2004 example, passing function error

[globalrev]

http://linuxgazette.net/109/pramode.html

>>>
>>>def sqr(x): return x*x

....

>>>def cube(x): return x*x*x

....

>>>sqr

<function sqr at 0x402ba10c>

>>>a = [sqr, cube]
>>>a[0](2)
>>>def compose(f, g): return f(g(x))

....

>>>compose(sq r, cube, 2)

64

>>>

but i get:

>>compose(sqr , cube, 2)

Traceback (most recent call last):
File "<pyshell#5 6>", line 1, in <module>
compose(sqr, cube, 2)
TypeError: compose() takes exactly 2 arguments (3 given)


this:

>>def compose(f, g, x): return f(g(x))
>>compose(sqr , cube, 2)

64

works though
.. so just a mistake on his part? but it looks like he just copied his
shell...has there been a change since 2004 inr egards to how you can
pass functions as arguments to functions??

[Carsten Haese]

Re: 2004 example, passing function error

globalrev wrote:

but it looks like he just copied his
shell...has there been a change since 2004 inr egards to how you can
pass functions as arguments to functions??

It looks like it's copy/pasted from a shell, but it's not. No past or
present Python interpreter could produce an interactive session like
that under any circumstances. My guess is that instead of copy/pasting,
the author simply retyped the session and made a mistake in doing so.

--
Carsten Haese

http://informixdb.sourceforge.net

[alex23]

Re: 2004 example, passing function error

On May 14, 11:40 am, globalrev <skanem...@yaho o.sewrote:

so just a mistake on his part? but it looks like he just copied his
shell...has there been a change since 2004 inr egards to how you can
pass functions as arguments to functions??

Adding the value argument (x) to the compose function really limits
its usefulness as well. Given the author was talking about functions
as first class objects, it makes more sense for compose to return a
function, rather than to perform several function calls and return a
value.

Here, compose creates a new function that performs first 'f' then 'g'
on a value:

>>def compose(f, g):

.... def _compose(x):
.... return f(g(x))
.... return _compose
....

>>compose(sqr , cube)

<function _compose at 0x00B7BEB0>

>>compose(sqr , cube)(2)

64

The advantage being you can easily create function chains this way:

>>sqrcube = compose(sqr, cube)
>>sqrcube(2)

64

And because they're functions, you can use them with compose to
construct more complex functions:

>>cubesqrcube = compose(cube, sqrcube)
>>cubesqrcube(2 )

262144

Just to show they're functionally the same:

>>cubesqrcube(2 ) == cube(sqrcube(2) ) == cube(sqr(cube(2 )))

True

I like this approach because it separates the function composition
from its execution. But you can also achieve much the same by using
functools.parti al (in 2.5+):

>>def compose(f, g, x): return f(g(x))

....

>>compose(sqr , cube, 2)

64

>>from functools import partial
>>sqrcube = partial(compose , sqr, cube)
>>sqrcube(2)

64

>>cubesqrcube = partial(compose , cube, sqrcube)
>>cubesqrcube(2 )

262144

Hope this helps.

- alex23