[Edu-sig] Calling functions...

[kirby urner]

Something I maybe hadn't been clear on
before cramming for a workshop:

Even functions with purely positional
parameters may be passed a dict, in
which case the positional parameters
will be treated like named ones, i.e.
the dict will map to them.

I'll use the new formating protocol:

>>> from __future__ import print_function

>>> def f(a, b, c):
	print("a={0}, b={1}, c={2}".format(a,b,c))

	
>>> f(1,2,3)
a=1, b=2, c=3

Here's what I'm illustrating:

>>> thedict = dict(c=5,a=10,b='cat')

>>> f(**thedict)
a=10, b=cat, c=5

Now what if we try to sneak in an argument
that doesn't map:

>>> thedict = dict(c=5,a=10,b='cat', d='dog')
>>> f(**thedict)

Traceback (most recent call last):
  File "<pyshell#11>", line 1, in <module>
    f(**thedict)
TypeError: f() got an unexpected keyword argument 'd'

The d is caught, an exception is raised.

So here we might add the all purpose "pooper scooper"
(yeah, scatological -- sometimes effective pedagogy):

>>> def f(a, b, c, **kwargs):
	print("a={0}, b={1}, c={2}".format(a,b,c))

The d now gets through, though nothing echoes:
	
>>> f(**thedict)
a=10, b=cat, c=5

Lets make sure we see what's in "overflow":

>>> def f(a, b, c, **kwargs):
	print("a={0}, b={1}, c={2}".format(a,b,c))
	print(kwargs)

	
>>> f(**thedict)
a=10, b=cat, c=5
{'d': 'dog'}

Is an all purpose pooper scooper allowed to have
defaults?  No, default arguments are what turn
parameters into named parameters.  *args and
**kwargs (often so named by convention) are
not named parameters so much as "collectors"
of "overflow" arguments (made-up terminology).

>>> def f(a, b, c, d='dog', *e):
	print("a={0}, b={1}, c={2} d={3} e={4}".format(a,b,c,d,e))

	
>>> f(**thedict)
a=10, b=cat, c=5 d=dog e=()
>>>

Speaking of made-up terminology, when I pass
**thedict as an argument, I tend to think of the
** as "explode".

If you don't double-star it, it goes through as just
the one argument and an exception gets raised:

>>> f(thedict)

Traceback (most recent call last):
  File "<pyshell#31>", line 1, in <module>
    f(thedict)
TypeError: f() takes at least 3 arguments (1 given)

The same "explode" operator works for tuples in
that it "unpacks" the one argument into len(tuple)
arguments.  The typical use is to make (x,y,z)-tuples
map to some function that expects x, y and z as
separate arguments.

The error messages remind us when we fail to
"explode" one argument into many:

>>> def f(x, y, z):
	return x**2 + y**2 + z**2

>>> f(1,2,3)
14
>>> coords = (1,2,3)
>>> f(coords)

Traceback (most recent call last):
  File "<pyshell#38>", line 1, in <module>
    f(coords)
TypeError: f() takes exactly 3 arguments (1 given)
>>> f(*coords)
14
>>> f(dict(y=2,x=1,z=3))

Traceback (most recent call last):
  File "<pyshell#40>", line 1, in <module>
    f(dict(y=2,x=1,z=3))
TypeError: f() takes exactly 3 arguments (1 given)
>>> f(**dict(y=2,x=1,z=3))
14

Kirby