List comprehension in if clause of another list comprehension

[Vedran Furac(]

Hi!

In [69]: a = 'a b c'
In [70]: b = 'a b, c d'

In [74]: [i for i in a.split() if i not in b.split()]
Out[74]: ['b']

Everything ok.

In [77]: b.split() == [i for i in b.split()]
Out[77]: True

As expected. Now, put this in the first list comprehension:

In [80]: [i for i in a.split() if i not in [i for i in b.split()] ]
Out[80]: ['d']

Hmmmm... why is that?

Regards!

 

[Peter Otten]

Hi!

In [69]: a = 'a b c'
In [70]: b = 'a b, c d'

In [74]: [i for i in a.split() if i not in b.split()]
Out[74]: ['b']

Everything ok.

In [77]: b.split() == [i for i in b.split()]
Out[77]: True

As expected. Now, put this in the first list comprehension:

In [80]: [i for i in a.split() if i not in [i for i in b.split()] ]
Out[80]: ['d']

Hmmmm... why is that?

a = "abc"
b = "a b, c d".split()
b ['a', 'b,', 'c', 'd']
[i for i in a if i not in b]

['b']

As expected, so far, but now comes the critical step:

[i for i in a if i not in [i for i in b]]

['d']

The problem is that list comprehensions do not introduce a new namespace. So
the inner and the outer list comp share the same i. You can either rename
the inner i

[i for i in a if i not in [k for k in b]]

['b']

or use a generator expression which does give a new namespace:

list(x for x in "abc") ['a', 'b', 'c']
x

Traceback (most recent call last):

[i for i in a if i not in (i for i in b)]

['b']

Peter

 

[Vedran Furac(]

The problem is that list comprehensions do not introduce a new namespace. So
the inner and the outer list comp share the same i. You can either rename
the inner i

[i for i in a if i not in [k for k in b]]

['b']

or use a generator expression which does give a new namespace:

list(x for x in "abc") ['a', 'b', 'c']
x

Traceback (most recent call last):

[i for i in a if i not in (i for i in b)]

['b']

Thanks!

 

[bearophileHUGS]

Peter Otten:

The problem is that list comprehensions do not introduce a new namespace.

I think Python3 fixes this bug.

Bye,
bearophile

 

[Marc 'BlackJack' Rintsch]

Peter Otten:

I think Python3 fixes this bug.

Or removes that feature. ;-)