Demonstration and workaround of Python bug 4806

issue4806.py

# written in python2.7


def bad(n):
    if n == 1:
        raise ValueError('n is 1!')
    if n == 2:
        raise TypeError('n is 2!')
    return n, 2*n


def bads(vs):
    for v in vs:
        yield bad(v)


print "using zip(*gen)"

a, b = zip(*bads([3]))
assert a == (3,)
assert b == (6,)
print "a and b as expected"

try:
    a, b = zip(*bads([1]))
except Exception as e:
    print repr(e)

try:
    a, b = zip(*bads([2]))
except Exception as e:
    print repr(e)

print "\nusing zip(*list(gen))"

a, b = zip(*list(bads([3])))
assert a == (3,)
assert b == (6,)
print "a and b as expected"

try:
    a, b = zip(*list(bads([1])))
except Exception as e:
    print repr(e)

try:
    a, b = zip(*list(bads([2])))
except Exception as e:
    print repr(e)

output.txt

using zip(*gen)
a and b as expected
ValueError('n is 1!',)
TypeError('zip() argument after * must be a sequence, not generator',)

using zip(*list(gen))
a and b as expected
ValueError('n is 1!',)
TypeError('n is 2!',)


===========
The moral is: convert calls from zip(*gen) to zip(*list(gen)) to get the correct errors
propagating up. It's a shame it has to be done but until http://bugs.python.org/issue4806
is fixed there is no alternative!