question about what lamda does

[nephish]

Hey there,
i have been learning python for the past few months, but i can seem to
get what exactly a lamda is for. What would i use a lamda for that i
could not or would not use a def for ? Is there a notable difference ?
I only ask because i see it in code samples on the internet and in
books.

thanks for any clarity

sk

 

[Dan Bishop]

Hey there,
i have been learning python for the past few months, but i can seem to
get what exactly a lamda is for.

It defines a function.

f = lambda x, y: expression

is equivalent to

def f(x, y):
return expression

Note that lambda is an expression while def is a statement.

What would i use a lamda for that i
could not or would not use a def for ? Is there a notable difference ?
I only ask because i see it in code samples on the internet and in
books.

Lambdas are typically used as parameters to functions that take
functions as arguments, like property() and reduce(). You never *need*
to use one, but sometimes it's convenient.

 

[nephish]

ok, i think i get it.
pretty cool.
thanks
-sk

It defines a function.

f = lambda x, y: expression

is equivalent to

def f(x, y):
return expression

Note that lambda is an expression while def is a statement.


Lambdas are typically used as parameters to functions that take
functions as arguments, like property() and reduce(). You never *need*
to use one, but sometimes it's convenient.

 

[Nick Vatamaniuc]

Use it anywhere a quick definition of a function is needed that can be
written as an expression. For example when a callback function is
needed you could say:
def callback(x,y):
return x*y
some_function(when_done_call_this=callback)
But with lambda you could just write
some_function(when_done_call_this=lambda x,y:x*y)
Note: because it is an _expression_ you cannot do stuff like 'if..else'
inside of lambda.

-Nick V.

 

[nephish]

so a lamda needs to stay at one expression, and use more than one lamda
for more expressions ?

i think i get it.

sk

 

[grebekel]

The two primary differences between using def and using lambda is that
lambda is limited to a single expression and def cannot be used within
another function.

Basically, use lambda when you need to define a small function within
another function. I've also used it to create 'shortcut' functions as:

bar = lambda a, b: object1.object2.function1( a ).function2( b )
foo = bar( somevalue1, somevalue2 )

It can save you a lot of typing if used wisely, and makes your code
smaller and neater too

On a real world example:

import random
roll_die = lambda sides=6: random.choice( range(1,sides+1) )
# roll dies with 4, 6, and 20 sides
print roll_die(4), roll_die(), roll_die(20)


Have fun with your lambdas.

greb

 

[K.S.Sreeram]

The two primary differences between using def and using lambda is that
lambda is limited to a single expression and def cannot be used within
another function.

'def' can certainly be used within another function :

def make_adder( delta ) :
def adder( x ) :
return x + delta
return adder

[sreeram;]


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[Steve Holden]

The two primary differences between using def and using lambda is that
lambda is limited to a single expression and def cannot be used within
another function.

Where on earth did you get that from? I presume you mean "You can't use
a def statement as an argument to a function"?

There is nothing wrong with

def adder(x):
def func(y):
return x+y
return func

for example. Then adder(5) returns a function that returns 5 more than
its argument.

regards
Steve

 

[tac-tics]

Hey there,
i have been learning python for the past few months, but i can seem to
get what exactly a lamda is for. What would i use a lamda for that i
could not or would not use a def for ? Is there a notable difference ?
I only ask because i see it in code samples on the internet and in
books.

Lambda is just as powerful as a function, but totally useless =-P

Lambda used to be handy before the introduction of list comprehensions.
Now, though, there primary use is obfuscating your code.

 

[grebekel]

I stand corrected. Not sure where I got that from, improper
defragmentation due to sleep depravation perhaps...

The two primary differences between using def and using lambda is that
lambda is limited to a single expression and def cannot be used within
another function.

'def' can certainly be used within another function :

def make_adder( delta ) :
def adder( x ) :
return x + delta
return adder

[sreeram;]


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[Steve Holden]

Lambda is just as powerful as a function, but totally useless =-P

Lambda used to be handy before the introduction of list comprehensions.
Now, though, there primary use is obfuscating your code.

