what is lambda used for in real code?

**Alex Martelli** · Jul 18 '05, 07:40 PM

Re: what is lambda used for in real code?

Steven Bethard <steven.bethard @gmail.com> wrote:
[color=blue]
> (2) lambda a: a.lower()
> My first thought here was to use str.lower instead of the lambda, but of
> course that doesn't work if 'a' is a unicode object:[/color]

Right, but string.lower works (after an 'import string'). More
generally, maybe it would be nice to have a way to say "call a method on
x" without x's type being checked, just like attrgetter says "fetch an
attribute on x" -- say s/thing like:

def methodcaller(me thod_name, *a, **k):
def callit(x):
return getattr(x, method_name)(*a , **k)
callit.__name__ = method_name
return callit

[color=blue]
> (3) self.plural = lambda n: int(n != 1)
> Note that this is *almost* writable with def syntax. If only we could do:
> def self.plural(n):
> int(n != 1)[/color]

Not sure about the context, but maybe we could use, at class-level:
@staticmethod
def plural(n):
return int(n != 1)

[color=blue]
> (4) objs.sort(lambd a a, b: cmp(getattr(a, 'lineno', 0),
> getattr(b, 'lineno', 0)))
> My first intuition here was to try something like:
> objs.sort(key=o perator.attrget ter('lineno'))
> but this doesn't work because then we don't get the default value of 0
> if the attribute doesn't exist. I wonder if operator.attrge tter should
> take an optional "default" parameter like getattr does:[/color]

The optional default parameter sounds like a good idea to me.

Even though a good number of lambda uses may be avoidable or removable
by such means, I think there's just slightly too much variety -- in some
cases, a def with a name will have to be best (just as it would even
today if, say, an if/else had to be part of the logic -- simulations of
ternary operators being rarely clearest and most solid).

Alex

**Diez B. Roggisch** · Jul 18 '05, 07:41 PM

Re: what is lambda used for in real code?

> So, those are my thoughts on how lambdas are "really" used. If others[color=blue]
> out there have real-life code that uses lambdas in interesting ways,
> feel free to share them here![/color]

I use them in conjunction with metaclasses and properties:

def _s_item(self, item):
""" saw::active """
self.__item = item
self.set_state( )
self.indexWidge t.setText("%i" % item.index)
created = item.created
dt = QDateTime(QDate (created.year, created.month, created.day),
QTime(created.h our, created.minute, created.second) )
self.createdWid get.setDateTime (dt)
self.set_text()
self.set_list_i tems(self.histo ry, item.history)
self.set_list_i tems(self.train log, item.train_log)
self.set_classi fication_result ()

self.adjust_hea der_sizes()

def _g_item(self):
return self.__item

# the lambda is needed for late-binding so that metaclass-wrapping will
# be in effect.
item = property(_g_ite m, lambda self, v: self._s_item(v) )

The doc string of _s_item contains a token the metaclass is aware of and
creates a wrapper around _s_item. That works nice on methods, but I found
that properties got bound to their functions _before_ the metaclass kicks
in, so the property wasn't called in the wrapped version, resulting in
errors. So I introduced the lambda that makes the method call "lazy". Of
course I could have introduced a

def _s_item_unwrapp ed(self, v):
self._s_item(v)

and used that in the property - but as there are lambdas, I use them :)

And the second def here is not more explanatory, as one has to graps the
internal details of python properties to understand why that extra hoop is
introduced in the first place.
--
Regards,

Diez B. Roggisch

**Steven Bethard** · Jul 18 '05, 07:41 PM

Re: what is lambda used for in real code?

Alex Martelli wrote:[color=blue]
> Steven Bethard <steven.bethard @gmail.com> wrote:
>[color=green]
>>(2) lambda a: a.lower()
>>My first thought here was to use str.lower instead of the lambda, but of
>>course that doesn't work if 'a' is a unicode object:[/color]
>
>
> Right, but string.lower works (after an 'import string'). More
> generally, maybe it would be nice to have a way to say "call a method on
> x" without x's type being checked, just like attrgetter says "fetch an
> attribute on x" -- say s/thing like:
>
> def methodcaller(me thod_name, *a, **k):
> def callit(x):
> return getattr(x, method_name)(*a , **k)
> callit.__name__ = method_name
> return callit[/color]

Yeah, that's exactly the kind of thing I was looking for. Very nice!
[color=blue][color=green]
>>(3) self.plural = lambda n: int(n != 1)
>>Note that this is *almost* writable with def syntax. If only we could do:
>> def self.plural(n):
>> int(n != 1)[/color]
>
>
> Not sure about the context, but maybe we could use, at class-level:
> @staticmethod
> def plural(n):
> return int(n != 1)[/color]

