List comprehension

Steve D'Aprano steve+python at pearwood.info
Fri Dec 30 19:37:55 EST 2016


On Sat, 31 Dec 2016 06:37 am, Jason Friedman wrote:

> $ python
> Python 3.6.0 (default, Dec 26 2016, 18:23:08)
> [GCC 4.8.4] on linux
> Type "help", "copyright", "credits" or "license" for more information.
>>>> data = (
> ... (1,2),
> ... (3,4),
> ... )
>>>> [a for a in data]
> [(1, 2), (3, 4)]
> 
> Now, this puzzles me:
> 
>>>> [x,y for a in data]
>   File "<stdin>", line 1
>     [x,y for a in data]
>            ^
> SyntaxError: invalid syntax
> 
> I expected:
> [(1, 2), (3, 4)]


Why would you expect that? I would expect the global variables x and y, or
if they don't exist, a NameError:

py> a = (1, 2)
py> x, y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

Python has no magic that "x and y mean the first two items of a".


Instead, you get a syntax error because the syntax is ambiguous:

[x,y for a in data]

looks like a list

[x, y]

except the second item is a syntax error "y for a in data". So let's fix the
syntax error:



py> data = [(1, 2), (3, 4)]
py> [a for a in data]
[(1, 2), (3, 4)]
py> [(x, y) for a in data]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
NameError: name 'x' is not defined


Okay, let's create global variables x and y:

py> x = 98
py> y = 99
py> [(x, y) for a in data]
[(98, 99), (98, 99)]


No surprises there. How about this instead:

py> [(x, y) for (x, y) in data]
[(1, 2), (3, 4)]


That's better!




-- 
Steve
“Cheer up,” they said, “things could be worse.” So I cheered up, and sure
enough, things got worse.



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