I do wish you could hold yourself back and stop muddying the waters.
Lambdas and list comprehensions have little or nothing to do with each
other. Unless you know something I don't ...

regards
Steve

 

[Iain King]

I do wish you could hold yourself back and stop muddying the waters.
Lambdas and list comprehensions have little or nothing to do with each
other. Unless you know something I don't ...

I think he meant that lambda's main use before was inside map and
filter; as stated earlier in the thread, lambda's main use was for
passing simple functions as arguments, and of these map and filter must
have made up a majority (and then I'd guess TKinter would be next).
List comprehensions replace map and filter, so...

I wouldn't put it as explosively as he has, but I find a lambda less
clear than a def too.

Iain

 

[danielx]

Hey there,
i have been learning python for the past few months, but i can seem to
get what exactly a lamda is for. What would i use a lamda for that i
could not or would not use a def for ? Is there a notable difference ?
I only ask because i see it in code samples on the internet and in
books.

thanks for any clarity

sk

hehe. Lambda's are kind of a sensative subject for pythoners who come
from Lisp. Guido being more of a C guy doesn't really like them, and
thought they should be removed in py3k. Last time I checked, he was
reconsidering because of public outcry, presumably from the Lisp crowd.

The standard reason for getting rid of it is "anywhere you need a
lambda, you can use a def". In addition to what has been said here,
there is another one small difference between lambda's and functions,
which is that when you use def, the object gets a name:

Whereas, a lambda has no name; it's "anonymous":

Many people who do not come from Lisp do not understand what the use of
a lambda is (and I have no idea what the purpose of having a name is).
Even people who do question whether it belongs in Python. In Lisp,
lambda's are the way things get done, because you can calculate
anything using just defines and expressions. This style does not fit
Python very well, since we do things using statements.

Python's lambda really can't be as powerful as Lisp's because Python
does not have expressions that do case analysis (this is not lambda's
fault, of course . The reason is that you really want to put each
case on its own set of lines. This enhances readability at the expense
of terseness. Since Python's statements are terminated by a newline, it
would be rather awkward to have a kind of expression where good style
calls for it to be spread out accross multiple lines.

You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:

def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )

The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want. This stay of
execution is accomplished by wrapping quotes around our expressions.
This example illustrates why we really need this kind of behavior,
because without it, we would fall into an infinite loop. Even if it
were safe to evaluate all cases, it's a big waste of time to do so.

Lastly, I think there is also a performance concern for certain uses of
lambda (correct me if I'm wrong). Say you have an expression with a
lambda in it where you could have used a def. Every time you evaluate
that expression, you have to construct a new lambda object, which takes
time. If you had used a def instead, you could hav avoided having to
construct multiple times.

 

[nephish]

hey thanks for that last post, although some of it was a bit over my
head.
i think i am getting more of the differences here.

thanks again,
sk

Hey there,
i have been learning python for the past few months, but i can seem to
get what exactly a lamda is for. What would i use a lamda for that i
could not or would not use a def for ? Is there a notable difference ?
I only ask because i see it in code samples on the internet and in
books.

thanks for any clarity

sk

hehe. Lambda's are kind of a sensative subject for pythoners who come
from Lisp. Guido being more of a C guy doesn't really like them, and
thought they should be removed in py3k. Last time I checked, he was
reconsidering because of public outcry, presumably from the Lisp crowd.

The standard reason for getting rid of it is "anywhere you need a
lambda, you can use a def". In addition to what has been said here,
there is another one small difference between lambda's and functions,
which is that when you use def, the object gets a name:

Whereas, a lambda has no name; it's "anonymous":

Many people who do not come from Lisp do not understand what the use of
a lambda is (and I have no idea what the purpose of having a name is).
Even people who do question whether it belongs in Python. In Lisp,
lambda's are the way things get done, because you can calculate
anything using just defines and expressions. This style does not fit
Python very well, since we do things using statements.

Python's lambda really can't be as powerful as Lisp's because Python
does not have expressions that do case analysis (this is not lambda's
fault, of course . The reason is that you really want to put each
case on its own set of lines. This enhances readability at the expense
of terseness. Since Python's statements are terminated by a newline, it
would be rather awkward to have a kind of expression where good style
calls for it to be spread out accross multiple lines.

