[Tutor] lamda in list comp

[Danny Yoo]

On Sat, Mar 28, 2015 at 6:25 PM, Jim Mooney <cybervigilante at gmail.com> wrote:
> Shouldn't this give me a list of squares?
> [lambda x: x**2 for x in range(10)]
>
> Instead I'm getting ten of these (with different addresses)
> <function <listcomp>.<lambda> at 0x01262A50>]


Mark Lawrence's answer shows how to correct this.


Let's look at the problem a little more closely.  To do so, one thing
we can do is rewrite to try removing the use of lambdas.  Here's a
fairly similar situation to what you had:

######################
    def square(x):
        return x * x

    [square for x in range(10)]
######################

If you saw that last expression, you might notice a mistake: we
haven't called the function on an argument!

We wanted to say:

    [square(x) for x in range(10)]

Once we understand this, let's go back to your original expression:

    [lambda x: x**2 for x in range(10)]

If we wanted to revise this so that it applied the function, we can do that:

    [(lambda x: x**2)(x) for x in range(10)]


If you're a functional programmer, Python's list comprehension syntax
will take a slight re-adjustment.  List comprehensions do sort-of the
same sort of thing as a functional map, but since it's built-in
syntax, it does a bit more, in that the "mapper" is an expression that
directly involves the mapped values.