The context was within the _parse method of GNUTranslations . Basically,
this method uses the fp passed in and a bunch of conditionals to
determine how to define the plural method. So I don't think it can be
done at the class level. Also, doesn't the assignment:
self.plural = lambda n: int(n != 1)
make this more like (at class level):
def plural(self, n):
return int(n != 1)
that is, isn't this an instance method, not a staticmethod?

py> class C(object):
.... def __init__(self):
.... self.plural = lambda n: int(n != 1)
....
py> c = C()
py> c.__class__.plu ral(1)
Traceback (most recent call last):
File "<interacti ve input>", line 1, in ?
AttributeError: type object 'C' has no attribute 'plural'
py> c.plural(1)
0
[color=blue]
> Even though a good number of lambda uses may be avoidable or removable
> by such means, I think there's just slightly too much variety -- in some
> cases, a def with a name will have to be best[/color]

Yup, that was my feeling. I was only able to rewrite as an expression
about 50% of the lambdas that I found. However, I (personally) don't
have much of a problem with adding a def in most of the other cases.
The only ones that make me a little nervous are examples like:

inspect.py: def formatargspec(a rgs, varargs=None, varkw=None,
...
formatvarargs=l ambda name: '*' + name,
formatvarkw=lam bda name: '**' + name,
formatvalue=lam bda value: '=' + repr(value),

where the lambdas are declaring functions as keyword arguments in a def.
I'm not sure how much I like adding to the module multiple function
defs that are really intended to be accessed only within formatargspec.
Still, were lambda to go away in Python 3000, it certainly wouldn't be
the end of the world. ;-)

Steve

**Alex Martelli** · Jul 18 '05, 07:41 PM

Re: what is lambda used for in real code?

Steven Bethard <steven.bethard @gmail.com> wrote:
[color=blue][color=green][color=darkred]
> >>(3) self.plural = lambda n: int(n != 1)
> >>Note that this is *almost* writable with def syntax. If only we could do:
> >> def self.plural(n):
> >> int(n != 1)[/color]
> >
> > Not sure about the context, but maybe we could use, at class-level:
> > @staticmethod
> > def plural(n):
> > return int(n != 1)[/color]
>
> The context was within the _parse method of GNUTranslations . Basically,
> this method uses the fp passed in and a bunch of conditionals to
> determine how to define the plural method. So I don't think it can be[/color]

Ah, OK -- I see, then you're probably quite right here!
[color=blue]
> done at the class level. Also, doesn't the assignment:
> self.plural = lambda n: int(n != 1)
> make this more like (at class level):
> def plural(self, n):
> return int(n != 1)
> that is, isn't this an instance method, not a staticmethod?[/color]

Apart from the different possible definitions (which are of course
crucial), I don't see that. Given the fact that, if you define plural
as an instancemethod, you're not using 'self' anyway, what usage would
break with a staticmethod? "Doesn't use 'self'" smells more like a
staticmethod to me, even if you always call it on an instance.
[color=blue]
> py> class C(object):
> ... def __init__(self):
> ... self.plural = lambda n: int(n != 1)
> ...
> py> c = C()
> py> c.__class__.plu ral(1)
> Traceback (most recent call last):
> File "<interacti ve input>", line 1, in ?
> AttributeError: type object 'C' has no attribute 'plural'
> py> c.plural(1)
> 0[/color]

This shows that staticmethod has slightly wider applicability, yes, but
I don't see this as a problem. IOW, I see no real use cases where it's
important that hasattr(C, 'plural') is false while hasattr(C(),
'plural') is true [I could of course be missing something!].

Alex

**Steven Bethard** · Jul 18 '05, 07:41 PM

Re: what is lambda used for in real code?

Alex Martelli wrote:[color=blue]
> Steven Bethard <steven.bethard @gmail.com> wrote:[color=green]
>>
>>py> class C(object):
>>... def __init__(self):
>>... self.plural = lambda n: int(n != 1)
>>...
>>py> c = C()
>>py> c.__class__.plu ral(1)
>>Traceback (most recent call last):
>> File "<interacti ve input>", line 1, in ?
>>AttributeErro r: type object 'C' has no attribute 'plural'
>>py> c.plural(1)
>>0[/color]
>
>
> This shows that staticmethod has slightly wider applicability, yes, but
> I don't see this as a problem. IOW, I see no real use cases where it's
> important that hasattr(C, 'plural') is false while hasattr(C(),
> 'plural') is true [I could of course be missing something!].[/color]

True, true. I guess I was just wrapped up in reproducing the class
behavior. Making it available as a staticmethod of the class would of
course only add functionality, not remove any.

