List comprehension

[Uchikago]

Please explain why this doesn't work

seq=[1,2,3]
def a(): return x+2
x=[a() for x in seq]

I thought a list comprehension create a new local scope for the expression itself, and when we use a(), x is searched by the LEGB rule and can find the x in the enclosing scope, am i correct ?

[perfringo]

Following is friendly banter. No offence intended.

These repeated questions reminded me 'sketch' from Three Men in a Boat by Jerome K Jerome.

One can read whole episode here, but for immediate usage I provide prescription:

1 lb. beefsteak, with
1 pt. bitter beer
every 6 hours.
1 ten-mile walk every morning.
1 bed at 11 sharp every night.
And don’t stuff up your head with things you don’t understand.

As for question: it's always good practice to use functions with parameters.

>>> seq = [1, 2, 3]                                                        
>>> def a(x): return x + 2                                                 
>>> lst = [a(num) for num in seq]                                          
>>> lst                                                                    
[3, 4, 5]

[ThomasL]

Please explain why this doesn't work

seq=[1,2,3]
def a(): return x+2
x=[a() for x in seq]

Sorry but this code is complete nonsense and that´s the reason it´s not working.
You definitely should watch some tutorial, e.g. this one here.

[scidam]

Not sure, is this question very simple, or very tricky...So, lets do some examples here.
First of all, lets start with the following example:

def a():
    return x + 2

for x in range(10):
    print(a(), globals().get('x'))

Output:
2 0
3 1
4 2
5 3
6 4
7 5
8 6
9 7
10 8
11 9

Everthing works as expected: for loop creates x-values in the global scope, and the variable x is visible from the a()-function.

List comprehensions are functions too. To show this, lets consider the following bytecode, e.g.

import dis
dis.dis("[x for x in range(5, dont_execute_me='I am in op-code')]")

Output:
  1           0 LOAD_CONST               0 (<code object <listcomp> at 0x7f8a0d898ed0, file "<dis>", line 1>)
              3 LOAD_CONST               1 ('<listcomp>')
              6 MAKE_FUNCTION            0
              9 LOAD_NAME                0 (range)
             12 LOAD_CONST               2 (5)
             15 LOAD_CONST               3 ('dont_execute_me')
             18 LOAD_CONST               4 ('I am in op-code')
             21 CALL_FUNCTION          257 (1 positional, 1 keyword pair)
             24 GET_ITER
             25 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             28 RETURN_VALUE

Line #25 executes list comprehension-function; line #21 creates range;

How to make it works...

We just need to declare something like this: [a() for x in range(5); global x]; but this isn't allowed by Python syntax.

Nevertheless, we can do some injection, e.g.

[(globals().update({"x":x}),  a())[-1] for x in range(3)]

Output:
[2, 3, 4]

Everything works fine! This is just for fun, don't do this in any valuable code.