Python: Nested list comprehension / Flatten, no access in transform to outer list

Question

(I Hope this is not a duplicate, but other questions are too many and badly named to find if so...)

(Tested in Python > 3, Solution - if any - must work in >= 2.7)

Assuming we have a list:

l = [[1,2], [3,4], [5,6]]

We attempt to flatten it using the standard list comprehension method:

[v for sl in l for v in sl] == [1,2,3,4,5,6]

Output:

True

We then attempt:

[(v, s1[0]) for sl in l for v in sl]

Which causes:

NameError: name 's1' is not defined

Why does this happen? Isn't s1 supposed to be accessible given that the for loops are evaluated as the loop should be evaluated as:

[<expression> for <outer> in <list> for <inner> in <outer>]

Or in other words:

r = []
for <outer> in <list>:
   for <inner> in <outer>:
      r.append(<inner>)

What am I missing?

Answer 1

It appears you are mixing up the names sl ("ess-ell") vs s1 ("ess-one").

As an addendum, you should consider switching to a programmer-specific font which emphasizes the differences between otherwise potentially ambiguous characters. One popular option is Source Code Pro.