Nested list comprehensions

[neildunn]

Hey guys:

[(i,j,k) for i in range(1,j) for j in range(1,k) for k in range(1,5)]

[(1, 1, 1), (1, 1, 2), (1, 1, 3), (1, 1, 4), (1, 2, 1), (1, 2, 2), (1,
2, 3), (1, 2, 4), (1, 3, 1), (1, 3, 2), (1, 3, 3), (1, 3, 4), (2, 1,
1), (2, 1, 2), (2, 1, 3), (2, 1, 4), (2, 2, 1), (2, 2, 2), (2, 2, 3),
(2, 2, 4), (2, 3, 1), (2, 3, 2), (2, 3, 3), (2, 3, 4)].... print [(j,k) for j in range(1,k) for k in range(1,5)]
....Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 2, in a
UnboundLocalError: local variable 'k' referenced before assignment

Why is it that I can execute the nested list comprehension in the
intepreter
but if the same line is within a method declaration I get an unbound
reference error?

Is this a bug or am I missing some deep rule?

Regards, Neil Dunn

 

[Carl Waldbieser]

Hey guys:

[(i,j,k) for i in range(1,j) for j in range(1,k) for k in range(1,5)]

[(1, 1, 1), (1, 1, 2), (1, 1, 3), (1, 1, 4), (1, 2, 1), (1, 2, 2), (1,
2, 3), (1, 2, 4), (1, 3, 1), (1, 3, 2), (1, 3, 3), (1, 3, 4), (2, 1,
1), (2, 1, 2), (2, 1, 3), (2, 1, 4), (2, 2, 1), (2, 2, 2), (2, 2, 3),
(2, 2, 4), (2, 3, 1), (2, 3, 2), (2, 3, 3), (2, 3, 4)]... print [(j,k) for j in range(1,k) for k in range(1,5)]
...Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 2, in a
UnboundLocalError: local variable 'k' referenced before assignment

Why is it that I can execute the nested list comprehension in the
intepreter
but if the same line is within a method declaration I get an unbound
reference error?

Is this a bug or am I missing some deep rule?

Regards, Neil Dunn

You may not be getting the name error in the interpreter because you already
defined those variables. Try this:

$ python

[(j,k) for j in range(1,k) for k in range(1,5)]

Traceback (most recent call last):

Looks like the variable was not defined here, either.

You are trying to use the variable "k" in the range() function before it has
been bound to anything, hence the error.

 

[Mike Meyer]

Hey guys:

[(i,j,k) for i in range(1,j) for j in range(1,k) for k in range(1,5)]

[(1, 1, 1), (1, 1, 2), (1, 1, 3), (1, 1, 4), (1, 2, 1), (1, 2, 2), (1,
2, 3), (1, 2, 4), (1, 3, 1), (1, 3, 2), (1, 3, 3), (1, 3, 4), (2, 1,
1), (2, 1, 2), (2, 1, 3), (2, 1, 4), (2, 2, 1), (2, 2, 2), (2, 2, 3),
(2, 2, 4), (2, 3, 1), (2, 3, 2), (2, 3, 3), (2, 3, 4)]

Give this a careful look: i varies slowest, and takes on the values 1
and 2. j varies next slowest, and takes on the values range(1, 4) in
both the i loops. k varies fastest, and goes through range(1, 5) in
all loops. I don't think it's doing what you intend.

... print [(j,k) for j in range(1,k) for k in range(1,5)]
...Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 2, in a
UnboundLocalError: local variable 'k' referenced before assignment

Why is it that I can execute the nested list comprehension in the
intepreter
but if the same line is within a method declaration I get an unbound
reference error?

Is this a bug or am I missing some deep rule?

What are the values of the global variables j and k before you enter
the loop? Wouldn't happen to be 2 and 4, would they? Deleting them
first gives different results:

del j
del k
[(i,j,k) for i in range(1,j) for j in range(1,k) for k in range(1,5)]

Traceback (most recent call last):
File "<stdin>", line 1, in ?
NameError: name 'j' is not defined

Methinks you want your for loops in the opposite order.

<mike