Develop Reference

Python code behaving differently depending on whether I pass a list directly into the constructor, or as a variable

I'm writing a simple bullet hell game using pygame. There is a point on the screen which I chose as an origin for my bullets. Originally, I passed the origin point to the constructor as a two element list directly, which works just fine. When I tried to assign the exact same list to the variable, and pass the variable to the constructor this happenes.
That's how I defined the class representing bullets:
class Bullet():
def __init__(self, pos, angle, birth, speed_mod, bullet_skin):
self.pos = pos
self.dir = Angle(angle)
self.angle = angle
self.birth = birth
self.image = bullet_skin
self.rotated = 0
self.speed_mod = speed_mod
def draw(self):
a = self.get_pos()
x = a[0]
y = a[1]
if not self.rotated:
self.image = pygame.transform.rotate(self.image, -self.angle + 90)
self.rotated = 1
screen.blit(self.image, (x - 10, y - 10))
def move(self, speed_mod):
self.pos[0] += speed_mod * bullet.dir[0]
self.pos[1] += speed_mod * bullet.dir[1]
def get_pos(self):
return [round(self.pos[0]), round(self.pos[1])]
def on_screen(pos):
return pos[0] > 0 and pos[0] < 800 and pos[1] > 10 and pos[1] < 600
def is_dead(self):
return pygame.time.get_ticks() - self.birth > 3000 or not Bullet.on_screen(self.get_pos())
That's how I defined class spawning bullets:
origin = [400, 20]
class Spawn():
global origin
def __init__(self, delay, state, offset):
self.delay = delay
self.state = state
self.offset = offset
def wave(self, Bullets): #the code works fine if you replace the origin with [400, 20] list here
for x in range(36):
Bullets.append(Bullet(origin, 10 * x + self.offset, pygame.time.get_ticks(), 5, blue_bullet))
def single(self, Bullets):
Bullets.append(Bullet([400, 20], randint(60, 120), pygame.time.get_ticks(), 3, purple_bullet))
def bomb(self, Bullets):
for x in range(18):
Bullets.append(Bullet([100, 100], 20 * x + self.offset, pygame.time.get_ticks(), 6, red_bullet))
My whole code can be found here

The problem you have is that you create a single list pointed by the variables origin. Then that list gets passed to every bullet you create without making a copy. When you modify the position of a bullet you are mutating that list and thus a different starting point is being used for the bullets. The easiest fix is to make a copy of the list:
Bullet(origin[:], 10 * x + self.offset, pygame.time.get_ticks(), 5, blue_bullet)
To see what is happening you can run the following example:
a = [0, 1, 2]
b = a
c = a[:]
c.append(3)
print(a, b, c)
b.append(4)
print(a, b, c)
Which of the operations affected a?

origin is a global list that you are passing to every new instance of Bullet.
Each Bullet instance stores a reference to this list as its pos. So they are all sharing one pos. When the list is altered, every Bullet is affected.
Possible fixes:
Pass origin[:] (a copy of the list) instead of origin to each bullet.
Have Bullet's __init__ method copy origin when it receives it.
Consider using an immutable type, like a tuple, for a constant like origin, to avoid mutation errors like this. You can reassign the pos instead of mutating it; or you can convert it to a list as Bullet receives it, if that's the way you prefer.

