FAQ
I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string) > maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY numeric
type and that constant will ALWAYS be larger!

## Search Discussions

•  at Feb 24, 2012 at 1:55 pm ⇧

On 24/02/2012 13:37, Rick Johnson wrote:
I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string)> maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY numeric
type and that constant will ALWAYS be larger!

Do you want to test for something that is larger than infinity?

--
Cheers.

Mark Lawrence.
•  at Feb 24, 2012 at 2:14 pm ⇧

On Feb 24, 7:55?am, Mark Lawrence wrote:
Do you want to test for something that is larger than infinity?
Not exactly. I want to set a constant that has a value of infinity and
then do comparisons against the constant.

##################
# Hypothetical 1 #
##################

def confine(string, maxlength=INFINITY):
return string[:maxlength]

py> confine('123')
'123'
py> confine('123', 1)
'1'

##################
# Hypothetical 2 #
##################

def confine(string, maxlength=INFINITY):
if len(string) < maxlength:
do_something()
else:
twiddle_thumbs()
•  at Feb 24, 2012 at 2:25 pm ⇧

On 2012-02-24, Rick Johnson wrote:
I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string) > maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY
numeric type and that constant will ALWAYS be larger!
What's the point of that?

The only time I've naively pined for such a thing is when
misapplying C idioms for finding a minimum value.

Python provides an excellent min implementation to use instead.

--
Neil Cerutti
•  at Feb 24, 2012 at 3:25 pm ⇧

On Feb 24, 8:25?am, Neil Cerutti wrote:

What Python needs is some constant that can be compared to ANY
numeric type and that constant will ALWAYS be larger!
What's the point of that?

The only time I've naively pined for such a thing is when
misapplying C idioms for finding a minimum value.
The best use case is for default arguments to constructors or func/
meths. If you set the argument to INFINITY instead of -1 (or some
other dumb string value to mean "unlimited") you can omit a useless
conditional block later. Observe:

if maxlength == -1 # unlimited length:
keep_going()
elif len(object) < maxlength:
stop() # because we reached the limit

I see tons and tons of old Python code that uses -1 as an "unlimited"
value, where positive numbers are meant to constrain dome value. I
have always found that to be intuitive; hence my question.
•  at Feb 24, 2012 at 5:32 pm ⇧

On Fri, Feb 24, 2012 at 9:25 AM, Neil Cerutti wrote:
The only time I've naively pined for such a thing is when
misapplying C idioms for finding a minimum value.

Python provides an excellent min implementation to use instead.
min can be a little inconvenient. As soon as anything complicated has
to be done during the min expression, you need to switch to using
something else for sanity's sake. In that vein, I do actually
sometimes use float('inf') (for numbers), or a custom max/min object.

----

Forgive me, I have spoken in error! min is the one true way, for you
can still do it with a little wrangling, as follows:

@operator.itemgetter(1)
@min
@apply
def closest_object():
for x in xrange(board_width)
for y in xrange(board_height):
try:
entity = board.get_entity(x, y)
except EntityNotFound:
pass
else:
yield distance(player.pos, entity.pos), entity

-- Devin
•  at Feb 25, 2012 at 1:26 am ⇧

On Fri, Feb 24, 2012 at 10:32 AM, Devin Jeanpierre wrote:
On Fri, Feb 24, 2012 at 9:25 AM, Neil Cerutti wrote:
The only time I've naively pined for such a thing is when
misapplying C idioms for finding a minimum value.

Python provides an excellent min implementation to use instead.
min can be a little inconvenient. As soon as anything complicated has
to be done during the min expression, you need to switch to using
something else for sanity's sake. In that vein, I do actually
sometimes use float('inf') (for numbers), or a custom max/min object.

