Referencing an appended list with user generated class objects from definition within a class?
Question:
I want to create a class Ball, and make a list of objects generated by user for that class. I will then reference this list in run() to draw user generated objects in pygame. My problems are as follows:
When I try to reference Ball(a, v, r...) it says Ball is referenced before my definition. Any help is appreciated!
HERE IS FULL ERROR:
File "/Users/.../Desktop/.../python/promot.py", line 66, in run
balls.append(Ball(a, v, r, c, x_pos, y_pos)) #here i get error
^^^^
UnboundLocalError: cannot access local variable 'Ball' where it is not associated with a value
import pygame
import math
pygame.init()
#screen
framespd = 30 #what is frame speed for gravity = 9.81
gravity = 1
t = 0.01
clock = pygame.time.Clock()
width, height = 1000, 1000
pygame.display.set_caption("Orbit Take 1")
screen = pygame.display.set_mode([width, height])
class Ball:
def __init__(self, angle, velocity, radius, color, x_pos, y_pos):
self.angle = angle
self.velocity = velocity
self.radius = radius
self.color = color
self.x_pos = x_pos
self.y_pos = y_pos
def draw(self):
self.circle = pygame.draw.circle(screen, self.color, (self.x_pos, self.y_pos), self.radius)
def true_velocity_x(self):
true_velocity_x = self.velocity * math.cos(self.angle)
def true_velocity_y(self):
true_velocity_y = self.velocity * math.sin(self.angle)
new_velocity_y = true_velocity_y + (gravity * t)
#def update position(self):
#x pos = x velocity
#y pos += y velocity
balls = []
def run():
value = True
while value:
clock.tick(framespd)
# Balls in sim
number_balls = int(input("How many balls: "))
while len(balls) < number_balls:
# Variables for Ball
a = input("Angle: ")
v = input("Velocity: ")
r = input("Radius: ")
c = input("Color: ")
x_pos = '0'
y_pos = '500'
balls.append(Ball(a, v, r, c, x_pos, y_pos)) #here i get error "local variable "Ball" defined in enclosing scope on line 24 referenced before assignment"
screen.fill('black')
#draw ball
for Ball in balls:
Ball.draw(pygame.display.set_mode((width, height)))
#update position
for event in pygame.event.get():
if event.type == pygame.QUIT:
value = False
pygame.display.flip()
pygame.quit()
run()
My first attempt is referenced in bold as "something else I tried", but I am having trouble using classes outside of their initialization with out errors. My next attempt was "current attempt" which does not show anything when I run.
Answers:
For the first ball this was working correctly but then it encountered the
for Ball in balls:
statement and then you had a Ball in your namespace so the global variable is overwritten, instead we can call the object ball and run .draw on the ball object.
There was a way around this (if you insist on calling the local variable Ball) by passing the class into the function that I demonstrated as well.