Pygame pixel perfect collision not working as expected

I am creating tetris using pygame. i want to use collision detection so that when the shape in play comes into contact with any other previously played shapes, i can stop the shape, as per the logic of tetris. i came across pixel perfect collision using masks. i have followed some tutorials online, however the pixel detection returns true every time a new shape comes into play, not when any shapes collide. sorry in advance for the long code, its the bare minimum for the code to actually and still containing the game element of it. i think there is something wrong with my approach which is causing this error. I basically have a function that everytime the shape in play comes into contact with the 'floor' that shape is held in that position and a new shape is created. i think ive overcomplicated it, in turn creating this error. thanks in advance
import pygame
import sys
import shapelogic
pygame.init()
screensize = width, height = 800, 595
screen = pygame.display.set_mode(screensize)
background_image =pygame.image.load("/Users/marceason/PycharmProjects/Tetris/Wooden_background.jpg").convert_alpha()
myshape = 0
stop_movement = 0
blit_count = 0
stored_shapes = pygame.sprite.Group()
stored_shapes_with_coords = []
extra_blit_required = False
index = 0
count = 0
listofshapes = []
class shapemanager():
def __init__(self):
self.listofshapes = []
def create_another_instance(self):
global count
count += 1
string = "Shape_{0},".format(count)
another_shape = Shape(string)
self.listofshapes.append(another_shape)
global index
object = self.listofshapes[index]
index += 1
return object
def load_shape(self):
shape = self.create_another_instance()
shape.load_shapes()
class Shape(pygame.sprite.Sprite):
def __init__(self, name):
pygame.sprite.Sprite.__init__(self)
self.name = name
self.x = 50
self.y = 100
self.move_event = pygame.USEREVENT + 1
self.reached_bottom_event = pygame.USEREVENT + 2
self.one_sec_timer = 1000
self.half_sec_timer = 500
self.reachbottomflag = False
self.movement_possible = True
self.image = pygame.image.load(
"/Users/marceason/PycharmProjects/Tetris/Tetris_Shapes/Green_Shape_1_Position_1.png")
self.mask = pygame.mask.from_surface(self.image)
self.rect = self.image.get_rect()
def move_shape(self):
if self.movement_possible:
key_input = pygame.key.get_pressed()
if key_input[pygame.K_LEFT]:
self.x -= 16
if key_input[pygame.K_RIGHT]:
self.x += 16
if not self.reachbottomflag:
if key_input[pygame.K_DOWN]:
self.y += 16
def reachbottom(self):
if self.y >= 560:
self.reachbottomflag = True
def no_movement_possible(self):
self.movement_possible = False
def assign_shape():
global myshape
global stop_movement
myshape = sl.create_another_instance()
pygame.time.set_timer(myshape.move_event, myshape.one_sec_timer)
stop_movement = pygame.time.set_timer(myshape.reached_bottom_event, myshape.half_sec_timer)
def blit_used_shapes():
global screen
global blit_count
blit_count = len(stored_shapes_with_coords)
local_count = 0
while local_count < blit_count:
screen.blit(stored_shapes_with_coords[local_count][0], (stored_shapes_with_coords[local_count][1], stored_shapes_with_coords[local_count][2]))
local_count += 1
sl = shapemanager()
##### HERE IS THE PIXEL DETECTION #####
result = pygame.sprite.spritecollide(myshape, stored_shapes, False, pygame.sprite.collide_mask)
## Main loop ##
assign_shape()
while True:
for event in pygame.event.get():
if event.type == pygame.QUIT: sys.exit()
screen.blit(background_image, (0, 0))
screen.blit(myshape.image, (myshape.x, myshape.y))
myshape.move_shape()
key_input = pygame.key.get_pressed()
if key_input[pygame.K_SPACE]:
myshape.rotate_shape()
myshape.reachbottom()
if myshape.reachbottomflag:
if event.type == myshape.reached_bottom_event:
myshape.no_movement_possible()
stored_shape_tuple = [myshape.image, myshape.x, myshape.y]
stored_shapes_with_coords.append(stored_shape_tuple)
stored_shapes.add(myshape)
extra_blit_required = True
assign_shape()
####### PIXEL DETECTION IS HERE IN FOR LOOP ####
if result:
print("this should only execute when two shapes touch!!")
if extra_blit_required:
blit_used_shapes()
pygame.display.update()

