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Date: Tue, 31 May 2011 13:05:25 +1000
Subject: Re: scope of function parameters (take two)
From: Chris Angelico <rosuav@gmail.com>
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On Tue, May 31, 2011 at 10:28 AM, Henry Olders <henry.olders@mcgill.ca> wro=
te:
> I don't believe I'm the only person who thinks this way. Here is a quote =
from wikipedia: "It is considered good programming practice to make the sco=
pe of variables as narrow as feasible so that different parts of a program =
do not accidentally interact with each other by modifying each other's vari=
ables. Doing so also prevents action at a distance. Common techniques for d=
oing so are to have different sections of a program use different namespace=
s, or to make individual variables "private" through either dynamic variabl=
e scoping or lexical variable scoping." (http://en.wikipedia.org/wiki/Varia=
ble_(programming)#Scope_and_extent).
>

Side point, on variable scope.

There is a philosophical split between declaring variables and not
declaring them. Python is in the latter camp; you are not required to
put "int a; char *b; float c[4];" before you use the integer, string,
and list/array variables a, b, and c. This simplifies code
significantly, but forces the language to have an unambiguous scoping
that doesn't care where you assign to a variable.

Example:

def f():
  x=3D1  # x is function-scope
  if cond: # cond is global
    x=3D2 # Same function-scope x as above
  print(x) # Function-scope, will be 2 if cond is true

This is fine, and is going to be what you want. Another example:

def f():
  print(x) # global
  # .... way down, big function, you've forgotten what you're doing
        for x in list_of_x:
          x.frob()

By using x as a loop index, you've suddenly made it take function
scope, which stops it from referencing the global. Granted, you
shouldn't be using globals with names like 'x', but it's not hard to
have duplication of variable names. As a C programmer, I'm accustomed
to being able to declare a variable in an inner block and have that
variable cease to exist once execution leaves that block; but that
works ONLY if you declare the variables where you want them.

Infinitely-nested scoping is simply one of the casualties of a
non-declarative language.

Chris Angelico