I just ran into a weird bug in my code due to a typo where I had used a comma in a conditional rather than an `and` keyword. See the code below for a watered down example:

```
def foo():
    s = set([1, 2, 3])
    a = 4
    b = 5
    if (a, b in s):      # Should have been: if (a and b in s):
        print "Foo"

foo()  # prints "Foo"

```

Why does the conditional evaluate to `True`? Even if `a` is `None`, "Foo" gets printed.