The issue is that you are not updating the sprites rect attribute. The sprites rects all have position (0, 0) (since you do not set it in the call to self.image.get_rect()) and as a result the masks will all overlap and collide.
If you read the docs for pygame.sprite.collide_mask you will note that it says that your sprites need to have mask and rect attributes. You have a rect in your sprite and you set it in the __init__(), but you do not keep it updated when you move the sprite. You just change the x and y attributes without adjusting the rect position. The reason that the collide_mask wants a rect is that it uses that to determine the offset parameter for the pygame.mask.Mask.overlap() call that it uses. The important thing to realize is that masks themselves do not have a position, they need the rects to determine the relative positions of the masks.
This is similar to images/surfaces not having a position and needing a rect to track that for them.
On a separate issue, the way you are blit'ing the sprites to the screen makes no sense. You are not using the abilities of the sprite groups to draw and worse you are keeping the image, x and y of the sprite in a separate list and not containing it in the sprite itself. You should go look at some examples of pygame sprite based code. There are lots of examples out there.

How to access attributes of the group's object? AttributeError: 'Group' object has no attribute 'risk_level'

I have two classes GeoFence and Worker. The class GeoFence has an attribute risk_level that is assigned randomly to all objects of this class.
I want to access this attribute risk_level from the class Worker. In particular, when I check the collision between the objects of these two classes (see the function update in Worker), I want to print the risk_level of particular geofence that the worker collides with.
Below I provide my code, but it fails with the error message:
AttributeError: 'Group' object has no attribute 'risk_level'
So, I understand that it's necessary to access objects of the group, because the group itself does not have the attribute risk_level. But how can I do it?
import pygame
class GeoFence(pygame.sprite.Sprite):
def __init__(self, rect, risk_level, *groups):
# we set a _layer attribute before adding this sprite to the sprite groups
self._layer = 1
pygame.sprite.Sprite.__init__(self, groups)
self.image = pygame.surface.Surface((rect.width, rect.height))
self.image.fill(GREEN)
self.rect = rect
self.risk_level = risk_level
class Worker(pygame.sprite.Sprite):
# we introduce to possible states: RUNNING and IDLE
RUNNING = 0
IDLE = 1
NUMBER_OF_ACCIDENTS = 0
def __init__(self, image_running, image_idle, location, *groups):
# each state has it's own image
self.images = {
Worker.RUNNING: pygame.transform.scale(get_image(image_running), (45, 45)),
Worker.IDLE: pygame.transform.scale(get_image(image_idle), (20, 45))
}
# we set a _layer attribute before adding this sprite to the sprite groups
# we want the workers on top
self._layer = 2
pygame.sprite.Sprite.__init__(self, groups)
# let's keep track of the state and how long we are in this state already
self.state = Worker.IDLE
self.ticks_in_state = 0
self.image = self.images[self.state]
self.rect = self.image.get_rect(topleft=location)
self.direction = pygame.math.Vector2(0, 0)
self.speed = random.randint(1, 3)
self.set_random_direction()
def set_random_direction(self):
# random new direction or standing still
vec = pygame.math.Vector2(random.randint(-100,100), random.randint(-100,100)) if random.randint(0, 5) > 1 else pygame.math.Vector2(0, 0)
# check the new vector and decide if we are running or fooling around
length = vec.length()
speed = sum(abs(int(v)) for v in vec.normalize() * self.speed) if length > 0 else 0
if length == 0 or speed == 0:
new_state = Worker.IDLE
self.direction = pygame.math.Vector2(0, 0)
else:
new_state = Worker.RUNNING
self.direction = vec.normalize()
self.ticks_in_state = 0
self.state = new_state
# use the right image for the current state
self.image = self.images[self.state]
def update(self, screen):
self.ticks_in_state += 1
# the longer we are in a certain state, the more likely is we change direction
if random.randint(0, self.ticks_in_state) > 70:
self.set_random_direction()
# now let's multiply our direction with our speed and move the rect
vec = [int(v) for v in self.direction * self.speed]
self.rect.move_ip(*vec)
if any(s for s in pygame.sprite.spritecollide(self, fences, False) if s != self):
print("RISK_LEVEL: ",fences.risk_level)
all_sprites = pygame.sprite.LayeredUpdates()
workers = pygame.sprite.Group()
fences = pygame.sprite.Group()
screen = pygame.display.set_mode((SCREENWIDTH, SCREENHEIGHT))
pygame.display.set_caption("TEST")
# create multiple workers
for pos in ((30,30), (50, 400), (200, 100), (700, 200)):
Worker("w1.png", "w2.png", pos, all_sprites, workers, fences)
# create multiple geo-fences
risks = ["HIGH","MEDIUM","LOW"]
for rect in (pygame.Rect(510,150,75,52), pygame.Rect(450,250,68,40), pygame.Rect(450,370,68,48)):
risk = risks[random.randint(0,2)]
GeoFence(rect, risk, all_sprites, fences)
UPDATE:
For #Prune: my previous thread that contains the whole code if there is any problem to reproduce this example.