def run(cls = Ball):
import pygame
import math
pygame.init()
# screen
framespd = 30 # what is frame speed for gravity = 9.81
gravity = 1
t = 0.01
clock = pygame.time.Clock()
width, height = 1000, 1000
pygame.display.set_caption("Orbit Take 1")
screen = pygame.display.set_mode([width, height])
class Ball:
def __init__(self, angle, velocity, radius, color, x_pos, y_pos):
self.angle = angle
self.velocity = velocity
self.radius = radius
self.color = color
self.x_pos = x_pos
self.y_pos = y_pos
self.balls = []
def draw(self, width, height):
# We need the inputs of width and height to pass to set_mode
pygame.display.set_mode((width, height))
#self.circle = pygame.draw.circle(screen, (0, 0, 255), (250, 250), (self.x_pos, self.y_pos), self.radius)
# below will always draw a blue circle - tweak color, center and radius to be variables so that you can
# generate multiple balls with different properties
self.circle = pygame.draw.circle(screen, color=(0, 0, 255), center=(250, 250), radius=75)
def true_velocity_x(self):
true_velocity_x = self.velocity * math.cos(self.angle)
def true_velocity_y(self):
true_velocity_y = self.velocity * math.sin(self.angle)
new_velocity_y = true_velocity_y + (gravity * t)
# def update position(self):
# x pos = x velocity
# y pos += y velocity
balls = []
def run():
value = True
while value:
clock.tick(framespd)
# Balls in sim
number_balls = int(input("How many balls: "))
while len(balls) < number_balls:
# Variables for Ball
a = input("Angle: ")
v = input("Velocity: ")
r = input("Radius: ")
c = input("Color: ")
x_pos = '0'
y_pos = '500'
balls.append(Ball(a, v, r, c, x_pos,
y_pos)) # here i get error "local variable "Ball" defined in enclosing scope on line 24 referenced before assignment"
screen.fill('black')
# draw ball
# call our ball 'ball' so it doesn't overwrite out global class
for ball in balls:
ball.draw(width, height)
# update position
for event in pygame.event.get():
if event.type == pygame.QUIT:
value = False
pygame.display.flip()
pygame.quit()
run()
Ball is the (global) name of your class.
In your run function, you wrote
for Ball in balls:
Ball.draw(pygame.display.set_mode((width, height)))
so the name Balls will be successively given to each of the balls in balls (and will no longer refer to the class, btw).
Python’s rule is simple: if you assign to a variable somewhere in the body of a function, then that variable is considered to be local to the function, unless declared otherwise. That’s what happens here to Ball.
So, as you try to use it before that line, it’s a local variable that hasn’t received any value, hence the error message:
UnboundLocalError: cannot access local variable 'Ball' where it is not associated with a value
The solution is simple anyway: don’t use this name which should only refer to your class. Just rewrite the loop using a new name for your variable:
for ball in balls:
ball.draw(pygame.display.set_mode((width, height)))
Note also that this name is compliant with the PEP8 recommendation for variable names, which states that variable names should be all lowercase. Capitalized names are for classes.
I want to create a class Ball, and make a list of objects generated by user for that class. I will then reference this list in run() to draw user generated objects in pygame. My problems are as follows:
When I try to reference Ball(a, v, r...) it says Ball is referenced before my definition. Any help is appreciated!
HERE IS FULL ERROR:
File "/Users/.../Desktop/.../python/promot.py", line 66, in run
balls.append(Ball(a, v, r, c, x_pos, y_pos)) #here i get error
^^^^
UnboundLocalError: cannot access local variable 'Ball' where it is not associated with a value
import pygame
import math
pygame.init()
#screen
framespd = 30 #what is frame speed for gravity = 9.81
gravity = 1
t = 0.01
clock = pygame.time.Clock()
width, height = 1000, 1000
pygame.display.set_caption("Orbit Take 1")
screen = pygame.display.set_mode([width, height])
class Ball:
def __init__(self, angle, velocity, radius, color, x_pos, y_pos):
self.angle = angle
self.velocity = velocity
self.radius = radius
self.color = color
self.x_pos = x_pos
self.y_pos = y_pos
def draw(self):
self.circle = pygame.draw.circle(screen, self.color, (self.x_pos, self.y_pos), self.radius)
def true_velocity_x(self):
true_velocity_x = self.velocity * math.cos(self.angle)
def true_velocity_y(self):
true_velocity_y = self.velocity * math.sin(self.angle)
new_velocity_y = true_velocity_y + (gravity * t)
#def update position(self):
#x pos = x velocity
#y pos += y velocity
balls = []
def run():
value = True
while value:
clock.tick(framespd)
# Balls in sim
number_balls = int(input("How many balls: "))
while len(balls) < number_balls:
# Variables for Ball
a = input("Angle: ")
v = input("Velocity: ")
r = input("Radius: ")
c = input("Color: ")
x_pos = '0'
y_pos = '500'
balls.append(Ball(a, v, r, c, x_pos, y_pos)) #here i get error "local variable "Ball" defined in enclosing scope on line 24 referenced before assignment"
screen.fill('black')
#draw ball
for Ball in balls:
Ball.draw(pygame.display.set_mode((width, height)))
#update position
for event in pygame.event.get():
if event.type == pygame.QUIT:
value = False
pygame.display.flip()
pygame.quit()
run()
My first attempt is referenced in bold as "something else I tried", but I am having trouble using classes outside of their initialization with out errors. My next attempt was "current attempt" which does not show anything when I run.