----

Forgive me, I have spoken in error! min is the one true way, for you
can still do it with a little wrangling, as follows:

? ?@operator.itemgetter(1)
? ?@min
? ?@apply
? ?def closest_object():
? ? ? ?for x in xrange(board_width)
? ? ? ? ? ?for y in xrange(board_height):
? ? ? ? ? ? ? ?try:
? ? ? ? ? ? ? ? ? ?entity = board.get_entity(x, y)
? ? ? ? ? ? ? ?except EntityNotFound:
? ? ? ? ? ? ? ? ? ?pass
? ? ? ? ? ? ? ?else:
? ? ? ? ? ? ? ? ? ?yield distance(player.pos, entity.pos), entity
Cute, but what's so terrible about:

def all_entities():
for x in xrange(board_width):
for y in xrange(board_height):
try:
yield board.get_entity(x, y)
except EntityNotFound:
pass

closest_object = min(all_entities,
key=lambda e: distance(player.pos, e.pos))

Especially given that all_entities should be reusable in other contexts.

Cheers,
Ian
•  at Feb 24, 2012 at 2:39 pm ⇧
float('infinity') should be good enough.
•  at Feb 24, 2012 at 3:18 pm ⇧

On Feb 24, 8:39?am, Miki Tebeka wrote:
float('infinity') should be good enough.
Yes, that is the answer however the implementation is inconsistent.

py> float("inf")
inf
py> float("infinity")
inf
py> int("inf")
Traceback (most recent call last):
File "<pyshell#2>", line 1, in <module>
int("inf")
ValueError: invalid literal for int() with base 10: 'inf'
py> int("infinity")
Traceback (most recent call last):
File "<pyshell#3>", line 1, in <module>
int("infinity")
ValueError: invalid literal for int() with base 10: 'infinity'

The best place for INFINITY is a constant of the math module.

# Hypothetical #
py> from math import INFINITY
py> 1 < INFINITY
True
py> 99999999999999999 < INFINITY
True
•  at Feb 24, 2012 at 3:21 pm ⇧

Rick Johnson wrote:

I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string) > maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY numeric
type and that constant will ALWAYS be larger!
Easily fixed:

class Greatest (object):
def __cmp__ (self, other):
if isinstance (other, Greatest):
return 0
return 1

def __hash__ (self):
return id (Greatest)

class Least (object):
def __cmp__ (self, other):
if isinstance (other, Least):
return 0
return -1

def __hash__ (self):
return id (Least)

Mel.
•  at Feb 24, 2012 at 3:31 pm ⇧

On Fri, 24 Feb 2012 10:21:45 -0500, Mel Wilson wrote:

Rick Johnson wrote:
I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string) > maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY numeric
type and that constant will ALWAYS be larger!
Easily fixed:

class Greatest (object):
def __cmp__ (self, other):
if isinstance (other, Greatest):
return 0
return 1

def __hash__ (self):
return id (Greatest)
__cmp__ no longer exists in Python 3, so this solution could only work in
Python 2.

Here's a version using rich comparisons:

class Greatest:
__eq__ = __le__ = lambda self, other: isinstance(other, type(self))
__ne__ = __gt__ = lambda self, othr: not isinstance(othr, type(self))
__lt__ = lambda self, other: False
__ge__ = lambda self, other: True
__hash__ = lambda self: 42

--
Steven
•  at Feb 24, 2012 at 3:34 pm ⇧

On Feb 24, 9:21?am, Mel Wilson wrote:

Easily fixed:

[...snip code...]
Yes i could write my own implementation of INFINITY if i wanted,
although i would have returned True and False as apposed to 1 and 0
AND used the identifiers Infinity and Infinitesimal, but i digress :-
P.

However, INFINITY is something i believe a language should provide;
which python does, albeit inconsistently.
•  at Feb 24, 2012 at 4:23 pm ⇧

On 02/24/2012 08:34 AM, Rick Johnson wrote:
Yes i could write my own implementation of INFINITY if i wanted,
although i would have returned True and False as apposed to 1 and 0
AND used the identifiers Infinity and Infinitesimal, but i digress :-
P.

