Why would I get a TypeEror?

[It's me]

For this code snip:

a=3
.....
b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]

Why would I get a TypeError from the len function?

Thanks,

[Peter Hansen]

Re: Why would I get a TypeEror?

It's me wrote:[color=blue]
> For this code snip:
>
> a=3
> ....
> b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]
>
> Why would I get a TypeError from the len function?[/color]

What did you expect the "length" of the integer 3 to be?

-Peter

[harold fellermann]

Re: Why would I get a TypeEror?


On 12.01.2005, at 18:35, It's me wrote:
[color=blue]
> For this code snip:
>
> a=3
> ....
> b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]
>
> Why would I get a TypeError from the len function?[/color]

because len() works only for sequence and mapping objects:
[color=blue][color=green][color=darkred]
>>> help(len)[/color][/color][/color]
Help on built-in function len in module __builtin__:

len(...)
len(object) -> integer

Return the number of items of a sequence or mapping.

- harold -

--
Ceci n'est pas une signature.
--

[harold fellermann]

Re: Why would I get a TypeEror?


On 12.01.2005, at 18:35, It's me wrote:
[color=blue]
> For this code snip:
>
> a=3
> ....
> b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]
>
> Why would I get a TypeError from the len function?[/color]

the problem is, that (1,len(a)) is evaluated, neither what type a
actually has
(python has no builtin lazy evaluation like ML). You have to do it this
way
instead:

a=3
....
b = isinstance(a,(l ist,tuple,dict) ) and len(a) or 1

- harold -

--
The opposite of a correct statement is a false statement.
But the opposite of a profound truth may be another profound truth.
-- Niels Bohr

[It's me]

Re: Why would I get a TypeEror?

Sorry if my question was a little "lazy" and yes, I was asking about the
"lazy evaluation". :=)

I am surprised about this (and this can be dangerous, I guess).

If this is true, I would run into trouble real quick if I do a:

(1/x,1.0e99)[x==0]

and that's not good.

Something to keep in mind. :-(


"harold fellermann" <harold.fellerm ann@upf.edu> wrote in message
news:mailman.57 8.1105553620.22 381.python-list@python.org ...[color=blue]
>
> On 12.01.2005, at 18:35, It's me wrote:
>[color=green]
> > For this code snip:
> >
> > a=3
> > ....
> > b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]
> >
> > Why would I get a TypeError from the len function?[/color]
>
> the problem is, that (1,len(a)) is evaluated, neither what type a
> actually has
> (python has no builtin lazy evaluation like ML). You have to do it this
> way
> instead:
>
> a=3
> ...
> b = isinstance(a,(l ist,tuple,dict) ) and len(a) or 1
>
> - harold -
>
> --
> The opposite of a correct statement is a false statement.
> But the opposite of a profound truth may be another profound truth.
> -- Niels Bohr
>[/color]

[Steve Holden]

Re: Why would I get a TypeEror?

It's me wrote:
[color=blue]
> For this code snip:
>
> a=3
> .....
> b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]
>
> Why would I get a TypeError from the len function?
>
> Thanks,
>
>[/color]
because the interpreter evaluates the tuple (1, len(a)) before applying
the indexing to it.

You are trying to be far too clever. The standard way to write this
would be:

a = 3
.....
b = 1
if isinstance(a,(l ist,tuple,dict) ):
b = len(a)

Is code length *really* so important? Think carefully ...

regards
Steve
--
Steve Holden http://www.holdenweb.com/
Python Web Programming http://pydish.holdenweb.com/
Holden Web LLC +1 703 861 4237 +1 800 494 3119

[Terry Reedy]

Re: Why would I get a TypeEror?


"Peter Hansen" <peter@engcorp. com> wrote in message
news:XOOdnSCB9N mJ-HjcRVn-2w@powergate.ca ...[color=blue]
> What did you expect the "length" of the integer 3 to be?[/color]

Perhaps 2 (bits in a minimal binary representation) .

I once, for maybe a minute, considered proposing this as an overloaded
meaning of len, but realized that that would more often mask errors than
save time.