The error will get raised here:
if any(s for s in pygame.sprite.spritecollide(self, fences, False) if s != self):
print("RISK_LEVEL: ", fences.risk_level)
fences is a sprite group and it doesn't have a risk_level attribute, only the GeoFence sprites in this group have this attribute. So you need to figure out which sprites have collided with the worker and then access the risk_level of these sprites. The simplest solution would be to iterate over the collided sprites with a for loop (replace the two lines above with):
for s in pygame.sprite.spritecollide(self, fences, False):
print("RISK_LEVEL: ", s.risk_level)

Understand object orientated python - initialization of objects in pygame

I'm learning Object Orientated Python and understand the main principals of classes and creating objects from classes however I need something explained Re: the pygame code below. I'm struggling to get my head around what's happening when sprite lists are being created and the two lines of code under the code which creates the ball object (allsprites.add etc). In other words what are sprites and why are lists of them created? Why isn't the ball object just created from the class on its own? Why does it need to be added to a sprite list?? What's going on? Any explanation would be greatly appreciated.
"""
Sample Breakout Game
Sample Python/Pygame Programs
Simpson College Computer Science
http://programarcadegames.com/
http://simpson.edu/computer-science/
"""
# --- Import libraries used for this program
import math
import pygame
# Define some colors
black = (0, 0, 0)
white = (255, 255, 255)
blue = (0, 0, 255)
# Size of break-out blocks
block_width = 23
block_height = 15
class Block(pygame.sprite.Sprite):
"""This class represents each block that will get knocked out by the ball
It derives from the "Sprite" class in Pygame """
def __init__(self, color, x, y):
""" Constructor. Pass in the color of the block,
and its x and y position. """
# Call the parent class (Sprite) constructor
pygame.sprite.Sprite.__init__(self)
# Create the image of the block of appropriate size
# The width and height are sent as a list for the first parameter.
self.image = pygame.Surface([block_width, block_height])
# Fill the image with the appropriate color
self.image.fill(color)
# Fetch the rectangle object that has the dimensions of the image
self.rect = self.image.get_rect()
# Move the top left of the rectangle to x,y.
# This is where our block will appear..
self.rect.x = x
self.rect.y = y
class Ball(pygame.sprite.Sprite):
""" This class represents the ball
It derives from the "Sprite" class in Pygame """
# Speed in pixels per cycle
speed = 10.0
# Floating point representation of where the ball is
x = 0.0
y = 180.0
# Direction of ball (in degrees)
direction = 200
width = 10
height = 10
# Constructor. Pass in the color of the block, and its x and y position
def __init__(self):
# Call the parent class (Sprite) constructor
pygame.sprite.Sprite.__init__(self)
# Create the image of the ball
self.image = pygame.Surface([self.width, self.height])
# Color the ball
self.image.fill(white)
# Get a rectangle object that shows where our image is
self.rect = self.image.get_rect()
# Get attributes for the height/width of the screen
self.screenheight = pygame.display.get_surface().get_height()
self.screenwidth = pygame.display.get_surface().get_width()
def bounce(self, diff):
""" This function will bounce the ball
off a horizontal surface (not a vertical one) """
self.direction = (180 - self.direction) % 360
self.direction -= diff
def update(self):
""" Update the position of the ball. """
# Sine and Cosine work in degrees, so we have to convert them
direction_radians = math.radians(self.direction)
# Change the position (x and y) according to the speed and direction
self.x += self.speed * math.sin(direction_radians)
self.y -= self.speed * math.cos(direction_radians)
# Move the image to where our x and y are
self.rect.x = self.x
self.rect.y = self.y
# Do we bounce off the top of the screen?
if self.y <= 0:
self.bounce(0)
self.y = 1
