To get a better output of unicode, instead of “\uxxx”, and also to follow the fastion, I was upgrading my server to Python 3.

I got weird result from a loop applying multiple filters. Example code:

n = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
ranges = ((3, 8), (6, 10))

for i in ranges:
    n = filter(lambda x: x > i[0] and x < i[1], n)
print(list(n))

ranges clamps n, so I was expecting the result to be [7], but it gave me [7, 8, 9], which was the result of the second range.

I investigated along, and found the filter returns an iterator in Python 3, instead of a list in Python 2. This means filter would be lazy evaluated, which causes a series of results.

Lazy evaluation means the parameters used in the filter function would be fetched from the scope when the filter function is executed. In the above example, i would be (6, 10) in both loops, as well as n be the last iterator. To demonstrate:

n = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
ranges = ((3, 8), (6, 10))


def f(x, i, n):
    print("I'm apply the filter to {} by {}".format(n, i))
    return x > i[0] and x < i[1]


for i in ranges:
    n = filter(lambda x: f(x, i, n), n)
print(list(n))

And I got:

I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)
I'm applying the filter to <filter object at 0x105849048> by (6, 10)

So in fact, n was clamped by (6, 10) twice.

This “problem” is described well in Problems with nested scopes and lazy evaluation. Well, that’s another topic.

I might need to follow 2to3 to replace filter with list comprehension.

It is those “surprises” that took me hours to debug to find out the reason, kind of frustrated me. I wish to upgrade, to follow the latest fasion, but to change is not fit to human’s nature, at least not to mine. I’m adapting to those details, and looking for an alternative. Rust is the new superstar, Nim also looks interesting.