[Tutor] lambda and map

[alan.gauld@bt.com]

> Okay, I am now what I would call competent in Python.  But, I keep
> noticing (learning) some new stuff every time I get an email from
> tutor.  There is, however, one thing I am not really sure about,
> lambda.  I don't quite understand what a lambda does or why 
> to use it. 

First, I will (as usual? :-) recommend you visit my tutor 
and read the Functional Programming advbanced topic. It 
explains how lambda works and to some extent why.

http://www.crosswinds.net/~agauld/

Now the short answer:

lambda is just a shortcut for defining a single 
line function. It saves us cluttering up our code with lots of little
functions that only get used one.

thus:

double = lambda x : x*2

is identical to:

def double(x): return x*2

But in situations where we need to pass a function object - like map() we
can create the lambda within the map function call instead of defining it as
a function first. Thus

def double(x): return x*2
twos = map(double, range(6))

is the same as but longer than:

twos = map(lambda x: x*2, range(6))

There is also another use, particularly in programming 
Tkinter where we use a lambda within a loop to create 
the command function for a set of widgets. If the 
function has a default parameter. You cant do that 
by defining a function because the default parameter
is set when you define it, however using lambda inside 
the loop we create a new default each time thru the loop.


(See my hmgui.py code in the Games framework on Useless 
  Python for an example of this use.)

HTH.

Personal aside to the list...
To be honest, in Python lamda is a bit limited 
- essentially to single line expressions.
I'm not sure why we can't do:

>>>modmult = lambda x,y:
         result = x * y
         if result > 0:
            return result
         else: return (-1 * result)

>>>modmult(4,3)
12
>>>modmult(4,-3)
12

I know that in this case a def modmult(x,y): would do 
the same(*) but given lambda's use of a colon I'd have 
thought the extra to allow block defintions would 
have been viable.

Alan G.