Example of list comprehensions doesn't work

[DeaD_EyE]

You get a

Error:
UnboundLocalError: local variable 'z' referenced before assignment

The cause is, you're referencing z before it has been assigned.
In this case it doesn't matter, that the name z is global.
In the context of the list comprehension the name z is a local variable, because it's assigned.
You may ask now, where the assignment happens:

for z in range(z+1)

The red one, is the assignment of local variable z. The green is read access to the variable.
Just change the z into z_ and the code works.

print([[x,y,z_] for i in range(x+1) for j in range(y+1) for z_ in range(z+1)])

[Truman]

@micseydel
At first I used nested loops but I read somewhere that that using nested loops in Python is considered a garbage code

@DeaD_EyE
Thank you, I completely missed that.

print([[x,y,z] for i in range(x+1) for j in range(y+1) for k in range(z+1)  if i+j+k != n])

works fine.

p.s. checking again it doesn't work as I expected.
The result is:

Output:
[[1, 1, 1], [1, 1, 1], [1, 1, 1], [1, 1, 1], [1, 1, 1]]

the input was: 1 1 1 2

[micseydel]

At first I used nested loops but I read somewhere that that using nested loops in Python is considered a garbage code

That doesn't make any sense to me. I'm a professional software engineer and if I saw that complicated of a comprehension during code review, I'd have them break it up. I had a Google interviewer suggest I not use a comprehension for something that would have been less complicated than that.

Comprehensions are an intermediate feature so there's some prestige associated with them I suppose, but you should use the right tool for the right job, and code at your level - as I said I don't think you'd have this problem with a plain old nested loop, so I suggest you stick to simpler concepts until you have more mastery. Even if you prefer to keep using comprehensions, I'd suggest in situations like this where you're struggling to debug the comprehension, break it up anyway. If you need to break it up to debug it, it probably should be broken up, but personally even if I want it to ultimately be a comprehension I'll simplify the code for debugging.

[Truman]

Thank you.

I would appreciate anyone's opinion on why my previous line of code doesn't work. It doesn't look complicated but still it doesn't give me the output that I want. Comprehension with x and y works fine, but the one with x,y and z doesn't although it's the same logic.

[wavic]

What you are doing is to sacrifice the readability of the code for some desire to be pythonic. Pythonic means just that, the right tool in the right place. As @micseydel mentioned it already.
One reason why Python has this popularity is that it's easy to read. And to write. You are struggling to write one piece of code. It should not be that way. If it's hard for you to write it, it will be hard for someone else to read it.

[buran]

print([[x,y,z] for i in range(x+1) for j in range(y+1) for k in range(z+1) if i+j+k != n])

I would appreciate anyone's opinion on why my previous line of code doesn't work. It doesn't look complicated but still it doesn't give me the output that I want.

well, that's the correct output.
Probably you were thinking of writing

print([[i,j,k] for i in range(x+1) for j in range(y+1) for k in range(z+1)  if i+j+k != n])

[volcano63]

print([[i,j,k] for i in range(x+1) for j in range(y+1) for k in range(z+1)  if i+j+k != n])

Being an extremely lazy person , I would have written that as

[coords for coords in itertools.product(range(x+1), range(y+1), range(z+1)) 
 if sum(coords) != n]

I know that it is actually longer - but I really hate one-letter variables (x, y and z I can forgive )

[buran]

I fully agree regarding one-letter variable names :-)