However, INFINITY is something i believe a language should provide;
which python does, albeit inconsistently.
How do you represent infinity as an binary integer number? Or are you
suggesting that the integer type (class) be modified to allow an
"infinity" state that really isn't a number at all (could not be stored
as a integer in C)?

Float is a different story because IEEE does define a binary
representation of infinity in the floating-point specification.

I know of no language that has any form of representation of infinity
for integers mainly because there's no way to represent infinity as a
standard twos-compliment binary number. In a language that deals
directly with types in memory such as C, having an infinity
representation would be possible but would make simple math really hard,
and much slower.

All this reminds me of the original cray supercomputers. They didn't
use twos compliment for integers so they had two representations of zero
(+0 and -0). Made programming a bit tricky. When asked why the cray
didn't just do two's compliment like everyone else, Seymour Cray
responded that when the computer was designed he simply didn't know
•  at Feb 24, 2012 at 4:59 pm ⇧

On 24/02/2012 16:23, Michael Torrie wrote:
On 02/24/2012 08:34 AM, Rick Johnson wrote:
Yes i could write my own implementation of INFINITY if i wanted,
although i would have returned True and False as apposed to 1 and 0
AND used the identifiers Infinity and Infinitesimal, but i digress :-
P.

However, INFINITY is something i believe a language should provide;
which python does, albeit inconsistently.
How do you represent infinity as an binary integer number? Or are you
suggesting that the integer type (class) be modified to allow an
"infinity" state that really isn't a number at all (could not be stored
as a integer in C)?
The C integer bit doesn't matter since e.g.
>>>
a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
a
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000L

And no, I'm not going to calculate how much memory I'd need to store a
string that's this long :)
Float is a different story because IEEE does define a binary
representation of infinity in the floating-point specification.

I know of no language that has any form of representation of infinity
for integers mainly because there's no way to represent infinity as a
standard twos-compliment binary number. In a language that deals
directly with types in memory such as C, having an infinity
representation would be possible but would make simple math really hard,
and much slower.

All this reminds me of the original cray supercomputers. They didn't
use twos compliment for integers so they had two representations of zero
(+0 and -0). Made programming a bit tricky. When asked why the cray
didn't just do two's compliment like everyone else, Seymour Cray
responded that when the computer was designed he simply didn't know
--
Cheers.

Mark Lawrence.
•  at Feb 24, 2012 at 11:16 pm ⇧

On 02/24/2012 09:59 AM, Mark Lawrence wrote:
The C integer bit doesn't matter since e.g.
a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
a
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000L

And no, I'm not going to calculate how much memory I'd need to store a
string that's this long :)
Sure but that doesn't answer the question posed. How does Rick plan to
represent an infinite integer? Obviously you've shown that with an
infinite amount of memory we could do it quite easily. But baring that,
how does Rick suggest we should represent an infinite integer?
•  at Feb 25, 2012 at 12:09 am ⇧

On Sat, Feb 25, 2012 at 10:16 AM, Michael Torrie wrote:
Sure but that doesn't answer the question posed. ?How does Rick plan to
represent an infinite integer? Obviously you've shown that with an
infinite amount of memory we could do it quite easily. ?But baring that,
how does Rick suggest we should represent an infinite integer?
Barring a suggestion from Rick, I think we should define the number 8
to be greater than all other integers. After all, Rick's very much in
favour of evolution, and what would better depict the evolution of
this glorious language than this notation, showing that the infinity
symbol is now walking erect!

