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]

ne*****@xit.net wrote:

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
Dan Bishop wrote:

ne*****@xit.net wrote:

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.

[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(w hen_done_call_t his=callback)
But with lambda you could just write
some_function(w hen_done_call_t his=lambda x,y:x*y)
Note: because it is an _expression_ you cannot do stuff like 'if..else'
inside of lambda.

-Nick V.

ne*****@xit.net wrote:

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

[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

Nick Vatamaniuc wrote:

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(w hen_done_call_t his=callback)
But with lambda you could just write
some_function(w hen_done_call_t his=lambda x,y:x*y)
Note: because it is an _expression_ you cannot do stuff like 'if..else'
inside of lambda.

-Nick V.

ne*****@xit.net wrote:

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

[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

ne*****@xit.net wrote:

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

[K.S.Sreeram]

gr******@gmail. com wrote:

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]

gr******@gmail. com wrote:

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
--
Steve Holden [phone] [phone]
Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

[tac-tics]

ne*****@xit.net wrote:

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...

K.S.Sreeram wrote:

gr******@gmail. com wrote:

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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