Steve

**Adam DePrince** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

On Fri, 2004-12-31 at 01:53, Steven Bethard wrote:[color=blue]
> I thought it might be useful to put the recent lambda threads into
> perspective a bit. I was wondering what lambda gets used for in "real"
> code, so I grepped my Python Lib directory. Here are some of the ones I
> looked, classified by how I would rewrite them (if I could):
>[/color]
<snipping wonderful verbosity>
[color=blue]
> So, those are my thoughts on how lambdas are "really" used. If others
> out there have real-life code that uses lambdas in interesting ways,
> feel free to share them here!
>[/color]

Lets not forget the "real reason" for lambda ... the elegance of
orthogonality. Why treat functions differently than any other object?

We can operate on every other class without having to involve the
namespace, why should functions be any different? Wouldn't it to awful
if we had to write:

x = 3 * y ** 2 + 4 * y + 5

as

a = 3
e = 2
b = 4
c = 5
x = a * y ** e + b * y + c

Everybody understand that sometimes a number just isn't important enough
to assign to the name space. And we all can understand that this
applies to other data types, for example:

print "The answer is", x

Unless internationaliz ation was a concern, few people would write:

THE_ANSWER_IS = "The answer is"
print THE_ANSWER_IS, x

But when we consider functions, we suddenly change. Understandably we
have a certain bias towards functions. When programming, the most
commonly constructed object is the function. We likely spend more time
crafting function objects than any other object. Our reflex to
economize on the programming process focuses on the reduction in
function code creation time, hence the a focus on reuseabiity and a
plethora of ways to categorize our code to achieve this end.

The notion that we would use a function exactly once is almost blasphemy
to such a perspective. But it is true ... there are times when a
programmer will want to construct and use a function in exactly one
place for one singular purpose. In my own code, this occurs most often
when the function is used as a parameters to another function.

Examples of this are the cmp parameters to [].sort. The function I
provide to cmp is only barely more important to preserve for posterity
than the value I might provide to the same functions "reverse"
parameter.

In sort, we must preserve the ability to create an anonymous function
simply because we can do so for every other object type, and functions
are not special enough to permit this special case.

Adam DePrince

**Steven Bethard** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

Adam DePrince wrote:[color=blue]
> Lets not forget the "real reason" for lambda ... the elegance of
> orthogonality. Why treat functions differently than any other object?
>
> We can operate on every other class without having to involve the
> namespace, why should functions be any different?[/color]

Yup. I think in most of the examples that I didn't know how to rewrite,
this was basically the issue. On the other hand, I do think that
lambdas get overused, as indicated by the number of examples I *was*
able to rewrite.[1]

Still, I have to admit that in some cases (especially those involving
reduce), I wish the coder had named the function -- it would have given
me a little bit more documentation as to what the code was trying to do.

On the other hand, in other cases, like when a function is a keyword
argument to another function (e.g. inspect.py's "def formatargspec.. ."
example) using a def statement and naming the function would be redundant.

Steve

[1] Note that this isn't entirely fair to the examples, some of which
were written before list comprehensions, generator expressions and
itemgetter/attrgetter.

**Hans Nowak** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

Adam DePrince wrote:
[color=blue]
> In sort, we must preserve the ability to create an anonymous function
> simply because we can do so for every other object type, and functions
> are not special enough to permit this special case.[/color]

Your reasoning makes sense... lambda enables you to create a function as
part of an expression, just like other types can be part of an
expression. However, by that same reasoning, maybe classes aren't
special enough either to warrant a special case. Where's the keyword to
create an anonymous class? :-)

--
Hans Nowak

zephyrfalcon.org

http://zephyrfalcon.org/

Memimpin Angin Perubahan Teknologi

**Steven Bethard** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

Hans Nowak wrote:[color=blue]
> Adam DePrince wrote:
>[color=green]
>> In sort, we must preserve the ability to create an anonymous function
>> simply because we can do so for every other object type, and functions
>> are not special enough to permit this special case.[/color]
>
>
> Your reasoning makes sense... lambda enables you to create a function as
> part of an expression, just like other types can be part of an
> expression. However, by that same reasoning, maybe classes aren't
> special enough either to warrant a special case. Where's the keyword to
> create an anonymous class? :-)[/color]

Well, no keyword, but you can use the type function:

py> d = dict(c=type('C' , (object,), dict(spam=42)),
.... d=type('D', (dict,), dict(badger=Tru e)))
py> d['c'].spam
42
py> d['c']()
<__main__.C object at 0x063F2DD0>

Steve

**Alex Martelli** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

Steven Bethard <steven.bethard @gmail.com> wrote:
...[color=blue][color=green]
> > Your reasoning makes sense... lambda enables you to create a function as
> > part of an expression, just like other types can be part of an
> > expression. However, by that same reasoning, maybe classes aren't
> > special enough either to warrant a special case. Where's the keyword to
> > create an anonymous class? :-)[/color]
>
> Well, no keyword, but you can use the type function:
>
> py> d = dict(c=type('C' , (object,), dict(spam=42)),
> ... d=type('D', (dict,), dict(badger=Tru e)))
> py> d['c'].spam
> 42
> py> d['c']()
> <__main__.C object at 0x063F2DD0>[/color]

