How much is set in stone?

[Jive Dadson]

Thanks.  Before I submit a formal proposal, let me run it by you guys.

I've looked through all the PEPS and I was surprised that my suggestion 
is not already there.  So, here it is.  Please comment.

Other languages have started life with the feature that the first 
occurrence of an identifier defines it.  The first time a programmer 
spends an afternoon chasing a bug introduced by misspelling an 
identifier, he begins to doubt the wisdom of that feature.  Suppose you 
are called on to modify a long and complex module written by someone 
else.  At some point you forget that the original programmer did not 
comply with your company's variable naming conventions.  He named a 
variable "keepRecord" when it should have been "keep_record".  After 
you've made your changes, the code usually works fine, but occasionally 
it deletes a record it is supposed to keep.  You stare at the code for 
hours and you can see absolutely nothing wrong with it.

Here's another one.  A programmer who is new to Python code might write 
the following:

initialized = 0
def init_everything():
	blah()
	blah()
	blah()
	initialized = 1
	
# Oh noooooo!

I would have complete sympathy for this hypothetical unfortunate fellow.

Visual Basic (and perhaps other Basics, so far as I know) implement a 
way to have your cake and yet not spoil the old cake.  It will 
correctly process code that is written to the old standard, but it 
allows you to require a defining occurrence of an identifier.  If you 
want it the new way, you put a line at the top of your module that says,

option explicit

Of course you must know to put that in your source code, but most books 
on Visual Basic tell you up front always to do it.  It gets to be a 
habit.

Here's how it might work in Python.  Again the module would have 
something at the top, like "option explicit" ("import explicit"? or 
what?) telling the interpreter that within that module, variables must 
be declared before use.  Then the defining occurrence of a variable 
could look similar to the declaration of a function:

option explicit

def initialized = 0 
def init_everything():
	blah() 
	blah() 
	blah() 
	initialized = 1
	
The above code would work as anticipated.  No pesky "global" qualifier.

The following would generate an error message, just like trying to call 
an undefined function does:

>>>option explicit 
>>>initialized = 0 
Traceback (most recent call last): 
  File "<stdin>", line 1, in ? 
NameError: initialized is not defined.


After due warning (a year or two), "option explicit" could become the 
default.  Old code might need "option implicit" or something added at 
the top to keep it working.