If case bits(i) is what the OP wants:

def bits(i):
i = abs(i)
b = 0
while i:
i >>= 1
b += 1
return b

Terry J. Reedy

[Steven Bethard]

Re: Why would I get a TypeEror?

It's me wrote:[color=blue]
> For this code snip:
>
> a=3
> ....
> b=(1,len(a))[isinstance(a,(l ist,tuple,dict) )]
>
> Why would I get a TypeError from the len function?[/color]

You're looking for lazy evaluation or short-circuiting behavior. Python
provides one form of short circuiting behavior with 'and' and 'or',
though you need to be careful. In your particular circumstances, you
could write this code as:

b = not isinstance(a, (list, tuple, dict)) and 1 or len(a)

Some example code:

py> def b(a):
.... return not isinstance(a, (list, tuple, dict)) and 1 or len(a)
....
py> b(3)
1
py> b([])
0
py> b([3, 4])
2

Note however that, due to how 'and' and 'or' short-circuit, you cannot
write your code as:

b = isinstance(a, (list, tuple, dict)) and len(a) or 1

because when len(a) is 0, 1 will be returned instead of 0:

py> def b(a):
.... return isinstance(a, (list, tuple, dict)) and len(a) or 1
....
py> b(3)
1
py> b([])
1
py> b([3, 4])
2

If you want lazy evaluation, you can do this with lambdas (though I
wouldn't advise it):

b = (lambda: 1, lambda: len(a))[isinstance(a,(l ist,tuple,dict) )]()

Note that I select which function using isinstance as you have before,
and then invoke the selected function with the final ().

Steve

[Reinhold Birkenfeld]

Re: Why would I get a TypeEror?

It's me wrote:[color=blue]
> Sorry if my question was a little "lazy" and yes, I was asking about the
> "lazy evaluation". :=)
>
> I am surprised about this (and this can be dangerous, I guess).
>
> If this is true, I would run into trouble real quick if I do a:
>
> (1/x,1.0e99)[x==0]
>
> and that's not good.
>
> Something to keep in mind. :-([/color]

Lazy evaluation: use the (x==0 and 1e99 or 1/x) form!

Reinhold

[It's me]

Re: Why would I get a TypeEror?

Say again???

"Reinhold Birkenfeld" <reinhold-birkenfeld-nospam@wolke7.n et> wrote in
message news:34ql1vF4dq d9pU1@individua l.net...[color=blue]
> It's me wrote:[color=green]
> > Sorry if my question was a little "lazy" and yes, I was asking about the
> > "lazy evaluation". :=)
> >
> > I am surprised about this (and this can be dangerous, I guess).
> >
> > If this is true, I would run into trouble real quick if I do a:
> >
> > (1/x,1.0e99)[x==0]
> >
> > and that's not good.
> >
> > Something to keep in mind. :-([/color]
>
> Lazy evaluation: use the (x==0 and 1e99 or 1/x) form!
>
> Reinhold[/color]

[Reinhold Birkenfeld]

Re: Why would I get a TypeEror?

It's me wrote:[color=blue]
> Say again???
>
> "Reinhold Birkenfeld" <reinhold-birkenfeld-nospam@wolke7.n et> wrote in
> message news:34ql1vF4dq d9pU1@individua l.net...[color=green]
>> It's me wrote:[color=darkred]
>> > Sorry if my question was a little "lazy" and yes, I was asking about the
>> > "lazy evaluation". :=)
>> >
>> > I am surprised about this (and this can be dangerous, I guess).
>> >
>> > If this is true, I would run into trouble real quick if I do a:
>> >
>> > (1/x,1.0e99)[x==0]
>> >
>> > and that's not good.
>> >
>> > Something to keep in mind. :-([/color]
>>
>> Lazy evaluation: use the (x==0 and 1e99 or 1/x) form!
>>
>> Reinhold[/color][/color]

Say what again?

Reinhold

PS: Please do not produce top-posting!

[Steven Bethard]

Re: Why would I get a TypeEror?

