A bit weird dictionary behavior

[Pekka Laukkanen]

Hello,

just noticed this:

Python 2.5.1 (r251:54863, Jan 17 2008, 19:35:17)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.{1: False}

This must be because
True

but it still doesn't feel exactly right. Would it be worth submitting a bug?

Cheers,
.peke

 

[Arnaud Delobelle]

Hello,

just noticed this:

Python 2.5.1 (r251:54863, Jan 17 2008, 19:35:17)
[GCC 4.0.1 (Apple Inc. build 5465)] on darwin
Type "help", "copyright", "credits" or "license" for more information.>>> {1: 2}
{1: 2}
{1: False}

This must be because

True

That's exactly the reason!

but it still doesn't feel exactly right. Would it be worth submitting a bug?

I don't think it can be considered as a bug, for the reason you gave
above and because dictionary keys are by definition unique with
respect to equality.

Perhaps you could call it a "surprising feature"

 

[bearophileHUGS]

Pekka Laukkanen:

but it still doesn't feel exactly right. Would it be worth submitting a bug?

It feels wrong because it is. In a tidier language (Pascal, Java, etc)
a boolean and an integer must be different types. Keeping booleans and
integers separated may avoid some bugs too (I don't know how many/
often).
But the Python language copies many things from C, where most things
are chosen for practical purposes and maximum efficiency (and from
much simpler Python implementations, where there was no boolean type,
so bools are grafted in), so it's not a bug, and I think it will not
be fixed soon (Python 3 was probably the last chance to change it, for
several years to come).
So you probably have to live with this illogical behavior.

On the other hand it has some little practical advantages, you can do:
sum(x == y for x in iterable)

That also equals to a more tidy:
sum(1 for x in iterable if x == y)

Regarding the dict, they are dynamically typed, but good programming
practice (that comes from experience of bugs that have bitten you)
tells you that generally it's better to be careful with the inserting
different types into a dict; often it's better to avoid doing it.

Bye,
bearophile

 

[bearophileHUGS]

Tino Wildenhain:

Wouldn't
len([x for x in iterable if x==y])
or even shorter:
iterable.count(y)
not work and read better anyway?

The first version creates an actual list just to take its length,
think about how much memory it may use.
The second version requires the 'iterable' object to have a count()
method, and in general this is false.

even calculating with boolean values isn't neccessary
since 'and' and 'foo if bar else blub' are working much better
so the type coalescing
bool - int - float can really go away.

I don't understand.

Bye,
bearophile

 

[Bruno Desthuilliers]

(e-mail address removed) a écrit :

Pekka Laukkanen:

It feels wrong because it is. In a tidier language (Pascal, Java, etc)
a boolean and an integer must be different types.

Some would argue (and some did by the time Python grew a 'bool' type)
that what is wrong is to have a bool type in a language that already
have a wider definition of the truth value of an expression...

(snip)

 

[Carl Banks]

(e-mail address removed) a écrit :



Some would argue (and some did by the time Python grew a 'bool' type)
that what is wrong is to have a bool type in a language that already
have a wider definition of the truth value of an expression...

And some would argue that it was wrong to have such a wide definition
for the truth value of an expression in the first place...


Carl Banks

 

[Grzegorz Staniak]

And some would argue that it was wrong to have such a wide definition
for the truth value of an expression in the first place...

Just out of idle curiosity, what could be the alternatives? Not to evaluate
e.g. strings to "true"? Aren't such conventions as "whatever is not empty,
is 'true'" popular in dynamic langauges?

GS

 

[Steven D'Aprano]

Tino Wildenhain:

Wouldn't
len([x for x in iterable if x==y])
or even shorter:
iterable.count(y)
not work and read better anyway?

The first version creates an actual list just to take its length, think
about how much memory it may use.

For many iterables, the amount of memory is not excessive and the
increase in readability of len() is to be preferred over the side-effect
of sum(1 for...).

But sure, in general you shouldn't try to count the number of items in an
arbitrary iterable unless you know how much time and resources it will
end up using. That's why I don't think len() should support arbitrary
iterables.

I don't understand.

I think Tino means that you don't need to cast items to bool since you
can use ints, floats etc. directly as truth values.

 

[Terry Reedy]

An alternate viewpoint is that only True and False 'have' a truth value.
So whenever the interpreter *needs* a truth value for conditional and
logical operations, and it has something else, it implicitly calls
bool(ob) to get one. This, or possibly a shortcut version thereof, is
what a Python interpreter effectively does.

From this viewpoint, objecters would instead have to argue that it is
wrong to have such implicit calls and that programmers should have to
put them in explicitly.

Just out of idle curiosity, what could be the alternatives? Not to evaluate
e.g. strings to "true"? Aren't such conventions as "whatever is not empty,
is 'true'" popular in dynamic langauges?

I do not know what is popular, but implicit bool call are darn handy.

tjr

 

[bearophileHUGS]

Steven D'Aprano:

For many iterables, the amount of memory is not excessive and the increase in readability of len() is to be preferred over the side-effect of sum(1 for...).<

With side-effects do you mean the possibility of exhausting a lazy
iterable?

The readability difference is little, and it's way safer because it
works well enough with very long iterables too, so leniter(..) is
better than len(list(...)).

Bye,
bearophile

 

[Bruno Desthuilliers]

Carl Banks a écrit :

And some would argue that it was wrong to have such a wide definition
for the truth value of an expression in the first place...

Indeed !-)

 

[Lawrence D'Oliveiro]

From this viewpoint, objecters would instead have to argue that it is
wrong to have such implicit calls and that programmers should have to
put them in explicitly.

But then again, you want to avoid unexpected restrictions like in Java,
where bool is a separate type, and while it is discrete, it cannot be used
to index arrays.

(Cf Pascal, where any discrete type could be used as an array index type.)