🪲 If-pressed assignment does not work in functions

[boryanagoncharenko]

Describe the bug In level 12, the following program outputs a instead of great or not great.

define turnz
    x is 'a'
    if x is pressed
        x is 'great'
    else
        x is 'not great'
    print x

call turnz

The trouble is that the if-pressed uses a global variable x while the function uses a local one. We could:

• change the function definition to use global variables, although this will have quite an impact on functions with arguments
• forbid if-pressed to be used in functions
• or?

[boryanagoncharenko]

Confirmed in the contributors meeting that functions should not use a global scope. This means that we are left with 2 options:

• Change the global scope to nonlocal scope. Unfortunately, this feature is not supported by Skulpt: https://stackoverflow.com/questions/62135489/how-do-i-fix-the-notimplementederror-in-python
• Forbid the usage of if-pressed in functions

[boryanagoncharenko]

Perhaps there is another way to support if-pressed statements in functions. We could put all variables in the current scope (global if not in a function or local if in a function) in a dictionary. Every variable definition, assignment or reference will just be substituted with access to the dictionary. In this way we will be able to bridge the gap between if-pressed definitions and their outer scope and still leave the local scope of functions intact. This is an example of using if-pressed not in a function:

__gs = dict()
__gs['var1'] = 1
__gs['var2'] = 10
def if_pressed_handler():
    __gs['var1'] = 2
    __gs['var2'] = __gs['var2'] + 1
    __gs['var3'] = 15
if_pressed_handler()
print(f'var1 is reassigned in if-pressed. Expected value is 2. Actual value is {__gs["var1"]}.')
print(f'var2 is accessed and reassigned in if-pressed. Expected value is 11. Actual value is {__gs["var2"]}.')
print(f'var3 is defined in if-pressed. Expected value is 15. Actual value is {__gs["var3"]}.')

And this is how if-pressed would appear in a function:

def turnz():
    __ls = dict()
    __ls['x'] = 'a'
    def if_pressed_x():
        __ls['x'] = 'great'
    def if_pressed_else():
        __ls['x'] = 'not great'
    if_pressed_x() # simulates pressing x
    print(__ls['x'])
turnz()

We could only use this approach if the program contains an if-pressed command. In this way we will limit the scope of the change. @jpelay what do you think? Do you see issues with this approach?

[jpelay]

Hi @boryanagoncharenko I think your idea is great, you're right with something like that I think we can sort the global scope situation. Although I tried this code on Skulpt and it didn't really work:

def turnz():
  __ls = dict()
  __ls['x'] = 'a'
  if_pressed_mapping = {"else": "if_pressed_default_else"}
  global if_pressed_x_
  if_pressed_mapping['x'] = 'if_pressed_x_'
  def if_pressed_x_():
    __ls['x'] = 'great'    
  global if_pressed_else_
  if_pressed_mapping['else'] = 'if_pressed_else_'
  def if_pressed_else_():    
    __ls['x'] = 'not great'      
  extensions.if_pressed(if_pressed_mapping)
  print(f'''{__ls['x']}''')
turnz()

Hopefully you can get it to work!

[jpelay]

The above snippet didn't work, because Skulpt doesn't like variables that start with double underscores, I think. Anyways, changing the snippet to this manages to get it working:

if_pressed_mapping = {"else": "if_pressed_default_else"}
def turnz():
  local_variables_xxxx = dict()
  local_variables_xxxx['x'] = 'a'
  if_pressed_mapping = {"else": "if_pressed_default_else"}
  global if_pressed_x_
  if_pressed_mapping['x'] = 'if_pressed_x_'
  def if_pressed_x_():
    local_variables_xxxx['x'] = 'great'
  global if_pressed_else_
  if_pressed_mapping['else'] = 'if_pressed_else_'
  def if_pressed_else_():    
    local_variables_xxxx['x'] = 'not great'      
  extensions.if_pressed(if_pressed_mapping)
  print(f'''{local_variables_xxxx['x']}''')
turnz()

Here's a video:

https://github.com/user-attachments/assets/e47fc43e-5f46-4d57-9a98-594a130fc443

I think the idea of isolating the dictionary to functions when using ifpressed is a good idea. How would it work with parameters and such? The same?

[boryanagoncharenko]

Thanks so much for pointing this out!

I have not thought about the double underscore prefix. My assumption is that Skulpt maintains everything in the global scope but it does not do much for local scopes. I tried looking into implementing support for the nonlocal keyword but it is hard because only the global scope seems to be accessible and correct.

My guess is that defining the __ls variable within the turnz function made it a local variable, so it was never added to the Sk.globals list and, in turn, could not be used in the if-pressed functions. The problem seems to be gone if (do not laugh please!) the local scope is defined globally.

global __ls
__ls = dict()
def turnz():
  __ls['x'] = 'a'
  if_pressed_mapping = {"else": "if_pressed_default_else"}
  global if_pressed_x_
  if_pressed_mapping['x'] = 'if_pressed_x_'
  def if_pressed_x_():
    global __ls
    __ls['x'] = 'great'
  global if_pressed_else_
  if_pressed_mapping['else'] = 'if_pressed_else_'
  def if_pressed_else_():
    global __ls
    __ls['x'] = 'not great'
  extensions.if_pressed(if_pressed_mapping)
  print(f'''{__ls['x']}''')
turnz()

Now the output becomes great / not great which is the correct behavior.

[boryanagoncharenko]

You are right, starting with double underscore basically Skulpt fail to create a closure from one local scope to another. Thanks, once again!

[jpelay]

You are right, starting with double underscore basically Skulpt fail to create a closure from one local scope to another. Thanks, once again!

Yes, it was a crazy bug!!!