ChrisA
•  at Feb 25, 2012 at 12:35 am ⇧

On 24/02/2012 23:16, Michael Torrie wrote:
On 02/24/2012 09:59 AM, Mark Lawrence wrote:
The C integer bit doesn't matter since e.g.
a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
a
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000L

And no, I'm not going to calculate how much memory I'd need to store a
string that's this long :)
Sure but that doesn't answer the question posed. How does Rick plan to
represent an infinite integer? Obviously you've shown that with an
infinite amount of memory we could do it quite easily. But baring that,
how does Rick suggest we should represent an infinite integer?
I understand that a Python integer can run to infinity. Quite how the
illustrious rr manages to test for the length of a string that's already
used all of the memory on his system has baffled me, but I'm sure that
all the people who frequent this list with their Phds, MScs or whatever
will soon correct me.

--
Cheers.

Mark Lawrence.
•  at Feb 25, 2012 at 12:37 am ⇧

On 24/02/2012 23:16, Michael Torrie wrote:
On 02/24/2012 09:59 AM, Mark Lawrence wrote:
The C integer bit doesn't matter since e.g.
a000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000
a
10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000L

And no, I'm not going to calculate how much memory I'd need to store a
string that's this long :)
Sure but that doesn't answer the question posed. How does Rick plan to
represent an infinite integer? Obviously you've shown that with an
infinite amount of memory we could do it quite easily. But baring that,
how does Rick suggest we should represent an infinite integer?
We already have arbitrarily long ints, so there could be a special
infinite int singleton (actually, 2 of them, one positive, the other
negative).
•  at Feb 25, 2012 at 1:22 am ⇧

On Saturday 25 Feb 2012 12:37:58 AM MRAB wrote:
We already have arbitrarily long ints, so there could be a special
infinite int singleton (actually, 2 of them, one positive, the other
negative).
Seconded. Although would a wish request to bugs.python.org saying "Allow
storage of the integer infinity" make any sense to the developers? :P
--
Fayaz Yusuf Khan
Cloud developer and architect
Dexetra SS, Bangalore, India
fayaz.yusuf.khan_AT_gmail_DOT_com
fayaz_AT_dexetra_DOT_com
+91-9746-830-823
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•  at Feb 25, 2012 at 1:50 am ⇧

On Sat, 25 Feb 2012 06:52:09 +0530, Fayaz Yusuf Khan wrote:
On Saturday 25 Feb 2012 12:37:58 AM MRAB wrote:
We already have arbitrarily long ints, so there could be a special
infinite int singleton (actually, 2 of them, one positive, the other
negative).
Seconded. Although would a wish request to bugs.python.org saying "Allow
storage of the integer infinity" make any sense to the developers? :P
If you explained it as a pair of special int values, INF and -INF, rather
than the storage of an infinite-sized integer, it would make perfect
sense.

But it would also be rejected, and rightly so, as unnecessary complexity
for the int type. There are already Decimal and float infinities, just
use one of them. Or make your own, it's not difficult. Publish it on
ActiveState, and if people flock to use it, then you will have a good
argument that this is useful and should be part of the Python built-ins.

--
Steven
•  at Feb 25, 2012 at 8:29 pm ⇧

On Feb 24, 7:50?pm, Steven D'Aprano <steve +comp.lang.pyt... at pearwood.info> wrote:

But it would also be rejected, and rightly so, as unnecessary complexity
for the int type. There are already Decimal and float infinities, just
use one of them.
Sure there are float INFINITIES that work fine for ints and floats,
but where is the consistency? INFINITY need not be a int or a float or
a str, or whatever. All it need be is a an object who always returns
itself as being larger in any comparison.
Or make your own, it's not difficult.
INFINITY should be at the very least a constant of the math module.
•  at Feb 27, 2012 at 2:32 am ⇧

