Question About Logic In Python

[James H.]

Greetings! I'm new to Python and am struggling a little with "and" and
"or" logic in Python. Since Python always ends up returning a value
and this is a little different from C, the language I understand best
(i.e. C returns non-zero as true, and zero as false), is there anything
I should be aware of given Python's different approach? Namely any
pitfalls or neat tricks that make the Python approach cool or save my
butt.

Thank you!

James

[Dan Bishop]

James H. wrote:

Greetings! I'm new to Python and am struggling a little with "and" and
"or" logic in Python. Since Python always ends up returning a value
and this is a little different from C, the language I understand best
(i.e. C returns non-zero as true, and zero as false), is there anything
I should be aware of given Python's different approach? Namely any
pitfalls or neat tricks that make the Python approach cool or save my
butt.

The most common use of this feature is "x = x or default_val" as a
shorthand for "if not x: x = default_val".

Also, you can emulate C's ternary operator (cond ? t : f) with
(cond and [t] or [f])[0].

[mensanator]

James H. wrote:

Greetings! I'm new to Python and am struggling a little with "and" and
"or" logic in Python. Since Python always ends up returning a value
and this is a little different from C, the language I understand best
(i.e. C returns non-zero as true, and zero as false), is there anything
I should be aware of given Python's different approach? Namely any
pitfalls or neat tricks that make the Python approach cool or save my
butt.

Thank you!

James

Booleans are a subtype of plain integers, so if you use them
in arithmetic expressions, they evaluate to 0 and 1.

def bool(): print "Boolean algebra"
print "%9s %9s | %9s %9s %9s" % ('A','B','A and B','A or B','A xor B')
print "-"*51
for a in [False,True]:
for b in [False,True]:
print "%9s %9s | %9s %9s %9s" % (a,b,(a and b),(a or b),((a and not
b) or (not a and b)))
print
print
print "Arithmetic "
print "%9s %9s | %9s %9s %9s" % ('A','B','A + B','A * B','A - B')
print "-"*51
for a in [False,True]:
for b in [False,True]:
print "%9s %9s | %9s %9s %9s" % (a,b,(a + b),(a * b),(a - b))

bool()

Boolean algebra
A B | A and B A or B A xor B
---------------------------------------------------
False False | False False False
False True | False True True
True False | False True True
True True | True True False
Arithmetic
A B | A + B A * B A - B
---------------------------------------------------
False False | 0 0 0
False True | 1 0 -1
True False | 1 0 1
True True | 2 1 0

[sven]

At 02:20 19.09.2005, James H. wrote:

Greetings! I'm new to Python and am struggling a little with "and" and
"or" logic in Python. Since Python always ends up returning a value
and this is a little different from C, the language I understand best
(i.e. C returns non-zero as true, and zero as false), is there anything
I should be aware of given Python's different approach? Namely any
pitfalls or neat tricks that make the Python approach cool or save my
butt.

to make sure that an operation yields a boolean value wrap a bool()
around an expression.
None, 0 and objects which's len is 0 yield False.
so you can also do stuff like that:

a = []
b = [1,2,3]
a or b [1, 2, 3]
class Foo: .... def __len__(self): return 0
.... class Bar: .... def __len__(self): return 1
.... foo = Foo()
bar = Bar()
foo or bar

<__main__.Bar instance at 0x7D289940>

sven.

[Steven D'Aprano]

On Mon, 19 Sep 2005 12:16:15 +0200, sven wrote:

to make sure that an operation yields a boolean value wrap a bool()
around an expression.
None, 0 and objects which's len is 0 yield False.
so you can also do stuff like that:

Are there actually any usage cases for *needing* a Boolean value? Any
object can be used for truth testing, eg:

if the_str

is to be preferred over:

if bool(the_str)

or even worse:

if bool(the_str != "")

Or wait, I have thought of one usage case: if you are returning a value
that you know will be used only as a flag, you should convert it into a
bool. Are there any other uses for bool()?

--
Steven.

[Bengt Richter]

On Mon, 19 Sep 2005 23:46:05 +1000, Steven D'Aprano <st***@REMOVETH IScyber.com.au> wrote:

On Mon, 19 Sep 2005 12:16:15 +0200, sven wrote:

to make sure that an operation yields a boolean value wrap a bool()
around an expression.
None, 0 and objects which's len is 0 yield False.
so you can also do stuff like that:

Are there actually any usage cases for *needing* a Boolean value? Any
object can be used for truth testing, eg:

if the_str

is to be preferred over:

if bool(the_str)

or even worse:

if bool(the_str != "")

Or wait, I have thought of one usage case: if you are returning a value
that you know will be used only as a flag, you should convert it into a
bool. Are there any other uses for bool()?

making an index (it's an int subclass), as in

things = None, 0, 1, 0.0, 5.0, '', 'a', [], [1], {}, {1:2}
for thing in things:

... print 'if %-6r would act like if %s' % (thing, ('False','True' )[bool(thing)])
...
if None would act like if False
if 0 would act like if False
if 1 would act like if True
if 0.0 would act like if False
if 5.0 would act like if True
if '' would act like if False
if 'a' would act like if True
if [] would act like if False
if [1] would act like if True
if {} would act like if False
if {1: 2} would act like if True