Well then, just call new.function to similarly create functions as part
of an expression, hm? Passing the bytecode in as a string isn't
incredibly legible, OK, but, we've seen worse...;-)

Alex

**Reinhold Birkenfeld** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

Adam DePrince wrote:
[color=blue][color=green]
>> So, those are my thoughts on how lambdas are "really" used. If others
>> out there have real-life code that uses lambdas in interesting ways,
>> feel free to share them here!
>>[/color]
>
> Lets not forget the "real reason" for lambda ...[/color]

I really hoped you would point out the _real_ reason for lambda...
[color=blue]
> the elegance of orthogonality.[/color]

.... but you didn't.

Everyone knows that lambda is there to help in one-liner contests and
code obfuscation.

Lambda is one of Python's very few instruments that assist in writing
code reaching Perl's unreadability, and as such it should be valued highly!

<big-evil-grin-wink>

Reinhold

**Terry Reedy** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

"Adam DePrince" <adam@cognitcor p.com> wrote in message
news:1104531721 .3724.18.camel@ localhost.local domain...[color=blue]
> In sort, we must preserve the ability to create an anonymous function
> simply because we can do so for every other object type, and functions
> are not special enough to permit this special case.[/color]

Please show me how to create an anonymous type, module, or class,
especially with an expression. Number, sequences, and dicts have easily
printable values. Functions, like classes and module do not*, so
definition names act as a standin and as a pointer to the source code that
produced the object. If functions are not special relative to classes and
modules, which is the grouping they belong with, then we should get rid of
lambda ;-)

*Now that memory is 'cheap', someone recently proposed that code objects
(and hence functions) get .source attribute. Maybe, someday...

Terry J. Reedy

**Aahz** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

In article <mailman.18.110 4531731.22381.p ython-list@python.org >,
Adam DePrince <adam@cognitcor p.com> wrote:[color=blue]
>
>Unless internationaliz ation was a concern, few people would write:
>
>THE_ANSWER_I S = "The answer is"
>print THE_ANSWER_IS, x[/color]

However, there's a moderately large (and growing!) set of people who
would argue that I18N is *always* a concern -- it's just that a lot of
people either don't know it yet or ignore it.
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

"19. A language that doesn't affect the way you think about programming,
is not worth knowing." --Alan Perlis

**Alex Martelli** · Jul 18 '05, 07:42 PM

Re: what is lambda used for in real code?

Terry Reedy <tjreedy@udel.e du> wrote:
[color=blue]
> "Adam DePrince" <adam@cognitcor p.com> wrote in message
> news:1104531721 .3724.18.camel@ localhost.local domain...[color=green]
> > In sort, we must preserve the ability to create an anonymous function
> > simply because we can do so for every other object type, and functions
> > are not special enough to permit this special case.[/color]
>
> Please show me how to create an anonymous type, module, or class,
> especially with an expression. Number, sequences, and dicts have easily[/color]

new.module('<th eta>') creates a module and can be used within an
expression. Of course 'anonymous' is doubtful, i.e.:
[color=blue][color=green][color=darkred]
>>> new.module('<th eta>').__name__[/color][/color][/color]
'<theta>'

but then
[color=blue][color=green][color=darkred]
>>> (lambda:23).__n ame__[/color][/color][/color]
'<lambda>'

So the parallel is there, roughly. You can create old-style classes
similarly, with new.classobj, and new-style ones by calling type, which
is more analogous to, say, creating sets by calling set, decimal numbers
by calling decimal.Decimal , and so on.

So the creating isn't a big problem -- by analogy with other types it
should be done by calling some callable, either built-in or from the
library. And, we do have new.function for that; problem is the code
object that you need to pass as the first argument... there's new.code,
too, but it wants a gazillion args, including a codestring that's not
nice to put together;-). "Not for the faint of heart" as the docs say.

Contents of modules and classes are less problematic. Well, new.module
should perhaps take an optional dict argument, like new.classobj or
calling type -- right now it's hard to create _and populate_ the module
within the same expression. Not that I have any use case for this,
though. And there's the rub...:
[color=blue]
> printable values. Functions, like classes and module do not*, so
> definition names act as a standin and as a pointer to the source code that
> produced the object. If functions are not special relative to classes and
> modules, which is the grouping they belong with, then we should get rid of
> lambda ;-)[/color]

Yes but... we DO have a few real use cases for functions, which we don't
really have for modules and classes, _and_ making the *contents* of a
function object is harder too...:-(

Alex

what is lambda used for in real code?

what is lambda used for in real code?

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