On Feb 26, 6:29?am, Rick Johnson wrote:
Sure there are float INFINITIES that work fine for ints and floats,
but where is the consistency?
Sure, there are all of the COMPLEXITIES of floating point arithmetic
but I want to ignore all of that and demand ridiculous consistencies.
Why should I have to do float(some_int) < float('inf') when it's a far
better use of my time to spend days if not weeks bemoaning yet another
language wart? Why should I be expected to know what float('inf')
actually represents before making stupid demands like:
INFINITY need not be a int or a float or
a str, or whatever.
Please provide a non-contrived use case of an "infinite string".
INFINITY should be at the very least a constant of the math module.
Why? This isn't a mathematical concept of 'infinite' when you're
talking about comparing against "str, or whatever". We need a more
shit.rick.wants into the stdlib.
•  at Feb 27, 2012 at 3:51 am ⇧

On Sun, 26 Feb 2012 18:32:27 -0800, alex23 wrote:
On Feb 26, 6:29?am, Rick Johnson wrote:
Sure there are float INFINITIES that work fine for ints and floats, but
where is the consistency?
Sure, there are all of the COMPLEXITIES of floating point arithmetic but
I want to ignore all of that and demand ridiculous consistencies. Why
should I have to do float(some_int) < float('inf')
Ints and floats can be compared directly, no need to convert the int to a
float first:
INF = float('inf')
23 < INF
True

Likewise Fractions and Decimals, at least in Python 3.2 (possibly not in
older versions):
from fractions import Fraction
from decimal import Decimal
Fraction(33, 5) < INF
True
Decimal("42.1568") < INF
True

when it's a far
better use of my time to spend days if not weeks bemoaning yet another
language wart? Why should I be expected to know what float('inf')
actually represents before making stupid demands like:
INFINITY need not be a int or a float or a str, or whatever.
Please provide a non-contrived use case of an "infinite string".
Any lazy stream of characters that potentially goes on forever could be
considered an infinite string. But that's not what Rick is talking about.

He's talking about having a pair of special values, say, BIGGEST and
SMALLEST, which compare larger and smaller to any other value, regardless
of type and including strings, not literally a string with an infinite
number of characters.

I can see some value for this as a convenience, but not enough to make it
a built-in language feature. Every developer should have at least one
utility module with all the trivial code snippets they frequently use.
This belongs in there.

--
Steven
•  at Feb 27, 2012 at 3:59 am ⇧

On Mon, Feb 27, 2012 at 2:51 PM, Steven D'Aprano wrote:
I can see some value for this as a convenience, but not enough to make it
a built-in language feature. Every developer should have at least one
utility module with all the trivial code snippets they frequently use.
+1. I used to call mine "oddsends" - it nicely fitted into eight
characters (yeah, was important then - I was on DOS), and I had quite
a few odds and ends in there.

ChrisA
•  at Feb 27, 2012 at 6:49 am ⇧

On Feb 27, 1:51?pm, Steven D'Aprano <steve +comp.lang.pyt... at pearwood.info> wrote:
Ints and floats can be compared directly, no need to convert the int to a
float first
Ah, cheers. You can see how often I use the two interchangeably :)
Please provide a non-contrived use case of an "infinite string".
Any lazy stream of characters that potentially goes on forever could be
considered an infinite string. But that's not what Rick is talking about.

He's talking about having a pair of special values, say, BIGGEST and
SMALLEST, which compare larger and smaller to any other value, regardless
of type and including strings, not literally a string with an infinite
number of characters.
Yeah, my point was more to highlight Rick's laziness in co-opting a
defined term - INFINITE - and trying to use it to mean something else
that he couldn't express clearly. His original post stressed numeric
comparison, the feature creep to include all other types happened
later. Not the sort of thing we've come to expect from the resident
linguist extraordinaire :)
I can see some value for this as a convenience, but not enough to make it
a built-in language feature.
For me, it feels like a step backwards to comparing different types:
1 < INFINITE
True
'string' < INFINITE
True
1 < 'string'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unorderable types: int() < str()
Every developer should have at least one
utility module with all the trivial code snippets they frequently use.
This belongs in there.
Agreed. Especially when it's so trivial:

class Bound(object):
def __init__(self, value=None, always_greater=False):
self.value = value
self.always_greater = always_greater

def __cmp__(self, other):
return True if self.always_greater else self.value.__cmp__(other)
upper = Bound(100)
101 > upper
True
101 < upper
False
infinite = Bound(always_greater=True)
101 > infinite
False
101 < infinite
True
upper < 101 < infinite
True
•  at Feb 25, 2012 at 3:32 pm ⇧