Regards,
Bengt Richter

[Ron Adam]

Steven D'Aprano wrote:

On Mon, 19 Sep 2005 12:16:15 +0200, sven wrote:

to make sure that an operation yields a boolean value wrap a bool()
around an expression.
None, 0 and objects which's len is 0 yield False.
so you can also do stuff like that:

Are there actually any usage cases for *needing* a Boolean value? Any
object can be used for truth testing, eg:

if the_str

is to be preferred over:

if bool(the_str)

or even worse:

if bool(the_str != "")

Or wait, I have thought of one usage case: if you are returning a value
that you know will be used only as a flag, you should convert it into a
bool. Are there any other uses for bool()?

Of course if any of the default False or True conditions are
inconsistent with the logic you use, you need to do explicit truth testing.

if val > -1:

Where 0 would be True condition.
if arg != None:

Where '' could be a True condition.
Also... you need to be careful what order you do your comparisons in as..

(1 and 2) != (2 and 1) # they are both True, but not equal.

bool(1 and 2) == bool(2 and 1)

(1 and 2) * value != (2 and 1) * value
# except if value is False.

bool(1 and 2) * value == bool(2 and 1) * value
So..

bool(a and b) * value

Would return value or zero, which is usually what I want when I do this
type of expression.

Cheers,
Ron

[Steven D'Aprano]

On Mon, 19 Sep 2005 22:31:05 +0000, Bengt Richter wrote:

On Mon, 19 Sep 2005 23:46:05 +1000, Steven D'Aprano <st***@REMOVETH IScyber.com.au> wrote:

Are there actually any usage cases for *needing* a Boolean value? Any
object can be used for truth testing, eg:
[snip]
making an index (it's an int subclass), as in
>>> things = None, 0, 1, 0.0, 5.0, '', 'a', [], [1], {}, {1:2}
>>> for thing in things:

... print 'if %-6r would act like if %s' % (thing, ('False','True' )[bool(thing)])
...

That's a pretty artificial example though. Your general index ranges from
0 to n inclusive, where n is unlikely to be 1. That limits the usefulness
of the idiom sequence_or_map ping[bool(thing)] to a tiny set of cases.

As near as I can tell, explicitly converting objects to booleans is mostly
useful for demonstrating that booleans aren't needed for truth testing.
--
Steven.

[Steven D'Aprano]

On Tue, 20 Sep 2005 03:03:15 +0000, Ron Adam wrote:

Steven D'Aprano wrote:

Are there actually any usage cases for *needing* a Boolean value? Any
object can be used for truth testing, eg:

[snip]
Of course if any of the default False or True conditions are
inconsistent with the logic you use, you need to do explicit truth testing.
[snip]
So..

bool(a and b) * value

Would return value or zero, which is usually what I want when I do this
type of expression.

That's all very interesting, and valuable advice for somebody who doesn't
understand how Python's logical operators work, but the question is, when
would you actually want that type of expression?

In practice, how often do you really care that your truth values have the
specific values 0 and 1 rather than anything false and anything true? In
what circumstances?

--
Steven.

[Bengt Richter]

On Wed, 21 Sep 2005 09:03:00 +1000, Steven D'Aprano <st***@REMOVETH IScyber.com.au> wrote:

On Tue, 20 Sep 2005 03:03:15 +0000, Ron Adam wrote:

Steven D'Aprano wrote:

Are there actually any usage cases for *needing* a Boolean value? Any
object can be used for truth testing, eg:

[snip]

Of course if any of the default False or True conditions are
inconsistent with the logic you use, you need to do explicit truth testing.

[snip]

So..

bool(a and b) * value

Would return value or zero, which is usually what I want when I do this
type of expression.

That's all very interesting, and valuable advice for somebody who doesn't
understand how Python's logical operators work, but the question is, when
would you actually want that type of expression?

In practice, how often do you really care that your truth values have the
specific values 0 and 1 rather than anything false and anything true? In
what circumstances?

When you want to use the value as an index fed to something that has a
__getitem__ for which only the values 0 and 1 are valid, e.g., a list
or tuple of length 2, as I tried to illustrate before ;-)

Also, since values 0 and 1 are the values of a bit, you can shift it
and create a mask that encodes many logical values at once, which can
be handy for truth table stuff or perhaps indexing a 2**nbits table
rather than using a tree of nested if/elses to select values.

BTW, you asked
"Are there actually any usage cases for *needing* a Boolean value?"
^^^ ;-)
AFAIK, "one" is enough to make the answer "yes" ;-)

Of course you can use other expressions than bool(x) to get the boolean
value, but you may have to think more about whether (x and 1) will
do it, or whether you should write (x!=0) or, in case x can be None,
perhaps settle on (x and 1 or 0) as an idiom to play safe.
Well, bool(x) is safe, and less typing ;-) OTOH, it's not a hammer for all nails.

Regards,
Bengt Richter