# Issue

*This Content is from Stack Overflow. Question asked by mins *

I’d like to build one of these lists of tuples:

`(a, 0), (-a, 0) (b, 0), (-b, 0)`

`(0, a), (0, -a) (0, b), (0, -b)`

from scalars `a`

and `b`

.

based on a condition:

`c = a > b`

This is my attempt:

```
a = 5
b = 2
c = a > b
# Try build two tuples per element, e.g. (5, 0), (-5, 0) (2, 0), (-2, 0)
# This syntax is illegal
#f2 = [(m,0), (-m,0) if c else (0,m), (-0,-m) for m in (a,b)]
# This syntax works but creates tuples of tuples
f2 = [tuple(((m,0), (-m,0))) if c else tuple(((0,m), (-0,-m))) for m in (a,b)]
print(*f2) # ((5, 0), (-5, 0)) ((2, 0), (-2, 0))
# This syntax is illegal
#f3 = [*tuple(((m,0), (-m,0))) if c else *tuple(((0,m), (-0,-m))) for m in (a,b)]
#print(*f3)
```

`f2`

builds a list of two tuples of two tuples: `((5, 0), (-5, 0)) ((2, 0), (-2, 0))`

.

Using `*`

operator in `f3`

to unpack the outer tuples triggers a syntax error.

What is the correct syntax?

# Solution

You can do it with lambda functions.

```
x1 = lambda x : (x,0)
y1 = lambda x : (-x,0)
x2 = lambda x : (0,x)
y2 = lambda x : (-0,-x)
f2 = [f1(m) if c else f2(m) for m in (a,b) for f1,f2 in zip((x1,y1),(x2,y2))]
```

Output:

`[(5, 0), (-5, 0), (2, 0), (-2, 0)]`

A little bit over-engineered but makes sense.

*EDIT*

Also you can use chain from itertools to put 2 items in a single list comprehension.

```
from itertools import chain
f2 = list(
chain.from_iterable(
((m,0), (-m,0)) if c else ((0,m), (-0,-m)) for m in (a,b)
))
```

This Question was asked in StackOverflow by mins and Answered by Revuimar It is licensed under the terms of CC BY-SA 2.5. - CC BY-SA 3.0. - CC BY-SA 4.0.