In Python 2.x the loop variable is part of the containing scope:

  >>> a
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'a' is not defined
  >>> [a*2 for a in (1,2,3)]
  [2, 4, 6]
  >>> a
  3

In Python 3.x, it is not:

  >>> a
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'a' is not defined
  >>> [a*2 for a in (1,2,3)]
  [2, 4, 6]
  >>> a
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  NameError: name 'a' is not defined

It is sometimes useful to know why something failed. The following Python 2 code will not work in Python 3:

  >>> import math
  >>> try:
  ...   values = [math.sqrt(x) for x in (1, 2, -3, 4)]
  ... except ValueError:
  ...   print("cannot compute sqrt(%r)" % x)
  ...
  cannot compute sqrt(-3)

I have mostly found this feature of Python 2 to be useful on the interactive shell, when trying to diagnose what caused an error in my list comprehension. Eg.

  >>> with open("tmp.dat") as input_file:
  ...   fields = [line.split()[3] for line in input_file]
  ...
  Traceback (most recent call last):
    File "<stdin>", line 2, in <module>
  IndexError: list index out of range
  >>> line
  'one two three\n'

Oops, there are only 3 columns, not 4.

When you use iPython you can just drop into the context of the error with %debug after it occurred, and then print the line there. No need for the bleeding scope on the language level.

In Python 3.8 you will be able to use assignment expressions:

    [a := x*2 for x in (1, 2, 3)]

See PEP 572 -- Assignment Expressions: https://www.python.org/dev/peps/pep-0572/

I’m the same, I would never use that side effect in production code but I do miss having it in ipython though my use is slight different.

I often did:

    >>> [d for d in drawings]
    >>> [d.<tab> ...]

Because I’m lazy and I want the autocomplete to be there for me. It’s actually the only thing I miss from python 2 and I will admit that a couple of times the sideffects have caught me out in production code so I’m happy with the change.

It’s worth noting that the latest python pep is going to give you a way to assign the variables in the outer scope again:

https://www.python.org/dev/peps/pep-0572/#simplifying-list-c...

In Python 2, "[x for x in range(10)]; print(x)" prints 9. x is available outside the list comprehension.

In Python 3, it raises a NameError, because the x in "print(x)" is not defined. It's contained in the list comprehension.

But I don't know a good use of the Python 2 behavior.