25.02.12 02:37, MRAB ???????(??):
We already have arbitrarily long ints, so there could be a special
infinite int singleton (actually, 2 of them, one positive, the other
negative).
float('inf') and float('-inf').
•  at Feb 25, 2012 at 8:25 pm ⇧

On Feb 24, 6:35?pm, Mark Lawrence wrote:

I understand that a Python integer can run to infinity. ?Quite how the
illustrious rr manages to test for the length of a string that's already
used all of the memory on his system has baffled me,
When did i ever say that i would need a string who's length is
INFINITY? In fact, i don't. My example was just that, as SIMPLIFIED
example of the problem that was the genesis of my question. In my
"real world" problem, i don't expect the string to EVER be more than
double digits in length. But as any good programmer knows, you never
want to solve problems as globally as possible. I need to do
comparisons on strings now, but maybe integers later, or who knows.
INFINITY comparisons are useful in many places.
but I'm sure that
all the people who frequent this list with their Phds, MScs or whatever
will soon correct me.
I don't believe you'd need a Phd to understand my problem.
•  at Feb 24, 2012 at 10:45 pm ⇧

On Fri, 24 Feb 2012 09:23:08 -0700, Michael Torrie wrote:

All this reminds me of the original cray supercomputers. They didn't
use twos compliment for integers so they had two representations of zero
(+0 and -0). Made programming a bit tricky.
While there is only one integer zero, I would like to point out that in
floating point, there are usually two zeroes, -0.0 and +0.0, and that
this is by design and a feature, not an accident or a bug.

Well-written floating point functions should keep the sign when they
underflow, e.g.:

py> 1e-200 * 1e-200
0.0
py> 1e-200 * -1e-200
-0.0

and well-written functions should honour those separate zeroes because
sometimes it makes a difference.

--
Steven
•  at Feb 25, 2012 at 8:18 am ⇧

Am 24.02.12 14:37, schrieb Rick Johnson:
I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string) > maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY numeric
type and that constant will ALWAYS be larger!

If there is no limit for len(string), why not simply use

# get_limit() returns None if there is no limit
maxlength = get_limit()
if maxlength and (len(string) <= maxlength):
allow_passage()
else:
deny_passage()

Wolfgang
•  at Feb 25, 2012 at 5:54 pm ⇧

On 25/02/2012 08:18, Wolfgang Meiners wrote:
Am 24.02.12 14:37, schrieb Rick Johnson:
I get sick and tired of doing this!!!

if maxlength == UNLIMITED:
allow_passage()
elif len(string)> maxlength:
deny_passage()

What Python needs is some constant that can be compared to ANY numeric
type and that constant will ALWAYS be larger!

If there is no limit for len(string), why not simply use

# get_limit() returns None if there is no limit
maxlength = get_limit()
if maxlength and (len(string)<= maxlength):
allow_passage()
else:
deny_passage()
That should be:

if maxlength is not None and len(string) <= maxlength:
•  at Feb 25, 2012 at 8:35 pm ⇧

On Feb 25, 11:54?am, MRAB wrote:
[...]
That should be:
if maxlength is not None and len(string) <= maxlength:
Using "imaginary" infinity values defiles the intuitive nature of your
code. What is more intuitive?

def confine_length(string, maxlength=INFINITY):
if string.length < maxlength:
do_something()

def confine_length(string, maxlength=None):
if maxlength is not None and len(string) <= maxlength:
do_something()
•  at Feb 26, 2012 at 1:16 pm ⇧