# Do we bounce off the left of the screen?
if self.x <= 0:
self.direction = (360 - self.direction) % 360
self.x = 1
# Do we bounce of the right side of the screen?
if self.x > self.screenwidth - self.width:
self.direction = (360 - self.direction) % 360
self.x = self.screenwidth - self.width - 1
# Did we fall off the bottom edge of the screen?
if self.y > 600:
return True
else:
return False
class Player(pygame.sprite.Sprite):
""" This class represents the bar at the bottom that the player controls. """
def __init__(self):
""" Constructor for Player. """
# Call the parent's constructor
pygame.sprite.Sprite.__init__(self)
self.width = 75
self.height = 15
self.image = pygame.Surface([self.width, self.height])
self.image.fill((white))
# Make our top-left corner the passed-in location.
self.rect = self.image.get_rect()
self.screenheight = pygame.display.get_surface().get_height()
self.screenwidth = pygame.display.get_surface().get_width()
self.rect.x = 0
self.rect.y = self.screenheight-self.height
def update(self):
""" Update the player position. """
# Get where the mouse is
pos = pygame.mouse.get_pos()
# Set the left side of the player bar to the mouse position
self.rect.x = pos[0]
# Make sure we don't push the player paddle
# off the right side of the screen
if self.rect.x > self.screenwidth - self.width:
self.rect.x = self.screenwidth - self.width
# Call this function so the Pygame library can initialize itself
pygame.init()
# Create an 800x600 sized screen
screen = pygame.display.set_mode([800, 600])
# Set the title of the window
pygame.display.set_caption('Breakout')
# Enable this to make the mouse disappear when over our window
pygame.mouse.set_visible(0)
# This is a font we use to draw text on the screen (size 36)
font = pygame.font.Font(None, 36)
# Create a surface we can draw on
background = pygame.Surface(screen.get_size())
# Create sprite lists
blocks = pygame.sprite.Group()
balls = pygame.sprite.Group()
allsprites = pygame.sprite.Group()
# Create the player paddle object
player = Player()
allsprites.add(player)
# Create the ball
ball = Ball()
allsprites.add(ball)
balls.add(ball)
# The top of the block (y position)
top = 80
# Number of blocks to create
blockcount = 32
# --- Create blocks
# Five rows of blocks
for row in range(5):
# 32 columns of blocks
for column in range(0, blockcount):
# Create a block (color,x,y)
block = Block(blue, column * (block_width + 2) + 1, top)
blocks.add(block)
allsprites.add(block)
# Move the top of the next row down
top += block_height + 2
# Clock to limit speed
clock = pygame.time.Clock()
# Is the game over?
game_over = False
# Exit the program?
exit_program = False
# Main program loop
while exit_program != True:
# Limit to 30 fps
clock.tick(30)
# Clear the screen
screen.fill(black)
# Process the events in the game
for event in pygame.event.get():
if event.type == pygame.QUIT:
exit_program = True
# Update the ball and player position as long
# as the game is not over.
if not game_over:
# Update the player and ball positions
player.update()
game_over = ball.update()
# If we are done, print game over
if game_over:
text = font.render("Game Over", True, white)
textpos = text.get_rect(centerx=background.get_width()/2)
textpos.top = 300
screen.blit(text, textpos)
# See if the ball hits the player paddle
if pygame.sprite.spritecollide(player, balls, False):
# The 'diff' lets you try to bounce the ball left or right
# depending where on the paddle you hit it
diff = (player.rect.x + player.width/2) - (ball.rect.x+ball.width/2)
# Set the ball's y position in case
# we hit the ball on the edge of the paddle
ball.rect.y = screen.get_height() - player.rect.height - ball.rect.height - 1
ball.bounce(diff)
# Check for collisions between the ball and the blocks
deadblocks = pygame.sprite.spritecollide(ball, blocks, True)
# If we actually hit a block, bounce the ball
if len(deadblocks) > 0:
ball.bounce(0)
# Game ends if all the blocks are gone
if len(blocks) == 0:
game_over = True
# Draw Everything
allsprites.draw(screen)
# Flip the screen and show what we've drawn
pygame.display.flip()
pygame.quit()