It's me wrote:[color=blue]
> Say again???[/color]

Please stop top-posting -- it makes it hard to reply in context.
[color=blue]
> "Reinhold Birkenfeld" wrote...[color=green]
>>It's me wrote:[color=darkred]
>>>If this is true, I would run into trouble real quick if I do a:
>>>
>>>(1/x,1.0e99)[x==0][/color]
>>
>>Lazy evaluation: use the (x==0 and 1e99 or 1/x) form![/color][/color]

If you want short-circuting behavior, where only one of the two branches
gets executed, you should use Python's short-circuiting boolean
operators. For example,

(x == 0 and 1.0e99 or 1/x)

says something like:

Check if x == 0.
If so, check if 1.0e99 is non-zero. It is, so return it.
If x != 0, see if 1/x is non-zero. It is, so return it.

Note that if you're not comfortable with short-circuiting behavior, you
can also code this using lazy evaluation:

(lambda: 1/x, lambda: 1.0e99)[x==0]()

This says something like:

Create two functions, one to produce 1/x and one to produce 1.0e99.
Select one of these functions depending on whether or not x==0
Invoke the chosen function.

HTH,

Steve

[Reinhold Birkenfeld]

Re: Why would I get a TypeEror?

Steven Bethard wrote:[color=blue]
> It's me wrote:[color=green]
>> Say again???[/color]
>
> Please stop top-posting -- it makes it hard to reply in context.
>[color=green]
>> "Reinhold Birkenfeld" wrote...[color=darkred]
>>>It's me wrote:
>>>>If this is true, I would run into trouble real quick if I do a:
>>>>
>>>>(1/x,1.0e99)[x==0]
>>>
>>>Lazy evaluation: use the (x==0 and 1e99 or 1/x) form![/color][/color]
>
> If you want short-circuting behavior, where only one of the two branches
> gets executed, you should use Python's short-circuiting boolean
> operators. For example,
>
> (x == 0 and 1.0e99 or 1/x)
>
> says something like:
>
> Check if x == 0.
> If so, check if 1.0e99 is non-zero. It is, so return it.
> If x != 0, see if 1/x is non-zero. It is, so return it.
>
> Note that if you're not comfortable with short-circuiting behavior, you
> can also code this using lazy evaluation:
>
> (lambda: 1/x, lambda: 1.0e99)[x==0]()[/color]

Or even

(x==0 and lambda: 1e99 or lambda: 1/x)()

Or ...


Reinhold

[Stian Soiland]

Re: Why would I get a TypeEror?

På 14. jan 2005 kl. 22:58 skrev Steven Bethard:

(Any mac users? How do I fix this to appear in Norwegian? =)
[color=blue]
> Note that if you're not comfortable with short-circuiting behavior,
> you can also code this using lazy evaluation:
>
> (lambda: 1/x, lambda: 1.0e99)[x==0]()[/color]

... and people wonder why so many Python people want to get rid of
Lambda =)

--
Stian Søiland You can't say civilization don't
Trondheim, Norway advance, however, for in every war
http://www.soiland.no/ they kill you in a new way. [Rogers]
=/\=

[Steven Bethard]

Re: Why would I get a TypeEror?

Stian Soiland wrote:[color=blue]
> På 14. jan 2005 kl. 22:58 skrev Steven Bethard:
>
> (Any mac users? How do I fix this to appear in Norwegian? =)
>[color=green]
>> Note that if you're not comfortable with short-circuiting behavior,
>> you can also code this using lazy evaluation:
>>
>> (lambda: 1/x, lambda: 1.0e99)[x==0]()[/color]
>
>
> .. and people wonder why so many Python people want to get rid of Lambda =)
>[/color]

Heh heh. No I don't. ;)

In fact, I don't ever use lambdas in any of my own "real" code. But I
don't mind being a little dirty when I post to c.l.py. ;) I guess I
could have written this as:

def inverse():
return 1/x
def largenum():
return 1.0e99
b = (inverse, largenum)[x==0]()

but I'm usually too lazy, and it's an ugly solution anyway, compared to
the simple one which the OP was apparently trying to avoid:

if x != 0:
b = 1/x
else:
b = 1.0e99

=)

Steve