You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:

def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )

The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want. This stay of
execution is accomplished by wrapping quotes around our expressions.
This example illustrates why we really need this kind of behavior,
because without it, we would fall into an infinite loop. Even if it
were safe to evaluate all cases, it's a big waste of time to do so.

Lastly, I think there is also a performance concern for certain uses of
lambda (correct me if I'm wrong). Say you have an expression with a
lambda in it where you could have used a def. Every time you evaluate
that expression, you have to construct a new lambda object, which takes
time. If you had used a def instead, you could hav avoided having to
construct multiple times.

 

[Bruno Desthuilliers]

danielx wrote:
(snip)

Python's lambda really can't be as powerful as Lisp's because Python
does not have expressions that do case analysis (this is not lambda's
fault, of course . The reason is that you really want to put each
case on its own set of lines. This enhances readability at the expense
of terseness. Since Python's statements are terminated by a newline, it
would be rather awkward to have a kind of expression where good style
calls for it to be spread out accross multiple lines.

You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:

def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )

Why do you want to use eval here ?

The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want.

What about:

def recursiveFunction(args):
... # do stuff...
... # that defines 'newArgs' and 'predicate' of course ...
return (recursiveFunction, lambda x: 0)[predicate](newArgs)

 

[danielx]

danielx wrote:
(snip)

Python's lambda really can't be as powerful as Lisp's because Python
does not have expressions that do case analysis (this is not lambda's
fault, of course . The reason is that you really want to put each
case on its own set of lines. This enhances readability at the expense
of terseness. Since Python's statements are terminated by a newline, it
would be rather awkward to have a kind of expression where good style
calls for it to be spread out accross multiple lines.

You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:

def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )

Why do you want to use eval here ?

The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want.

What about:

def recursiveFunction(args):
... # do stuff...
... # that defines 'newArgs' and 'predicate' of course ...
return (recursiveFunction, lambda x: 0)[predicate](newArgs)

Sure, that works, but don't take things so literally. For instance, if
you have a bunch of cases, you might not way to apply the same set of
arguments to all of them.

Also, let's not get distracted from the main point about how doing case
analysis in an expression is ugly, making lambda's weaker in Python
than in the language which inspired them.

 

[Bruno Desthuilliers]

An expression can span several lines.

This enhances readability at the expense

or by a ';'

it

I must be pretty dumb, but I don't see how this relate to the problem of
case analysis in lambda expressions ?

You can try to simulate these kinds expressions using into a list or
dictionary, but this becomes rather messy. I think the only way to get
this done properly is to use eval. For example:

def recursiveFunction(args):
... # do stuff...
choices = { True:"0", False:"recurisveFunction(newArgs)" }
return eval( choices[predicate] )

Why do you want to use eval here ?

The reason that you need eval is that you want to prevent any cases
from being executed until you decide which one you want.

What about:

def recursiveFunction(args):
... # do stuff...
... # that defines 'newArgs' and 'predicate' of course ...
return (recursiveFunction, lambda x: 0)[predicate](newArgs)

Sure, that works, but don't take things so literally.

Sorry for being pragmatic !-)

For instance, if
you have a bunch of cases, you might not way to apply the same set of
arguments to all of them.

return {
'case1' : lambda: someFunc(args1),
'case2' : lambda: someFunc(args2),
'case3' : lambda: someOtherFunc(args1, arg42),
}.get(predicate, lambda: 0)()

Still no need for eval()...

Now of course there are limits to the exercice, and we're still far away
from ML-like pattern matching or Lisp 'case' forms. As you noted, Python
is a statement-based language, not an expression-based one like Lisp.
This makes a definitive difference.

Also, let's not get distracted from the main point about how doing case
analysis in an expression is ugly,

making lambda's weaker in Python
than in the language which inspired them.

The fact is that Python "lambdas" are *not* Lisp lambdas. Python
"lambdas" are mostly a handy trick to turn a *simple* expression into a
closure - and definitively not a basic building block of the language.

Daniel, I of course do agree that Python lambdas are nothing near Lisp
lambdas - FWIW, Python is not Lisp neither -, but that looks like an
apple and banana comparison to me... IMHO, the most obvious problem with
Python lambdas is the word "lambda" !-)