numpy - Python builtin "all" with generators

Question

I have the following problem with python's "all" and generators:

G = (a for a in [0,1])
all(list(G))   # returns False - as I expected

But:

G = (a for a in [0,1])
all(G)         # returns True!

Can anybody explain that?

UPDATE: I swear I get this! Check this out:

In [1]: G = (a for a in [0,1])

In [2]: all(G)
Out[2]: True

I am using Python 2.6.6 with IPython 0.10.2, all installed within the Python(x,y) package. Strangely enough, I get "True" (above) when I use the Spider IDE, and "False" in pure console...

UPDATE 2: As DSM pointed out, it seems to be a numpy problem. Python(x,y) loads numpy, and all(G) was actually calling numpy.all(G) rather then the builtin all(). A quick workaround is to write:

__builtins__.all(G)

Thank you all for your help!

-maciej

Answer 1

Aha!

Does Python(x,y) happen to import numpy? [It looks like it.]

Python 2.7.2 (v2.7.2:8527427914a2, Jun 11 2011, 15:22:34) 
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> 
>>> 
>>> G = (a for a in [0,1])
>>> all(G)
False
>>> from numpy import all
>>> 
>>> G = (a for a in [0,1])
>>> all(G)
True
>>> 

Here's an explanation by Robert Kern:

It [all --ed] works on arrays and things it can turn into arrays by calling the C API equivalent of numpy.asarray(). There's a ton of magic and special cases in asarray() in order to interpret nested Python sequences as arrays. That magic works fairly well when we have sequences with known lengths; it fails utterly when given an arbitrary iterator of unknown length. So we punt. Unfortunately, what happens then is that asarray() sees an object that it can't interpret as a sequence to turn into a real array, so it makes a rank-0 array with the iterator object as the value. This evaluates to True.