Am 25.02.12 21:35, schrieb Rick Johnson:
On Feb 25, 11:54 am, MRAB wrote:
[...]
That should be:
if maxlength is not None and len(string) <= maxlength:
Using "imaginary" infinity values defiles the intuitive nature of your
code. What is more intuitive?

def confine_length(string, maxlength=INFINITY):
if string.length < maxlength:
do_something()

def confine_length(string, maxlength=None):
if maxlength is not None and len(string) <= maxlength:
do_something()
I just had a closer look at it. It seems to be more complicated than i
thougth: You will have to write

def confine_length(string, maxlength=None):
if maxlength: # maxlength exists, comparison possible
if len(string) <= maxlength:
do_something()
else: # maxlength does not exist, so always do something
do_something()

you migth also write

def confine_length(str, maxlength=None):
do_it = (len(str) <= maxlength) if maxlength else True
if do_it:
do_something()

but it really does not look intuitive. Hmm. My idea was that None is a
perfect Value for infinity since there is no infinitely large number.
But as i see it, you must have two comparisons then. Maybe someone has a
better idea?

Wolfgang
•  at Feb 26, 2012 at 1:38 pm ⇧

Am 26.02.12 14:16, schrieb Wolfgang Meiners:
I just had a closer look at it. It seems to be more complicated than i
thougth: You will have to write
Obviously not close enough, as i just learned.
def confine_length(string, maxlength=None):
if maxlength: # maxlength exists, comparison possible
if maxlength is not None: # maxlength exists, comparison possible
if len(string) <= maxlength:
do_something()
else: # maxlength does not exist, so always do something
do_something()

you migth also write

def confine_length(str, maxlength=None):
do_it = (len(str) <= maxlength) if maxlength else True
do_it = (len(str) <= maxlength) if maxlength is not None else True
if do_it:
do_something()
I hope, it's correct now.
Wolfgang
•  at Feb 26, 2012 at 1:44 pm ⇧

On 26 February 2012 13:38, Wolfgang Meiners wrote:
? ? ?do_it = (len(str) <= maxlength) if maxlength is not None else True
That's a funny way to spell:

do_it = maxlength is None or len(str) <= maxlength

--
Arnaud
•  at Feb 26, 2012 at 1:50 pm ⇧

On Sun, 26 Feb 2012 14:16:24 +0100, Wolfgang Meiners wrote:

I just had a closer look at it. It seems to be more complicated than i
thougth: You will have to write

def confine_length(string, maxlength=None):
if maxlength: # maxlength exists, comparison possible
if len(string) <= maxlength:
do_something()
else: # maxlength does not exist, so always do something
do_something()
No, that still takes the wrong branch for maxlength = 0.

Be explicit in your code. If you want maxlength=None to be a sentinel for
"avoid the length test", then explicitly test for maxlength is None,
don't be tempted to take short-cuts that can fail.

def confine_length(string, maxlength=None):
if maxlength is None: # no length comparison needed
do_something()
elif len(string) <= maxlength:
do_something()

This can be simplified to:

def confine_length(string, maxlength=None):
if maxlength is None or len(string) <= maxlength:
do_something()

Or even simpler:

def confine_length(string, maxlength=float('inf')):
if len(string) <= maxlength:
do_something()

--
Steven
•  at Feb 27, 2012 at 12:25 pm ⇧

On 2012-02-26, Wolfgang Meiners wrote:
but it really does not look intuitive. Hmm. My idea was that
None is a perfect Value for infinity since there is no
infinitely large number. But as i see it, you must have two
comparisons then. Maybe someone has a better idea?
I do. A truncated string with a maxlength of INFINITY is just a
string.

