[Tutor] Why are expressions not allowed as parameters in function definition statements?

[Steven D'Aprano]

On Sat, Jun 18, 2016 at 02:04:46PM -0500, boB Stepp wrote:
> I have (Finally!) gotten a bit of time to look at Peter's answer to my
> Model-View-Controller question from May 29th, particularly his
> CircleImageView class to which he added a "#FIXME" comment.  I thought
> it would be helpful to abbreviate his distance function in the
> interpreter while I played around with pencil and graph paper.  I got:
> 
> Python 3.5.1 (v3.5.1:37a07cee5969, Dec  6 2015, 01:54:25) [MSC v.1900
> 64 bit (AMD64)] on win32
> Type "help", "copyright", "credits" or "license" for more information.
> py3: def d(row, col/2, radius=5):
>   File "<stdin>", line 1
>     def d(row, col/2, radius=5):
>                   ^
> SyntaxError: invalid syntax
> 
> And this surprised me.

I'm surprised that you're surprised. I don't even know what you expect a 
parameter col/2 would even mean.

> It seems that only identifiers are allowed as
> parameters in a function definition statement, and I cannot help but
> wonder why?

Because they are parameters, which by definition are variable names, 
i.e. identifiers. What else could they be?


def foo(x, 1+2, y):
    # how do I refer to the second parameter here?

foo(1000, 2000, 3000)  # what happens to the second argument here?


Can you explain what you expected

def d(row, col/2)

to mean? I have literally no idea.

> It seems that in most other places in Python's syntax it
> will allow one to insert almost any kind of object or expression.

You can't use arbitrary expressions on the left hand side of 
assignment:

1 + 2 = "some value"
x/2 = y

Function parameters are a form of assignment.


-- 
Steve