How to limit the size of a comprehension?

I have a list and want to build (via a comprehension) another list. I would like this new list to be limited in size, via a condition

The following code will fail:

a = [1, 2, 1, 2, 1, 2]
b = [i for i in a if i == 1 and len(b) < 3]

with

Traceback (most recent call last):
  File "compr.py", line 2, in <module>
    b = [i for i in a if i == 1 and len(b) < 3]
  File "compr.py", line 2, in <listcomp>
    b = [i for i in a if i == 1 and len(b) < 3]
NameError: name 'b' is not defined

because b is not defined yet at the time the comprehension is built.

Is there a way to limit the size of the new list at build time?

Note: I could break the comprehension into a for loop with the proper break when a counter is reached but I would like to know if there is a mechanism which uses a comprehension.

Solution

You can use a generator expression to do the filtering, then use islice() to limit the number of iterations:

from itertools import islice

filtered = (i for i in a if i == 1)
b = list(islice(filtered, 3))

This ensures you don't do more work than you have to to produce those 3 elements.

Note that there is no point anymore in using a list comprehension here; a list comprehension can't be broken out of, you are locked into iterating to the end.