If I try

>>> from pylab import *
>>> b = [2, 3, 4, 5, 6, 7]
>>> a = any(x < 0 for x in b)  
>>> print(a)

it doesn't return True or False.

It returns

<generator object <genexpr> at 0x7fbd62129ab0>

You are using a numpy.any() instead of the built-in any(). Most probably you have from numpy import any or from numpy import *, which causes this behavior.

Why that happens?

According to the documentation, any tests if any element evaluates the condition. However, if you look into the source code, it actually returns a asanarray() result which is a generator.

How to avoid it?

It is always a good idea to import only scope rather than the method itself, like so: import numpy as np

:)

UPDATE 1

Personally, I have never used iPython, but thanks to comments by @Praveen and @hpaulj, if you use --pylab flag with ipython, you will see the same behavior, and you can turn that behavior off - never knew it! :)))