You don't need to add the balls and blocks to sprite lists - it's just a matter of convenience. You could manually check each ball for a collision, but it's easier to just tell pygame to check them all for you
# See if the ball hits the player paddle
if pygame.sprite.spritecollide(player, balls, False):
# The 'diff' lets you try to bounce the ball left or right
# depending where on the paddle you hit it
diff = (player.rect.x + player.width/2) - (ball.rect.x+ball.width/2)
# Set the ball's y position in case
# we hit the ball on the edge of the paddle
ball.rect.y = screen.get_height() - player.rect.height - ball.rect.height - 1
ball.bounce(diff)
You could draw each thing to the screen separately on each frame, but it's easier just to tell pygame to do it for you:
# Draw Everything
allsprites.draw(screen)
Things can be in more than one list as required, for example a ball is added to the balls list so that you can easily check for collisions, but also added to the allsprites list so that you can easily draw everything on the screen
# Create the ball
ball = Ball()
allsprites.add(ball)
balls.add(ball)
Edit:
An important distinction is that allsprites is actually a sprite.Group. It has a list of sprites inside it, but it also has other methods like draw.
To address your question of "what is a Sprite", it's simply a thing that gets drawn on screen. pygame methods like sprite.Group.draw expect a list of things with certain attributes - such as update. The easiest way to make sure that you provide all of those attributes with the right names is to subclass Sprite, however this is also a (strongly recommended) convenience thing - for instance, this is from the pygame source code:
While it is possible to design sprite and group classes that don't
derive from the Sprite and AbstractGroup classes below, it is strongly
recommended that you extend those when you add a Sprite or Group
class.
So what specifically does subclassing Sprite get you? Let's take a look at the source. Here's how to find the source code for a python module:
>>> import pygame.sprite
>>> pygame.sprite.__file__
'c:\\Python27\\lib\\site-packages\\pygame\\sprite.py'
>>>
Every python module has a __file__ attribute that tells you where the source is located (well not quite every). If you open it up in your editor, and scroll down, you see the class definition for Sprite:
class Sprite(object):
"""simple base class for visible game objects
pygame.sprite.Sprite(*groups): return Sprite
The base class for visible game objects. Derived classes will want to
override the Sprite.update() and assign a Sprite.image and
Sprite.rect attributes. The initializer can accept any number of
Group instances to be added to.
When subclassing the Sprite, be sure to call the base initializer before
adding the Sprite to Groups.
"""
def __init__(self, *groups):
self.__g = {} # The groups the sprite is in
if groups: self.add(groups)
def add(self, *groups):
"""add the sprite to groups
Sprite.add(*groups): return None
Any number of Group instances can be passed as arguments. The
Sprite will be added to the Groups it is not already a member of.
"""
has = self.__g.__contains__
for group in groups:
if hasattr(group, '_spritegroup'):
if not has(group):
group.add_internal(self)
self.add_internal(group)
else: self.add(*group)
def remove(self, *groups):
"""remove the sprite from groups
Sprite.remove(*groups): return None
Any number of Group instances can be passed as arguments. The Sprite will
be removed from the Groups it is currently a member of.
"""
has = self.__g.__contains__
for group in groups:
if hasattr(group, '_spritegroup'):
if has(group):
group.remove_internal(self)
self.remove_internal(group)
else: self.remove(*group)
def add_internal(self, group):
self.__g[group] = 0
def remove_internal(self, group):
del self.__g[group]
def update(self, *args):
"""method to control sprite behavior
Sprite.update(*args):
The default implementation of this method does nothing; it's just a
convenient "hook" that you can override. This method is called by
Group.update() with whatever arguments you give it.
There is no need to use this method if not using the convenience
method by the same name in the Group class.
"""
pass
def kill(self):
"""remove the Sprite from all Groups
Sprite.kill(): return None
The Sprite is removed from all the Groups that contain it. This won't
change anything about the state of the Sprite. It is possible to continue
to use the Sprite after this method has been called, including adding it
to Groups.
"""
for c in self.__g.keys():
c.remove_internal(self)
self.__g.clear()
def groups(self):
"""list of Groups that contain this Sprite
Sprite.groups(): return group_list
Return a list of all the Groups that contain this Sprite.
"""
return self.__g.keys()
def alive(self):
"""does the sprite belong to any groups
Sprite.alive(): return bool
Returns True when the Sprite belongs to one or more Groups.
"""
return (len(self.__g) != 0)
def __repr__(self):
return "<%s sprite(in %d groups)>" % (self.__class__.__name__, len(self.__g))
So in summary, you don't have to subclass Sprite - you could just provide all of these methods on your own - but it's easier if you do ;)

