_scroll(self, event, offset_x, offset_y):
        """Called when the mouse wheel is scrolled.
            event ( The base event, as passed by SDL2.
                The other arguments should be used for a higher level
                interaction, unless specifically needed.
            offset_x (int): the amount scrolled horizontally, positive to the
                right and negative to the left.
            offset_y (int): the amount scrolled vertically, positive away
                from the user and negative toward the user.
    def on_update(self):
        """Graphical logic."""

Ta-da! No, no character on screen yet. But things are about to start running smoothly from now on! To test if everything works so far, add the lines below to the end of Cross your fingers and run it! An ugly green screen should appear and, if you press Escape, it should quit quietly.

if __name__ == '__main__':
    # example, with a borderless yet ugly green window
    m = Manager(window_color=(0, 255, 0, 255))

Here's a rundown of the whole code so far.

Go on to the next part.

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