while var, but var ==16 != true

[maestro]

why does this work? "while p" = "while p != 0" ? 1 is True and 0 is
false in python but other numbers have no boolean value so why doesnt
it abort.

>>p=16
>>p

16

>>while p:

print p
p -= 1


16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1

>>>

i can also do:

>>k=[]
>>while k:

k.pop()

>>k=[1,2,3]
>>while k:

k.pop()


3
2
1

>>>

so obv while var means while not empty or why not zero but it isnt
something youd guess unless youd beeen shown it.

[Larry Bates]

Re: while var, but var ==16 != true

maestro wrote:

why does this work? "while p" = "while p != 0" ? 1 is True and 0 is
false in python but other numbers have no boolean value so why doesnt
it abort.
>
>
>

>>>p=16
>>>p

16

>>>while p:

print p
p -= 1
>
>
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
>
>
i can also do:
>

>>>k=[]
>>>while k:

k.pop()
>
>

>>>k=[1,2,3]
>>>while k:

k.pop()
>
>
3
2
1
>
>
so obv while var means while not empty or why not zero but it isnt
something youd guess unless youd beeen shown it.

Python manuals outline that 0, None, empty list, empty dictionary are False
anything else evaluates to True. This is all over this list as well.

-Larry

[Tim Roberts]

Re: while var, but var ==16 != true

maestro <notnorwegian@y ahoo.sewrote:

>
>why does this work? "while p" = "while p != 0" ? 1 is True and 0 is
>false in python but other numbers have no boolean value so why doesnt
>it abort.

Because your statement is incorrect. Everything has a boolean value in
Python. 0, None, False, '' (empty string), [] (empty list), () (empty
tuple), and {} (empty dictionary) all have a False value. Everything else
has a True value.

Python didn't even have a boolean type (True and False) until rather
recently (2.2?).

This is a very handy feature, and it's one of the things I love about
Python.

>so obv while var means while not empty or why not zero but it isnt
>something youd guess unless youd beeen shown it.

It's clearly stated in the documentation. I don't know how you concluded
that True and False were the only boolean values.
--
Tim Roberts, timr@probo.com
Providenza & Boekelheide, Inc.

[John Machin]

Re: while var, but var ==16 != true

On Jul 14, 3:32 pm, Tim Roberts <t...@probo.com wrote:

maestro <notnorweg...@y ahoo.sewrote:
>

why does this work?  "while p" = "while p != 0" ? 1 is True and 0 is
false in python but other numbers have no boolean value so why doesnt
it abort.

>
Because your statement is incorrect.  Everything has a boolean value in
Python.  0, None, False, '' (empty string), [] (empty list), () (empty
tuple), and {} (empty dictionary) all have a False value.  Everything else
has a True value.

Not quite; for example:

>>bool(set())

False

>>>

According to section 5.10 of the Reference Manual:
"""
In the context of Boolean operations, and also when expressions are
used by control flow statements, the following values are interpreted
as false: False, None, numeric zero of all types, and empty strings
and containers (including strings, tuples, lists, dictionaries, sets
and frozensets). All other values are interpreted as true.
"""

... and for the definition for what a user-written class needs to do,
see section 3.4.1:
"""
__nonzero__( self)

Called to implement truth value testing, and the built-in operation
bool(); should return False or True, or their integer equivalents 0 or
1. When this method is not defined, __len__() is called, if it is
defined (see below). If a class defines neither __len__() nor
__nonzero__(), all its instances are considered true.
"""

Cheers,
John

[bruno.desthuilliers@gmail.com]

Re: while var, but var ==16 != true

On 14 juil, 07:32, Tim Roberts <t...@probo.com wrote:
(snip)

Everything has a boolean value in
Python. 0, None, False, '' (empty string), [] (empty list), () (empty
tuple), and {} (empty dictionary) all have a False value. Everything else
has a True value.

Unless the author of the class specified it otherwise (implementing
the appropriate __magic_method_ _).

[Jeffrey Froman]

Re: while var, but var ==16 != true

Tim Roberts wrote:

Everything has a boolean value in
Python.  0, None, False, '' (empty string), [] (empty list), () (empty
tuple), and {} (empty dictionary) all have a False value.  Everything else
has a True value.

Empty set objects also evaluate as false in a boolean context.


Jeffrey