--
Neil Cerutti
•  at Feb 27, 2012 at 11:55 am ⇧

Rick Johnson wrote:
On Feb 25, 11:54 am, MRAB wrote:

[...]
That should be:
if maxlength is not None and len(string) <= maxlength:
Using "imaginary" infinity values defiles the intuitive nature of your
code. What is more intuitive?

def confine_length(string, maxlength=INFINITY):
if string.length < maxlength:
do_something()

def confine_length(string, maxlength=None):
if maxlength is not None and len(string) <= maxlength:
do_something()
This one:

def confine_length(string, maxlength=None):
"""Confine the length.

@param maxlength: the maximum length allowed, set it to None to allow any length.
"""
if maxlength is not None and len(string) <= maxlength:
do_something()

I'm just feeding the troll, I know ... :-/

JM
•  at Feb 26, 2012 at 11:56 am ⇧

Am 25.02.12 18:54, schrieb MRAB:
If there is no limit for len(string), why not simply use

# get_limit() returns None if there is no limit
maxlength = get_limit()
if maxlength and (len(string)<= maxlength):
allow_passage()
else:
deny_passage()
That should be:

if maxlength is not None and len(string) <= maxlength:
Take a look at

http://docs.python.org/library/stdtypes.html

=========================================================================
Any object can be tested for truth value, for use in an if or while
condition or as operand of the Boolean operations below. The following
values are considered false:

None

False

zero of any numeric type, for example, 0, 0L, 0.0, 0j.

any empty sequence, for example, '', (), [].

any empty mapping, for example, {}.

instances of user-defined classes, if the class defines
__nonzero__() or __len__() method, when that method returns the
integer zero or bool value False. [1]

All other values are considered true ? so objects of many types are
always true.
==========================================================================

That means:
if maxlength and (len(string) <= maxlength):

is equivalent to
if (maxlength is not None) and (len(string) <= maxlength):

which is more complicated to type and -in my opinion- not so intuitive.
But because it is equivalent, it is a matter of taste, what to use.

Wolfgang
•  at Feb 26, 2012 at 12:52 pm ⇧

On Sun, Feb 26, 2012 at 10:56 PM, Wolfgang Meiners wrote:
That means:
if maxlength and (len(string) <= maxlength):

is equivalent to
if (maxlength is not None) and (len(string) <= maxlength):
On the contrary, it means they are distinctly NOT equivalent. The
shorter form would treat a maximum length of 0 as meaning "unlimited".
Now, that's an understandable notation, but it's not what's given
here; if None means unlimited, then 0 should enforce that string ==
"".

ChrisA
•  at Feb 26, 2012 at 1:19 pm ⇧

On Sun, 26 Feb 2012 12:56:46 +0100, Wolfgang Meiners wrote:

That means:
if maxlength and (len(string) <= maxlength):

is equivalent to
if (maxlength is not None) and (len(string) <= maxlength):

which is more complicated to type and -in my opinion- not so intuitive.
But because it is equivalent, it is a matter of taste, what to use.
Incorrect. The two are *not* equivalent.

def test(maxlength, string):
flag1 = maxlength and (len(string) <= maxlength)
flag2 = (maxlength is not None) and (len(string) <= maxlength)
return bool(flag1), bool(flag2) # normalise to booleans

test(0, '')
(False, True)

So the two forms will take opposite branches of the if statement when
maxlength is 0 and string is the empty string.

--
Steven
•  at Feb 26, 2012 at 1:32 pm ⇧

Am 26.02.12 13:52, schrieb Chris Angelico:
On Sun, Feb 26, 2012 at 10:56 PM, Wolfgang Meiners
wrote:
That means:
if maxlength and (len(string) <= maxlength):

is equivalent to
if (maxlength is not None) and (len(string) <= maxlength):
On the contrary, it means they are distinctly NOT equivalent. The
shorter form would treat a maximum length of 0 as meaning "unlimited".
Now, that's an understandable notation, but it's not what's given
here; if None means unlimited, then 0 should enforce that string ==
"".

ChrisA
You are right. It seems I did not get the line
zero of any numeric type, for example, 0, 0L, 0.0, 0j.
right.

Wolfgang

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