question about what lamda does

[danielx]

nephish at 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

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:

>>> def foo(): pass
...
>>> foo
<function foo at 0x009D8230>
#             ^ foo knows its own name
>>> bar
<function foo at 0x009D8230>
#             ^ see ;)
>>>

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

>>> spam = lambda: 1
>>> spam
<function <lambda> at 0x009D80F0>
#             ^ spam has an identity crisis ;)
>>>

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.