Python: Why Does a Method Behave Differently with an Added Parameter?

I have a method in a Pygame Sprite subclass, defined as such:
def walk(self):
"""move across screen"""
displacement = self.rect.move((self.move, 0))
if self.rect.left < self.area.left or self.rect.right > self.area.right:
self.move = -self.move
displacement = self.rect.move((self.move, 0))
self.rect = displacement
I modified it, adding a parameter speed_x, and now the program is broken.
def walk(self, speed_x):
"""move across screen"""
displacement = self.rect.move((speed_x, 0))
if self.rect.left < self.area.left or self.rect.right > self.area.right:
speed_x = -speed_x
displacement = self.rect.move((speed_x, 0))
self.rect = displacement
Before I called the method like this:
def update(self):
self.walk()
Now I do:
def update(self):
self.walk(self.move)
Why doesn't this work?

You don't explain how it's "broken", but the main difference is that
speed_x = -speed_x
which you have in your second version, is only changing the local variable (arguments are local variables!) speed_x, so that changed value does not persist.
In the first version,
self.move = -self.move
does alter self (specifically one of its attriubtes) and the alteration "persists" in future method calls on the object which is here accessed as self.
Just one of the many key differences between bare names (like speed_x) and qualified names (line self.move), and, I suspect, what's biting you here (hard as you may make it to guess by not saying how the second version is failing your expectations).

You are no storing the offset back in to self.move.

If you want to use the second version of your code, try adding this line:
self.move = speed_x
At the bottom of your function.

As mentioned by others, you are not changing the value of self.move in your new code. I assume the reason you modified this function was so you could reuse this function for values other than self.move.
If you want to be able to pass different arguments into your function and modify them as well, you could pass the modified value of speed_x back as a return value:
def walk(self, speed_x):
"""move across screen"""
displacement = self.rect.move((speed_x, 0))
if self.rect.left < self.area.left or self.rect.right > self.area.right:
speed_x = -speed_x
displacement = self.rect.move((speed_x, 0))
self.rect = displacement
return speed_x
And call the function like this as:
def update(self):
self.move = self.walk(self.move)
Note: This answer assumes that self.move should not always be updated when calling walk. If this assumption is false and self.move should in fact be updated every time walk is run, then you should instead use Xavier Ho's answer.