For the first ball this was working correctly but then it encountered the
for Ball in balls:
statement and then you had a Ball in your namespace so the global variable is overwritten, instead we can call the object ball and run .draw on the ball object.
There was a way around this (if you insist on calling the local variable Ball) by passing the class into the function that I demonstrated as well.
def run(cls = Ball):
import pygame
import math
pygame.init()
# screen
framespd = 30 # what is frame speed for gravity = 9.81
gravity = 1
t = 0.01
clock = pygame.time.Clock()
width, height = 1000, 1000
pygame.display.set_caption("Orbit Take 1")
screen = pygame.display.set_mode([width, height])
class Ball:
def __init__(self, angle, velocity, radius, color, x_pos, y_pos):
self.angle = angle
self.velocity = velocity
self.radius = radius
self.color = color
self.x_pos = x_pos
self.y_pos = y_pos
self.balls = []
def draw(self, width, height):
# We need the inputs of width and height to pass to set_mode
pygame.display.set_mode((width, height))
#self.circle = pygame.draw.circle(screen, (0, 0, 255), (250, 250), (self.x_pos, self.y_pos), self.radius)
# below will always draw a blue circle - tweak color, center and radius to be variables so that you can
# generate multiple balls with different properties
self.circle = pygame.draw.circle(screen, color=(0, 0, 255), center=(250, 250), radius=75)
def true_velocity_x(self):
true_velocity_x = self.velocity * math.cos(self.angle)
def true_velocity_y(self):
true_velocity_y = self.velocity * math.sin(self.angle)
new_velocity_y = true_velocity_y + (gravity * t)
# def update position(self):
# x pos = x velocity
# y pos += y velocity
balls = []
def run():
value = True
while value:
clock.tick(framespd)
# Balls in sim
number_balls = int(input("How many balls: "))
while len(balls) < number_balls:
# Variables for Ball
a = input("Angle: ")
v = input("Velocity: ")
r = input("Radius: ")
c = input("Color: ")
x_pos = '0'
y_pos = '500'
balls.append(Ball(a, v, r, c, x_pos,
y_pos)) # here i get error "local variable "Ball" defined in enclosing scope on line 24 referenced before assignment"
screen.fill('black')
# draw ball
# call our ball 'ball' so it doesn't overwrite out global class
for ball in balls:
ball.draw(width, height)
# update position
for event in pygame.event.get():
if event.type == pygame.QUIT:
value = False
pygame.display.flip()
pygame.quit()
run()
Ball is the (global) name of your class.
In your run function, you wrote
for Ball in balls:
Ball.draw(pygame.display.set_mode((width, height)))
so the name Balls will be successively given to each of the balls in balls (and will no longer refer to the class, btw).
Python’s rule is simple: if you assign to a variable somewhere in the body of a function, then that variable is considered to be local to the function, unless declared otherwise. That’s what happens here to Ball.
So, as you try to use it before that line, it’s a local variable that hasn’t received any value, hence the error message:
UnboundLocalError: cannot access local variable 'Ball' where it is not associated with a value
The solution is simple anyway: don’t use this name which should only refer to your class. Just rewrite the loop using a new name for your variable:
for ball in balls:
ball.draw(pygame.display.set_mode((width, height)))
Note also that this name is compliant with the PEP8 recommendation for variable names, which states that variable names should be all lowercase. Capitalized names are for classes.