[Tutor] The Boolean operator "and"

[Glen Wheeler]

> I have a question about one sentence in the fine Python book I'm
> presently studying, Practical Python, by Magnus Lie Hetland (Apress). The
> sentence is "Actually, if x is false, it returns x--otherwise it returns
> y." This sentence is in the paragraph on p. 109 that begins,
>
> 'The Boolean operators have one interesting property: They only evaluate
> what they need to. For instance, the expression  x and y  requires both x
> and y to be true; so if x is false, the expression returns false
> immediately, without worrying about y. Actually, if x is false, it
> returns x--otherwise it returns y. (Can you see how this give the
> expected meaning?) This behavior is called short-circuit logic: the
> Boolean operators are often called logical operators, and as you can see,
> the second value is sometimes "short-circuited." This works with or, too.
> In the expression x or y, if x is true, it is returned, otherwise y is
> returned. (Can you see how this makes sense?)'
>
> In the sentence I'm questioning, "Actually, if x is false, it returns
> x--otherwise it returns y", it seems to me that if x evaluates to False,
> False is returned, not x itself, (which might be "3 > 4").
>
> Am I confused? Or quibbling?
>

  I'd say a little bit of both ;).

  Mr Hetland is (of course) correct in his statement you quote above.  The
key part is where he states ``if x is false''.  This is not ``x evaluates to
False''.

  A quick interpreter session to hopefully clarify what he is trying to say:

>>> x = 3 < 4
>>> x
True
>>> x is True
True
# Note how x actually *is* True
>>> x is False
False
>>>

  The point is that any logical expression is not stored as a string, but as
either True or False.  This is neat, since it implies that the expected
value of any logical expression will be True or False.  Thus, we have if
statements.
  Any logical statement or expression is evaluated before assignment to the
variable holding that statement actually occurs.

>>> x = 1 > 0
>>> y = 1 < 0
>>> x and y
False
>>> x or y
True
>>> x is False
False
>>> x is True
True

  I hope this helps clear things up.

  Glen